Algorithm to return all combinations of k elements from n

Question

I want to write a function that takes an array of letters as an argument and a number of those letters to select    Say you provide an array of 8 letters and want to select 3 letters from that  Then you should get   8      8 - 3     3     56   Arrays  or words  in return consisting of 3 letters each

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Following Haskell code calculate the combination number and combinations at the same time  and thanks to Haskell s laziness  you can get one part of them without calculating the other   import Data Semigroup import Data Monoid  data Comb   MkComb  count    Int  combinations      Int    deriving  Show  Eq  Ord   instance Semigroup Comb where      MkComb c1 cs1   lt  gt   MkComb c2 cs2    MkComb  c1   c2   cs1    cs2   instance Monoid Comb where     mempty   MkComb 0     addElem    Comb - gt  Int - gt  Comb addElem  MkComb c cs  x   MkComb c  map  x    cs   comb    Int - gt  Int - gt  Comb comb n k   n  lt  0    k  lt  0   error  error in  comb n k   n and k should be natural number  comb n k   k    0    k    n   MkComb 1   take k  k-1 k-2  0    comb n k   n  lt  k   mempty comb n k   comb  n-1  k  lt  gt   comb  n-1   k-1   addElem   n-1     It works like    Main gt  comb 0 1 MkComb  count   0  combinations         Main gt  comb 0 0 MkComb  count   1  combinations           Main gt  comb 1 1 MkComb  count   1  combinations     0      Main gt  comb 4 2 MkComb  count   6  combinations     1 0   2 0   2 1   3 0   3 1   3 2      Main gt  count  comb 10 5  252

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We can use the concept of bits to do this   Let we have a string of  abc   and we want to have all combinations of the elements with length 2  i e  ab     ac   bc       We can find the set bits in numbers ranging from 1 to 2 n  exclusive   Here 1 to 7  and wherever we have set bits   2  we can print the corresponding value from string   for example    1 - 001  2 - 010 3 - 011 -  print ab  str 0    str 1   4 - 100  5 - 101 -  print ac  str 0    str 2   6 - 110 -  print ab  str 1    str 2   7 - 111     Code sample   public class StringCombinationK          static void combk String s   int k           int n   s length            int num   1 lt  lt n          int j 0          int count 0           for int i 0 i lt num i                 if  countSet i   k                   setBits i j s                   count                    System out println                                     System out println count              static void setBits int i int j String s      print the corresponding string value j represent the index of set bit         if i  0               return                     if i 2  1               System out print s charAt j                                         setBits i 2 j 1 s              static int countSet int i     count number of set bits         if  i  0               return 0                     return  i 2  0  0 1    countSet i 2              public static void main String   arhs           String s    abcdefgh           int k 3          combk s k

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I was looking for a similar solution for PHP and came across the following  class Combinations implements Iterator       protected  c   null      protected  s   null      protected  n   0      protected  k   0      protected  pos   0       function   construct  s   k            if is array  s                  this- gt s   array values  s                this- gt n   count  this- gt s             else                this- gt s    string   s               this- gt n   strlen  this- gt s                      this- gt k    k           this- gt rewind              function key             return  this- gt pos            function current              r   array            for  i   0   i  lt   this- gt k   i                 r      this- gt s  this- gt c  i            return is array  this- gt s     r   implode      r             function next             if  this- gt  next                 this- gt pos            else              this- gt pos   -1            function rewind              this- gt c   range 0   this- gt k            this- gt pos   0            function valid             return  this- gt pos  gt   0             protected function  next              i    this- gt k - 1          while   i  gt   0  amp  amp   this- gt c  i      this- gt n -  this- gt k    i               i--          if  i  lt  0              return false           this- gt c  i             while  i    lt   this- gt k - 1               this- gt c  i     this- gt c  i - 1    1          return true            foreach new Combinations  1234567   5  as  substring      echo  substring         source  I m not to sure how efficient the class is  but I was only using it for a seeder

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Lets say your array of letters looks like this   ABCDEFGH   You have three indices  i  j  k  indicating which letters you are going to use for the current word  You start with    A B C D E F G H       i j k   First you vary k  so the next step looks like that    A B C D E F G H         i j   k   If you reached the end you go on and vary j and then k again    A B C D E F G H         i   j k  A B C D E F G H           i   j   k   Once you j reached G you start also to vary i    A B C D E F G H           i j k  A B C D E F G H             i j   k       function initializePointers  cnt         pointers            for  i 0   i lt  cnt   i               pointers      i             return  pointers          function incrementPointers  amp  pointers   amp  arrLength        for  i 0   i lt count  pointers    i               currentPointerIndex   count  pointers  -  i - 1           currentPointer    pointers  currentPointerIndex            if  currentPointer  lt   arrLength -  i - 1                  pointers  currentPointerIndex               for  j 1    currentPointerIndex  j  lt count  pointers    j                       pointers  currentPointerIndex  j     pointers  currentPointerIndex   j                           return true                       return false     function getDataByPointers  amp  arr   amp  pointers         data            for  i 0   i lt count  pointers    i               data      arr  pointers  i               return  data     function getCombinations  arr   cnt         len   count  arr        result            pointers   initializePointers  cnt        do            result     getDataByPointers  arr   pointers         while incrementPointers  pointers  count  arr          return  result      result   getCombinations  0  1  2  3  4  5   3   print r  result     Based on https   stackoverflow com a 127898 2628125  but more abstract  for any size of pointers

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I d like to present my solution  No recursive calls  nor nested loops in next  The core of code is next   method   public class Combinations       final int pos        final List lt Object gt  set       public Combinations List lt   gt  l  int k            pos   new int k           set new ArrayList lt Object gt  l           reset              public void reset             for  int i 0  i  lt  pos length    i  pos i  i            public boolean next             int i   pos length-1          for  int maxpos   set size  -1  pos i   gt   maxpos  --maxpos                if  i  0  return false              --i                      pos i           while    i  lt  pos length              pos i  pos i-1  1          return true             public void getSelection List lt   gt  l             SuppressWarnings  unchecked           List lt Object gt  ll    List lt Object gt  l          if  ll size    pos length                ll clear                for  int i 0  i  lt  pos length    i                  ll add set get pos i                       else               for  int i 0  i  lt  pos length    i                  ll set i  set get pos i                         And usage example   static void main String   args        List lt Character gt  l   new ArrayList lt Character gt         for  int i 0  i  lt  32    i  l add  char   a  i        Combinations comb   new Combinations l 5       int n 0      do             n          comb getSelection l             Log debug   d   s   n  l toString           while  comb next         Log debug  num    d   n

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Here is a simple und understandable recursive C   solution   include lt vector gt  using namespace std   template lt typename T gt  void ksubsets const vector lt T gt  amp  arr  unsigned left  unsigned idx      vector lt T gt  amp  lst  vector lt vector lt T gt  gt  amp  res        if  left  lt  1            res push back lst           return            for  unsigned i   idx  i  lt  arr size    i              lst push back arr i            ksubsets arr  left - 1  i   1  lst  res           lst pop back             int main         vector lt int gt  arr     1  2  3  4  5        unsigned left   3      vector lt int gt  lst      vector lt vector lt int gt  gt  res      ksubsets lt int gt  arr  left  0  lst  res          now res has all the combinations

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Simple recursive algorithm in Haskell  import Data List  combinations 0 lst        combinations n lst   do      x xs   lt - tails lst     rest    lt - combinations  n-1  xs     return   x   rest   We first define the special case  i e  selecting zero elements  It produces a single result  which is an empty list  i e  a list that contains an empty list    For n   0  x goes through every element of the list and xs is every element after x   rest picks n - 1 elements from xs using a recursive call to combinations  The final result of the function is a list where each element is x   rest  i e  a list which has x as head and rest as tail  for every different value of x and rest    gt  combinations 3  abcde    abc   abd   abe   acd   ace   ade   bcd   bce   bde   cde     And of course  since Haskell is lazy  the list is gradually generated as needed  so you can partially evaluate exponentially large combinations    gt  let c   combinations 8  abcdefghijklmnopqrstuvwxyz   gt  take 10 c   abcdefgh   abcdefgi   abcdefgj   abcdefgk   abcdefgl   abcdefgm   abcdefgn     abcdefgo   abcdefgp   abcdefgq

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void combine char a    int N  int M  int m  int start  char result          if  0    m            for  int i   M - 1  i  gt   0  i--              std  cout  lt  lt  result i           std  cout  lt  lt  std  endl          return            for  int i   start  i  lt   N - m   1   i              result m - 1    a i           combine a  N  M  m-1  i 1  result            void combine char a    int N  int M        char  result   new char M       combine a  N  M  M  0  result       delete   result      In the first function  m denotes how many more you need to choose  and start denotes from which position in array you must start choosing

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JavaScript  generator-based  recursive approach    x000D   x000D  function  nCk n k   x000D    for var i n-1 i gt  k-1 --i  x000D      if k   1  x000D        yield  i   x000D      else x000D        for var temp of nCk i k-1    x000D          temp unshift i   x000D          yield temp  x000D          x000D    x000D   x000D  function test    x000D    try  x000D      var n parseInt ninp value   x000D      var k parseInt kinp value   x000D      log innerText     x000D      var stop Date now   1000  x000D      if k gt  1  x000D        for var res of nCk n k   x000D          if Date now   lt stop  x000D            log innerText  JSON stringify res       x000D          else  x000D            log innerText   1 second passed  stopping here    x000D            break  x000D            x000D     catch ex    x000D    x000D  n  lt input id  ninp  oninput  test    gt  x000D   amp gt   k  lt input id  kinp  oninput  test    gt   amp gt   1 x000D   lt div id  log  gt  lt  div gt  x000D   x000D   x000D    This way  decreasing i and unshift    it produces combinations and elements inside combinations in decreasing order  somewhat pleasing the eye  Test stops after 1 second  so entering weird numbers is relatively safe

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static IEnumerable lt string gt  Combinations List lt string gt  characters  int length        for  int i   0  i  lt  characters Count  i                     only want 1 character  just return this one         if  length    1              yield return characters i               want more than one character  return this one plus all combinations one shorter            only use characters after the current one for the rest of the combinations         else             foreach  string next in Combinations characters GetRange i   1  characters Count -  i   1    length - 1                   yield return characters i    next

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Very fast combinations for MetaTrader MQL4 implemented as iterator object   The code is so simple to understand   I benchmarked a lot of algorithms  this one is really very fast - about 3x faster than most next combination   functions    x000D   x000D  class CombinationsIterator x000D    x000D  private  x000D   int input array        1 2 3 4 5 x000D   int index array        i j k x000D   int m elements         N x000D   int m indices          K x000D   x000D  public  x000D   CombinationsIterator int  amp src data    int k  x000D     x000D    m indices   k  x000D    m elements   ArraySize src data   x000D    ArrayCopy input array  src data   x000D    ArrayResize index array  m indices   x000D   x000D       create initial combination  0  k-1  x000D    for  int i   0  i  lt  m indices  i    x000D      x000D     index array i    i  x000D      x000D     x000D   x000D      https   stackoverflow com questions 5076695 x000D      bool next combination int  amp item    int k  int N  x000D   bool advance   x000D     x000D    int N   m elements  x000D    for  int i   m indices - 1  i  gt   0  --i  x000D      x000D     if  index array i   lt  --N  x000D       x000D        index array i   x000D      for  int j   i   1  j  lt  m indices    j  x000D        x000D       index array j    index array j - 1    1  x000D        x000D      return true  x000D       x000D      x000D    return false  x000D     x000D   x000D   void getItems int  amp items    x000D     x000D       fill items   from input array x000D    for  int i   0  i  lt  m indices  i    x000D      x000D     items i    input array index array i    x000D      x000D     x000D     x000D   x000D   x000D    A driver program to test the above iterator class    x000D   x000D     ------------------------------------------------------------------  x000D                                                                         x000D     ------------------------------------------------------------------  x000D     driver program to test above class x000D   x000D   define N 5 x000D   define K 3 x000D   x000D  void OnStart   x000D    x000D   int myset N     1  2  3  4  5   x000D   int items K   x000D   x000D   CombinationsIterator comboIt myset  K   x000D   x000D   do x000D     x000D    comboIt getItems items   x000D   x000D    printf   s   ArrayToString items    x000D   x000D     while  comboIt advance     x000D   x000D    x000D   x000D   x000D     x000D   x000D  Output  x000D  1 2 3  x000D  1 2 4  x000D  1 2 5  x000D  1 3 4  x000D  1 3 5  x000D  1 4 5  x000D  2 3 4  x000D  2 3 5  x000D  2 4 5  x000D  3 4 5 x000D   x000D   x000D

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Lets say your array of letters looks like this   ABCDEFGH   You have three indices  i  j  k  indicating which letters you are going to use for the current word  You start with    A B C D E F G H       i j k   First you vary k  so the next step looks like that    A B C D E F G H         i j   k   If you reached the end you go on and vary j and then k again    A B C D E F G H         i   j k  A B C D E F G H           i   j   k   Once you j reached G you start also to vary i    A B C D E F G H           i j k  A B C D E F G H             i j   k       Written in code this look something like that  void print combinations const char  string        int i  j  k      int len   strlen string        for  i   0  i  lt  len - 2  i                  for  j   i   1  j  lt  len - 1  j                          for  k   j   1  k  lt  len  k                    printf   c c c n   string i   string j   string k

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And here comes granddaddy COBOL  the much maligned language   Let s assume an array of 34 elements of 8 bytes each  purely arbitrary selection    The idea is to enumerate all possible 4-element combinations and load them into an array   We use 4 indices  one each for each position in the group of 4  The array is processed like this       idx1   1     idx2   2     idx3   3     idx4   4   We vary idx4 from 4 to the end   For each idx4 we get a unique combination  of groups of four  When idx4 comes to the end of the array  we increment idx3 by 1 and set idx4 to idx3 1  Then we run idx4 to the end again   We proceed in this manner  augmenting idx3 idx2  and idx1 respectively until the position of idx1 is less than 4 from the end of the array   That finishes the algorithm   1          --- pos 1 2          --- pos 2 3          --- pos 3 4          --- pos 4 5 6 7 etc    First iterations   1234 1235 1236 1237 1245 1246 1247 1256 1257 1267 etc    A COBOL example   01  DATA ARAY      05  FILLER     PIC X 8     VALUE   VALUE 01       05  FILLER     PIC X 8     VALUE   VALUE 02     etc  01  ARAY DATA    OCCURS 34      05  ARAY ITEM       PIC X 8    01  OUTPUT ARAY   OCCURS  50000   PIC X 32    01   MAX NUM   PIC 99 COMP VALUE 34   01  INDEXXES  COMP      05  IDX1            PIC 99      05  IDX2            PIC 99      05  IDX3            PIC 99      05  IDX4            PIC 99      05  OUT IDX   PIC 9 9    01  WHERE TO STOP SEARCH          PIC 99  COMP        Stop the search when IDX1 is on the third last array element   COMPUTE WHERE TO STOP SEARCH   MAX VALUE - 3       MOVE 1 TO IDX1  PERFORM UNTIL IDX1  gt  WHERE TO STOP SEARCH    COMPUTE IDX2   IDX1   1    PERFORM UNTIL IDX2  gt  MAX NUM       COMPUTE IDX3   IDX2   1       PERFORM UNTIL IDX3  gt  MAX NUM          COMPUTE IDX4   IDX3   1          PERFORM UNTIL IDX4  gt  MAX NUM             ADD 1 TO OUT IDX             STRING  ARAY ITEM IDX1                      ARAY ITEM IDX2                      ARAY ITEM IDX3                      ARAY ITEM IDX4                      INTO OUTPUT ARAY OUT IDX              ADD 1 TO IDX4          END-PERFORM          ADD 1 TO IDX3       END-PERFORM       ADD 1 TO IDX2    END PERFORM    ADD 1 TO IDX1 END-PERFORM

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Here s my JavaScript solution that is a little more functional through use of reduce map  which eliminates almost all variables   x000D   x000D  function combinations arr  size    x000D    var len   arr length  x000D   x000D    if  size  gt  len  return     x000D    if   size  return       x000D    if  size    len  return  arr   x000D   x000D    return arr reduce function  acc  val  i    x000D      var res   combinations arr slice i   1   size - 1  x000D         map function  comb    return  val  concat comb       x000D       x000D      return acc concat res   x000D            x000D    x000D   x000D  var combs   combinations  1 2 3 4 5 6 7 8  3   x000D  combs map function  comb    x000D    document body innerHTML    comb toString       lt br   gt    x000D      x000D   x000D  document body innerHTML      lt br   gt  Total combinations       combs length  x000D   x000D   x000D

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Clojure version    defn comb  k l     if    1 k   map vector l         apply concat               map-indexed                 map  fn  x   conj x  2                        comb  dec k   drop  inc  1  l                  l

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Here is an algorithm i came up with for solving this problem  Its written in c    but can be adapted to pretty much any language that supports bitwise operations   void r nCr const unsigned int  amp startNum  const unsigned int  amp bitVal  const unsigned int  amp testNum     Should be called with arguments  2 r -1  2  r-1   2  n-1        unsigned int n    startNum - bitVal   lt  lt  1      n    bitVal   1   0       for  unsigned int i   log2 testNum    1  i  gt  0  i--     Prints combination as a series of 1s and 0s         cout  lt  lt   n  gt  gt   i - 1   amp  1       cout  lt  lt  endl       if    n  amp  testNum   amp  amp  n    startNum          r nCr n  bitVal  testNum        if  bitVal  amp  amp  bitVal  lt  testNum          r nCr startNum  bitVal  gt  gt  1  testNum       You can see an explanation of how it works here

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The following recursive algorithm picks all of the k-element combinations from an ordered set    choose the first element i of your combination combine i with each of the combinations of k-1 elements chosen recursively from the set of elements larger than i    Iterate the above for each i in the set   It is essential that you pick the rest of the elements as larger than i  to avoid repetition  This way  3 5  will be picked only once  as  3  combined with  5   instead of twice  the condition eliminates  5     3    Without this condition you get variations instead of combinations

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Here is a Lisp approach using a macro  This works in Common Lisp and should work in other Lisp dialects   The code below creates  n  nested loops and executes an arbitrary chunk of code  stored in the body variable  for each combination of  n  elements from the list lst  The variable var points to a list containing the variables used for the loops    defmacro do-combinations   var lst num   amp body body     loop with syms    loop repeat num collect  gensym           for i on syms         for k     loop for   car i  on  cdr   cadr i                            do  let    var  list    reverse syms      progn   body                    then   loop for   car i  on   if  cadr i    cdr   cadr i   lst  do  k          finally  return k      Let s see      macroexpand-1   do-combinations  p   1 2 3 4 5 6 7  4   pprint  mapcar   car p       LOOP FOR   G3217 ON   1 2 3 4 5 6 7  DO   LOOP FOR   G3216 ON  CDR   G3217  DO    LOOP FOR   G3215 ON  CDR   G3216  DO     LOOP FOR   G3214 ON  CDR   G3215  DO      LET   P  LIST   G3217   G3216   G3215   G3214          PROGN  PPRINT  MAPCAR   CAR P           do-combinations  p   1 2 3 4 5 6 7  4   pprint  mapcar   car p      1 2 3 4   1 2 3 5   1 2 3 6        Since combinations are not stored by default  storage is kept to a minimum  The possibility of choosing the body code instead of storing all results also affords more flexibility

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Short example in Python   def comb sofar  rest  n       if n    0          print sofar     else          for i in range len rest                comb sofar   rest i   rest i 1    n-1    gt  gt  gt  comb      abcde   3  abc abd abe acd ace ade bcd bce bde cde   For explanation  the recursive method is described with the following example    Example  A B C D E All combinations of 3 would be    A with all combinations of 2 from the rest  B C D E  B with all combinations of 2 from the rest  C D E  C with all combinations of 2 from the rest  D E

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Short fast C implementation   include  lt stdio h gt   void main int argc  char  argv        const int n   6     The size of the set  for  1  2  3  4  it s 4      const int p   4     The size of the subsets  for  1  2    1  3       it s 2      int comb 40     0      comb i  is the index of the i-th element in the combination       int i   0    for  int j   0  j  lt   n  j    comb j    0    while  i  gt   0        if  comb i   lt  n   i - p   1           comb i            if  i    p - 1    for  int j   0  j  lt  p  j    printf   d    comb j    printf   n             else              comb   i    comb i - 1           else i--        To see how fast it is  use this code and test it   include  lt time h gt   include  lt stdio h gt   void main int argc  char  argv        const int n   32     The size of the set  for  1  2  3  4  it s 4      const int p   16     The size of the subsets  for  1  2    1  3       it s 2      int comb 40     0      comb i  is the index of the i-th element in the combination       int c   0  int i   0    for  int j   0  j  lt   n  j    comb j    0    while  i  gt   0        if  comb i   lt  n   i - p   1           comb i               if  i    p - 1    for  int j   0  j  lt  p  j    printf   d    comb j    printf   n                if  i    p - 1  c           else              comb   i    comb i - 1           else i--      printf   d  d     d combination s  in  15 3f second s  n    p  n  c  clock   1000 0       test with cmd exe  windows    Microsoft Windows XP  Version 5 1 2600   C  Copyright 1985-2001 Microsoft Corp   c  Program Files lcc projects gt combination 16 32    601080390 combination s  in          5 781 second s   c  Program Files lcc projects gt    Have a nice day

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Since programming language is not mentioned I am assuming that lists are OK too  So here s an OCaml version suitable for short lists  non tail-recursive   Given a list l of elements of any type and an integer n it will return a list of all possible lists containing n elements of l if we assume that the order of the elements in the outcome lists is ignored  i e  list   a   b   is the same as   b   a   and will reported once  So size of resultant list will be   List length l  Choose n    The intuition of the recursion is the following  you take the head of the list and then make  two recursive calls    recursive call 1  RC1   to the tail of the list  but choose n-1 elements recursive call 2  RC2   to the tail of the list  but choose n elements   to combine the recursive results  list-multiply  please bear the odd name  the head of the list with the results of RC1  and then append     the results of RC2  List-multiply is the following operation lmul   a lmul   l1   l2   l3     a  l1   a  l2   a  l3    lmul is implemented in the code below as  List map  fun x - gt  h  x    Recursion is terminated when the size of the list equals the number of elements you want to choose  in which case you just return the list itself   So here s a four-liner in OCaml that implements the above algorithm       let rec choose l n   match l   List length l  with                                           lsize  when n  lsize  - gt   l                                        h  t    - gt   List map  fun x- gt  h  x   choose t  n-1       choose t n                 - gt

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Here is a code I recently wrote in Java  which calculates and returns all the combination of  num  elements from  outOf  elements      author  Sourabh Bhat  heySourabh gmail com   public class Testing       public static void main String   args            Test case num   5  outOf   8           int num   5          int outOf   8          int     combinations   getCombinations num  outOf           for  int i   0  i  lt  combinations length  i                          for  int j   0  j  lt  combinations i  length  j                                  System out print combinations i  j                                    System out println                         private static int     getCombinations int num  int outOf                int possibilities   get nCr outOf  num           int     combinations   new int possibilities  num           int arrayPointer   0           int   counter   new int num            for  int i   0  i  lt  num  i                          counter i    i                    breakLoop  while  true                           Initializing part             for  int i   1  i  lt  num  i                                  if  counter i   gt   outOf -  num - 1 - i                       counter i    counter i - 1    1                                Testing part             for  int i   0  i  lt  num  i                                  if  counter i   lt  outOf                                        continue                    else                                       break breakLoop                                                  Innermost part             combinations arrayPointer    counter clone                arrayPointer                    Incrementing part             counter num - 1                 for  int i   num - 1  i  gt   1  i--                                if  counter i   gt   outOf -  num - 1 - i                       counter i - 1                                      return combinations             private static int get nCr int n  int r                if r  gt  n                        throw new ArithmeticException  r is greater then n                      long numerator   1          long denominator   1          for  int i   n  i  gt   r   1  i--                        numerator    i                    for  int i   2  i  lt   n - r  i                          denominator    i                     return  int   numerator   denominator

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yet another recursive solution  you should be able to port this to use letters instead of numbers  using a stack  a bit shorter than most though     stack       def choose n x      r 0 0 n 1 x   def r p  c  n x      if x-c    0        print stack       return     for i in range p  n- x-1  c         stack append i        r i 1 c 1 n x        stack pop     4 choose 3 or I want all 3 combinations of numbers starting with 0 to 4  choose 4 3     0  1  2   0  1  3   0  1  4   0  2  3   0  2  4   0  3  4   1  2  3   1  2  4   1  3  4   2  3  4

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Here is my proposition in C    I tried to impose as little restriction on the iterator type as i could so this solution assumes just forward iterator  and it can be a const iterator  This should work with any standard container  In cases where arguments don t make sense it throws std  invalid argumnent   include  lt vector gt   include  lt stdexcept gt   template  lt typename Fci gt     Fci - forward const iterator std  vector lt std  vector lt Fci gt   gt  enumerate combinations Fci begin  Fci end  unsigned int combination size        if begin    end  amp  amp  combination size  gt  0u          throw std  invalid argument  empty set and positive combination size         std  vector lt std  vector lt Fci gt   gt  result     empty set of combinations     if combination size    0u  return result     there is exactly one combination of                                                  size 0 - emty set     std  vector lt Fci gt  current combination      current combination reserve combination size   1u      I reserve one aditional slot                                                            in my vector to store                                                            the end sentinel there                                                             The code is cleaner thanks to that     for unsigned int i   0u  i  lt  combination size  amp  amp  begin    end    i    begin                current combination push back begin      Construction of the first combination              Since I assume the itarators support only incrementing  I have to iterate over        the set to get its size  which is expensive  Here I had to itrate anyway to          produce the first cobination  so I use the loop to also check the size      if current combination size    lt  combination size          throw std  invalid argument  combination size  gt  set size         result push back current combination      Store the first combination in the results set     current combination push back end      Here I add mentioned earlier sentinel to                                            simplyfy rest of the code  If I did it                                             earlier  previous statement would get ugly      while true                unsigned int i   combination size          Fci tmp                                Thanks to the sentinel I can find first         do                                     iterator to change  simply by scaning                                                from right to left and looking for the             tmp   current combination --i      first  bubble   The fact  that it s                tmp                              a forward iterator makes it ugly but I                                                can t help it          while i  gt  0u  amp  amp  tmp    current combination i   1u                Here is probably my most obfuscated expression             Loop above looks for a  bubble   If there is no  bubble   that means  that            current combination is the last combination  Expression in the if statement            below evaluates to true and the function exits returning result             If the  bubble  is found however  the ststement below has a sideeffect of             incrementing the first iterator to the left of the  bubble           if   current combination i     current combination i   1u               return result             Rest of the code sets posiotons of the rest of the iterstors             if there are any   that are to the right of the incremented one             to form next combination          while   i  lt  combination size                        current combination i    current combination i - 1u                 current combination i                        Below is the ugly side of using the sentinel  Well it had to haave some             disadvantage  Try without it          result push back std  vector lt Fci gt  current combination begin                                              current combination end   - 1

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include  lt stdio h gt   unsigned int next combination unsigned int  ar  size t n  unsigned int k        unsigned int finished   0      unsigned int changed   0      unsigned int i       if  k  gt  0            for  i   k - 1   finished  amp  amp   changed  i--                if  ar i   lt   n - 1  -  k - 1    i                       Increment this element                    ar i                     if  i  lt  k - 1                           Turn the elements after it into a linear sequence                        unsigned int j                      for  j   i   1  j  lt  k  j                              ar j    ar j - 1    1                                                          changed   1                            finished   i    0                    if   changed                   Reset to first combination                for  i   0  i  lt  k  i                      ar i    i                                    return changed     typedef void  printfn  const void    FILE      void print set const unsigned int  ar  size t len  const void   elements      const char  brackets  printfn print  FILE  fptr        unsigned int i      fputc brackets 0   fptr       for  i   0  i  lt  len  i              print elements ar i    fptr           if  i  lt  len - 1                fputs       fptr                       fputc brackets 1   fptr      int main void        unsigned int numbers       0  1  2        char  elements        a    b    c    d    e         const unsigned int k   sizeof numbers    sizeof unsigned int       const unsigned int n   sizeof elements    sizeof const char         do           print set numbers  k   void  elements         printfn fputs  stdout           putchar   n          while  next combination numbers  n  k        getchar        return 0

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And here s a Clojure version that uses the same algorithm I describe in my OCaml implementation answer    defn select     items        select items 0  inc  count items         items n1 n2        reduce concat               map   select   items                     range n1  inc n2          n items        let              lmul  fn  a list-of-lists-of-bs                        map   cons a    list-of-lists-of-bs                        if    n  count items             list items            if  empty  items             items             concat              select n  rest items                lmul  first items   select  dec n   rest items             It provides three ways to call it    a  for exactly n selected items as the question demands     user  gt   count  select 3  abcdefgh      56    b  for between n1 and n2 selected items   user  gt   select   1 2 3 4  2 3    3 4   2 4   2 3   1 4   1 3   1 2   2 3 4   1 3 4   1 2 4   1 2 3      c  for between 0 and the size of the collection selected items   user  gt   select   1 2 3        3   2   1   2 3   1 3   1 2   1 2 3

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I m aware that there are a LOT of answers to this already  but I thought I d add my own individual contribution in JavaScript  which consists of two functions - one to generate all the possible distinct k-subsets of an original n-element set  and one to use that first function to generate the power set of the original n-element set   Here is the code for the two functions     Generate combination subsets from a base set of elements  passed as an array   This function should generate an   array containing nCr elements  where nCr   n   r   n-r        Arguments      1  baseSet       The base set to create the subsets from  e g     a    b    c    d    e    f       2  cnt           The number of elements each subset is to contain  e g   3   function MakeCombinationSubsets baseSet  cnt        var bLen   baseSet length      var indices           var subSet           var done   false      var result                Contains all the combination subsets generated     var done   false      var i   0      var idx   0      var tmpIdx   0      var incr   0      var test   0      var newIndex   0      var inBounds   false      var tmpIndices           var checkBounds   false         First  generate an array whose elements are indices into the base set          for  i 0  i lt cnt  i             indices push i          Now create a clone of this array  to be used in the loop itself              tmpIndices                tmpIndices   tmpIndices concat indices          Now initialise the loop          idx   cnt - 1         point to the last element of the indices array     incr   0      done   false      while   done              Create the current subset              subSet            Make sure we begin with a completely empty subset before continuing              for  i 0  i lt cnt  i                 subSet push baseSet tmpIndices i          Create the current subset  using items selected from the                                                       base set  using the indices array  which will change as we                                                       continue scanning             Add the subset thus created to the result set              result push subSet          Now update the indices used to select the elements of the subset  At the start  idx will point to the       rightmost index in the indices array  but the moment that index moves out of bounds with respect to the       base set  attention will be shifted to the next left index           test   tmpIndices idx    1           if  test  gt   bLen                      Here  we re about to move out of bounds with respect to the base set  We therefore need to scan back            and update indices to the left of the current one  Find the leftmost index in the indices array that           isn t going to  move out of bounds with respect to the base set                  tmpIdx   idx - 1              incr   1               inBounds   false          Assume at start that the index we re checking in the loop below is out of bounds             checkBounds   true               while  checkBounds                                if  tmpIdx  lt  0                                        checkBounds   false       Exit immediately at this point                                   else                                       newIndex   tmpIndices tmpIdx    1                      test   newIndex   incr                       if  test  gt   bLen                                              Here  incrementing the current selected index will take that index out of bounds  so                       we move on to the next index to the left                              tmpIdx--                          incr                                              else                                             Here  the index will remain in bounds if we increment it  so we                       exit the loop and signal that we re in bounds                              inBounds   true                          checkBounds   false                         End if else                                          End if                                                 End while                     At this point  if we er still in bounds  then we continue generating subsets  but if not  we abort immediately               if   inBounds                  done   true              else                             Here  we re still in bounds  We need to update the indices accordingly  NOTE  at this point  although a               left positioned index in the indices array may still be in bounds  incrementing it to generate indices to               the right may take those indices out of bounds  We therefore need to check this as we perform the index               updating of the indices array                   tmpIndices tmpIdx    newIndex                   inBounds   true                  checking   true                  i   tmpIdx   1                   while  checking                                        test   tmpIndices i - 1    1      Find out if incrementing the left adjacent index takes it out of bounds                      if  test  gt   bLen                                                inBounds   false              If we move out of bounds  exit NOW                             checking   false                                            else                                               tmpIndices i    test          Otherwise  update the indices array                              i                             Now move on to the next index to the right in the indices array                              checking    i  lt  cnt           And continue until we ve exhausted all the indices array elements                           End if else                                         End while                                     At this point  if the above updating of the indices array has moved any of its elements out of bounds                    we abort subset construction from this point                     if   inBounds                      done   true                End if else                                 else                     Here  the rightmost index under consideration isn t moving out of bounds with respect to the base set when           we increment it  so we simply increment and continue the loop                 tmpIndices idx    test            End if                 End while           return result     End function     function MakePowerSet baseSet        var bLen   baseSet length      var result           var i   0      var partialSet            result push           add the empty set to the power set      for  i 1  i lt bLen  i                  partialSet   MakeCombinationSubsets baseSet  i           result   result concat partialSet         End i loop             Now  finally  add the base set itself to the power set to make it complete          partialSet           partialSet push baseSet       result   result concat partialSet        return result         End function     I tested this with the set   a    b    c    d    e    f   as the base set  and ran the code to produce the following power set        a     b     c     d     e     f     a   b     a   c     a   d     a   e     a   f     b   c     b   d     b   e     b   f     c   d     c   e     c   f     d   e     d   f     e   f     a   b   c     a   b   d     a   b   e     a   b   f     a   c   d     a   c   e     a   c   f     a   d   e     a   d   f     a   e   f     b   c   d     b   c   e     b   c   f     b   d   e     b   d   f     b   e   f     c   d   e     c   d   f     c   e   f     d   e   f     a   b   c   d     a   b   c   e     a   b   c   f     a   b   d   e     a   b   d   f     a   b   e   f     a   c   d   e     a   c   d   f     a   c   e   f     a   d   e   f     b   c   d   e     b   c   d   f     b   c   e   f     b   d   e   f     c   d   e   f     a   b   c   d   e     a   b   c   d   f     a   b   c   e   f     a   b   d   e   f     a   c   d   e   f     b   c   d   e   f     a   b   c   d   e   f     Just copy and paste those two functions  as is   and you ll have the basics needed to extract the distinct k-subsets of an n-element set  and generate the power set of that n-element set if you wish   I don t claim this to be elegant  merely that it works after a lot of testing  and turning the air blue during the debugging phase

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In Python like Andrea Ambu  but not hardcoded for choosing three   def combinations list  k          Choose combinations of list  choosing k elements no repeats         if len list   lt  k          return        else          seq    i for i in range k           while seq              print  list index  for index in seq              seq   get next combination len list   k  seq   def get next combination num elements  k  seq           index to move   find index to move num elements  seq          if index to move    None              return None         else              seq index to move     1               for every element past this sequence  move it down             for i  elem in enumerate seq  index to move 1                      seq i   1   index to move    seq index to move    i   1              return seq  def find index to move num elements  seq              Tells which index should be moved            for rev index  elem in enumerate reversed seq                if elem  lt   num elements - rev index - 1                   return len seq  - rev index - 1         return None

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Another one solution with C     static List lt List lt T gt  gt  GetCombinations lt T gt  List lt T gt  originalItems  int combinationLength                if  combinationLength  lt  1                        return null                     return CreateCombinations lt T gt  new List lt T gt     0  combinationLength  originalItems           static List lt List lt T gt  gt  CreateCombinations lt T gt  List lt T gt  initialCombination  int startIndex  int length  List lt T gt  originalItems                List lt List lt T gt  gt  combinations   new List lt List lt T gt  gt             for  int i   startIndex  i  lt  originalItems Count - length   1  i                          List lt T gt  newCombination   new List lt T gt  initialCombination               newCombination Add originalItems i                if  length  gt  1                                List lt List lt T gt  gt  newCombinations   CreateCombinations newCombination  i   1  length - 1  originalItems                   combinations AddRange newCombinations                             else                               combinations Add newCombination                                    return combinations          Example of usage      List lt char gt  initialArray   new List lt char gt       a   b   c   d       int combinationLength   3     List lt List lt char gt  gt  combinations   GetCombinations initialArray  combinationLength

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Here s a C   solution i came up with using recursion and bit-shifting  It may work in C as well   void r nCr unsigned int startNum  unsigned int bitVal  unsigned int testNum     Should be called with arguments  2 r -1  2  r-1   2  n-1        unsigned int n    startNum - bitVal   lt  lt  1      n    bitVal   1   0       for  unsigned int i   log2 testNum    1  i  gt  0  i--     Prints combination as a series of 1s and 0s         cout  lt  lt   n  gt  gt   i - 1   amp  1       cout  lt  lt  endl       if    n  amp  testNum   amp  amp  n    startNum          r nCr n  bitVal  testNum        if  bitVal  amp  amp  bitVal  lt  testNum          r nCr startNum  bitVal  gt  gt  1  testNum       You can find an explanation of how this works here

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Here s a coffeescript implementation   combinations   list  n  - gt          permuations   Math pow 2  list length  - 1         out              combinations               while permuations             out                   for i in  0  list length                  y     1  lt  lt  i                   if  y  amp  permuations and  y isnt permuations                       out push list i                if out length  lt   n and out length  gt  0                 combinations push out               permuations--          return combinations

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This is my contribution in javascript  no recursion   set     q0    q1    q2    q3   collector        function comb num      results        one comb        for  i   set length - 1  i  gt   0  --i        tmp   Math pow 2  i      quotient   parseInt num   tmp      results push quotient      num   num   tmp       k   0   for  i   0  i  lt  results length    i      if  results i             k       one comb push set i           if  collector k     undefined      collector k         collector k  push one comb      sum   0 for  i   0  i  lt  set length    i    sum    Math pow 2  i   for  ii   sum  ii  gt  0  --ii    comb ii   cnt   0 for  i   1  i  lt  collector length    i      n   0   for  j   0  j  lt  collector i  length    j      document write   cnt    -        n      -    collector i  j     lt br gt      document write   lt hr gt

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A concise Javascript solution   Array prototype combine function combine k           var toCombine this      var last      function combi n comb                        var combs             for   var x 0 y comb length x lt y x                 for   var l 0 m toCombine length l lt m l                           combs push comb x  toCombine l                                               if  n lt k-1               n                combi n combs             else last combs             combi 1 toCombine       return last       Example     var toCombine   a   b   c       var results toCombine combine 4

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Algorithm    Count from 1 to 2 n  Convert each digit to its binary representation  Translate each  on  bit to elements of your set  based on position    In C    void Main         var set   new      A    B    C    D          E    F    G    H    I    J          var kElement   2       for var i   1  i  lt  Math Pow 2  set Length   i              var result   Convert ToString i  2  PadLeft set Length   0            var cnt   Regex Matches Regex Escape result     1   Count           if  cnt    kElement                for int j   0  j  lt  set Length  j                    if   Char GetNumericValue result j      1                      Console Write set j                Console WriteLine                        Why does it work   There is a bijection between the subsets of an n-element set and n-bit sequences   That means we can figure out how many subsets there are by counting sequences    e g   the four element set below can be represented by  0 1  X  0  1  X  0  1  X  0  1   or 2 4  different sequences   So - all we have to do is count from 1 to 2 n to find all the combinations   We ignore the empty set   Next  translate the digits to their binary representation  Then substitute elements of your set for  on  bits   If you want only k element results  only print when k bits are  on      If you want all subsets instead of k length subsets  remove the cnt kElement part      For proof  see MIT free courseware Mathematics for Computer Science  Lehman et al  section 11 2 2  https   ocw mit edu courses electrical-engineering-and-computer-science 6-042j-mathematics-for-computer-science-fall-2010 readings

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Simple but slow C   backtracking algorithm    include  lt iostream gt   void backtrack int  numbers  int n  int k  int i  int s        if  i    k                for  int j   0  j  lt  k    j                        std  cout  lt  lt  numbers j                     std  cout  lt  lt  std  endl           return             if  s  gt  n                return             numbers i    s      backtrack numbers  n  k  i   1  s   1       backtrack numbers  n  k  i  s   1      int main int argc  char  argv          int n   5      int k   3       int  numbers   new int k        backtrack numbers  n  k  0  1        delete   numbers       return 0

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Short javascript version  ES 5   x000D   x000D  let combine    list  n    gt    n    0                  list flatMap  e  i    gt        combine          list slice i   1           n - 1         map c   gt   e  concat c           let res   combine  1 2 3 4   3   res forEach e   gt  console log e join      x000D   x000D   x000D

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How about this answer    this prints all combinations of length 3    and it can generalised for any length     Working code       include lt iostream gt   include lt string gt  using namespace std   void combination string a string dest   int l   dest length    if a empty    amp  amp  l     3     cout lt  lt dest lt  lt endl   else    if  a empty    amp  amp  dest length    lt  3         combination a substr 1 a length    dest a 0       if  a empty    amp  amp  dest length    lt   3          combination a substr 1 a length    dest             int main     string demo  abcd     combination demo       return 0

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In Python  taking advantage of recursion and the fact that everything is done by reference   This will take a lot of memory for very large sets  but has the advantage that the initial set can be a complex object   It will find only unique combinations   import copy  def find combinations  length  set  combinations   None  candidate   None          recursive function to calculate all unique combinations of unique values       from  set   given combinations of  length    The result is populated       into the  combinations  list            if combinations    None          combinations          if candidate    None          candidate           for item in set          if item in candidate                this item already appears in the current combination somewhere                skip it             continue          attempt   copy deepcopy candidate          attempt append item            sorting the subset is what gives us completely unique combinations            so that  1  2  3  and  1  3  2  will be treated as equals         attempt sort            if len attempt   lt  length                the current attempt at finding a new combination is still too               short  so add another item to the end of the set               yay recursion              find combinations  length  set  combinations  attempt           else                the current combination attempt is the right length   If it               already appears in the list of found combinations then we ll               skip it              if attempt in combinations                  continue             else                    otherwise  we append it to the list of found combinations                   and move on                  combinations append attempt                  continue     return len combinations    You use it this way   Passing  result  is optional  so you could just use it to get the number of possible combinations    although that would be really inefficient  it s better done by calculation    size   3 set    1  2  3  4  5  result       num   find combinations  size  set  result    print  size  d results in  d sets     size  num  print  result   s     result     You should get the following output from that test data   size 3 results in 10 sets result    1  2  3    1  2  4    1  2  5    1  3  4    1  3  5    1  4  5    2  3  4    2  3  5    2  4  5    3  4  5     And it will work just as well if your set looks like this   set            vanilla    cupcake            chocolate    pudding            vanilla    pudding            chocolate    cookie            mint    cookie

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I found this thread useful and thought I would add a Javascript solution that you can pop into Firebug   Depending on your JS engine  it could take a little time if the starting string is large   function string recurse active  rest        if  rest length    0            console log active         else           string recurse active   rest charAt 0   rest substring 1  rest length            string recurse active  rest substring 1  rest length            string recurse      abc      The output should be as follows   abc ab ac a bc b c

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Array prototype combs   function num         var str   this          length   str length          of   Math pow 2  length  - 1          out  combinations            while of             out                for var i   0  y  i  lt  length  i                   y    1  lt  lt  i                if y  amp  of  amp  amp   y     of                   out push str i                        if  out length  gt   num               combinations push out                      of--             return combinations

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Below is an iterative algorithm in C   that does not use the STL nor recursion nor conditional nested loops  It is faster that way  it does not perform any element swaps and it does not burden the stack with recursion and it can also be easily ported to ANSI C by substituting mallloc    free   and printf   for new  delete and std  cout  respectively  If you want to display the elements with a different or longer alphabet then change the  alphabet parameter to point to a different string than  quot abcdefg quot   void OutputArrayChar unsigned int  ka  size t n  const char  alphabet        for  int i   0  i  lt  n  i            std  cout  lt  lt  alphabet ka i    lt  lt   quot   quot       std  cout  lt  lt  endl          void GenCombinations const unsigned int N  const unsigned int K  const char  alphabet        unsigned int  ka   new unsigned int  K      dynamically allocate an array of UINTs     unsigned int ki   K-1                       Point ki to the last elemet of the array     ka ki    N-1                                Prime the last elemet of the array           while  true            unsigned int tmp   ka ki      Optimization to prevent reading ka ki  repeatedly          while  ki                     Fill to the left with consecutive descending values  blue squares              ka --ki    --tmp          OutputArrayChar ka  K  alphabet                while  --ka ki     ki         Decrement and check if the resulting value equals the index  bright green squares              OutputArrayChar ka  K  alphabet               if    ki    K           Exit condition  all of the values in the array are flush to the left                  delete   ka                  return                                                           int main int argc  char  argv          GenCombinations 7  4   quot abcdefg quot        return 0     IMPORTANT  The  alphabet parameter must point to a string with at least N characters  You can also pass an address of a string which is defined somewhere else  Combinations   Out of  quot 7 Choose 4 quot

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If you can use SQL syntax - say  if you re using LINQ to access fields of an structure or array  or directly accessing a database that has a table called  Alphabet  with just one char field  Letter   you can adapt following code   SELECT A Letter  B Letter  C Letter FROM Alphabet AS A  Alphabet AS B  Alphabet AS C WHERE A Letter lt  gt B Letter AND A Letter lt  gt C Letter AND B Letter lt  gt C Letter AND A Letter lt B Letter AND B Letter lt C Letter   This will return all combinations of 3 letters  notwithstanding how many letters you have in table  Alphabet   it can be 3  8  10  27  etc     If what you want is all permutations  rather than combinations  i e  you want  ACB  and  ABC  to count as different  rather than appear just once  just delete the last line  the AND one  and it s done   Post-Edit  After re-reading the question  I realise what s needed is the general algorithm  not just a specific one for the case of selecting 3 items  Adam Hughes  answer is the complete one  unfortunately I cannot vote it up  yet   This answer s simple but works only for when you want exactly 3 items

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Short java solution   import java util Arrays   public class Combination       public static void main String   args           String   arr     A   B   C   D   E   F            combinations2 arr  3  0  new String 3               static void combinations2 String   arr  int len  int startPosition  String   result           if  len    0               System out println Arrays toString result                return                           for  int i   startPosition  i  lt   arr length-len  i                 result result length - len    arr i               combinations2 arr  len-1  i 1  result                              Result will be   A  B  C   A  B  D   A  B  E   A  B  F   A  C  D   A  C  E   A  C  F   A  D  E   A  D  F   A  E  F   B  C  D   B  C  E   B  C  F   B  D  E   B  D  F   B  E  F   C  D  E   C  D  F   C  E  F   D  E  F

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Art of Computer Programming Volume 4  Fascicle 3 has a ton of these that might fit your particular situation better than how I describe   Gray Codes  An issue that you will come across is of course memory and pretty quickly  you ll have problems by 20 elements in your set -- 20C3   1140  And if you want to iterate over the set it s best to use a modified gray code algorithm so you aren t holding all of them in memory  These generate the next combination from the previous and avoid repetitions  There are many of these for different uses  Do we want to maximize the differences between successive combinations  minimize  et cetera   Some of the original papers describing gray codes     Some Hamilton Paths and a Minimal Change Algorithm Adjacent Interchange Combination Generation Algorithm   Here are some other papers covering the topic    An Efficient Implementation of the Eades  Hickey  Read Adjacent Interchange Combination Generation Algorithm  PDF  with code in Pascal  Combination Generators Survey of Combinatorial Gray Codes  PostScript  An Algorithm for Gray Codes   Chase s Twiddle  algorithm   Phillip J Chase   Algorithm 382  Combinations of M out of N Objects   1970   The algorithm in C     Index of Combinations in Lexicographical Order  Buckles Algorithm 515   You can also reference a combination by its index  in lexicographical order    Realizing that the index should be some amount of change from right to left based on the index we can construct something that should recover a combination   So  we have a set  1 2 3 4 5 6     and we want three elements  Let s say  1 2 3  we can say that the difference between the elements is one and in order and minimal   1 2 4  has one change and is lexicographically number 2  So the number of  changes  in the last place accounts for one change in the lexicographical ordering  The second place  with one change  1 3 4  has one change but accounts for more change since it s in the second place  proportional to the number of elements in the original set    The method I ve described is a deconstruction  as it seems  from set to the index  we need to do the reverse     which is much trickier  This is how Buckles solves the problem  I wrote some C to compute them  with minor changes     I used the index of the sets rather than a number range to represent the set  so we are always working from 0   n  Note    Since combinations are unordered   1 3 2     1 2 3  --we order them to be lexicographical  This method has an implicit 0 to start the set for the first difference    Index of Combinations in Lexicographical Order  McCaffrey   There is another way   its concept is easier to grasp and program but it s without the optimizations of Buckles  Fortunately  it also does not produce duplicate combinations   The set  that maximizes   where    For an example  27   C 6 4    C 5 3    C 2 2    C 1 1   So  the 27th lexicographical combination of four things is   1 2 5 6   those are the indexes of whatever set you want to look at  Example below  OCaml   requires choose function  left to reader      this will find the  x  combination of a  set  list when taking  k  elements    let combination maccaffery set k x          maximize function -- maximize a that is aCb                        return largest c where c  lt  i and choose c i   lt   z               let rec maximize a b x           if  choose a b    lt   x then a else maximize  a-1  b x     in     let rec iterate n x i   match i with           0 - gt               i - gt              let max   maximize n i x in             max    iterate n  x -  choose max i    i-1      in     if x  lt  0 then failwith  errors  else     let idxs    iterate  List length set  x k in     List map  List nth set   List sort  -  idxs    A small and simple combinations iterator  The following two algorithms are provided for didactic purposes  They implement an iterator and  a more general  folder overall combinations   They are as fast as possible  having the complexity O nCk   The memory consumption is bound by k    We will start with the iterator  which will call a user provided function for each combination  let iter combs n k f     let rec iter v s j       if j   k then f v     else for i   s to n - 1 do iter  i  v   i 1   j 1  done in   iter    0 0   A more general version will call the user provided function along with the state variable  starting from the initial state  Since we need to pass the state between different states we won t use the for-loop  but instead  use recursion   let fold combs n k f x     let rec loop i s c x       if i  lt  n then       loop  i 1  s c          let c   i  c and s   s   1 and i   i   1 in       if s  lt  k then loop i s c x else f c x     else x in   loop 0 0    x

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Here is an elegant  generic implementation in Scala  as described on 99 Scala Problems   object P26     def flatMapSublists A B  ls  List A   f   List A     gt  List B    List B         ls match         case Nil   gt  Nil       case sublist       tail    gt  f sublist      flatMapSublists tail  f           def combinations A  n  Int  ls  List A    List List A         if  n    0  List Nil      else flatMapSublists ls    sl   gt        combinations n - 1  sl tail  map  sl head

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Perhaps I ve missed the point  that you need the algorithm and not the ready made solution   but it seems that scala does it out of the box  now    def combis str String  k Int  Array String        str combinations k  toArray      Using the method like this     println combis  abcd  2  toList    Will produce     List ab  ac  ad  bc  bd  cd

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Jumping on the bandwagon  and posting another solution  This is a generic Java implementation  Input   int k  is number of elements to choose and  List lt T gt  list  is the list to choose from  Returns a list of combinations  List lt List lt T gt  gt     public static  lt T gt  List lt List lt T gt  gt  getCombinations int k  List lt T gt  list        List lt List lt T gt  gt  combinations   new ArrayList lt List lt T gt  gt         if  k    0            combinations add new ArrayList lt T gt              return combinations            for  int i   0  i  lt  list size    i              T element   list get i           List lt T gt  rest   getSublist list  i 1           for  List lt T gt  previous   getCombinations k-1  rest                 previous add element               combinations add previous                       return combinations     public static  lt T gt  List lt T gt  getSublist List lt T gt  list  int i        List lt T gt  sublist   new ArrayList lt T gt         for  int j   i  j  lt  list size    j              sublist add list get j              return sublist

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Here is my Scala solution   def combinations A  s  List A   k  Int   List List A        if  k  gt  s length  Nil   else if  k    1  s map List       else combinations s tail  k - 1  map s head           combinations s tail  k

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In VB Net  this algorithm collects all combinations of n numbers from a set of numbers  PoolArray   e g  all combinations of 5 picks from  8 10 20 33 41 44 47    Sub CreateAllCombinationsOfPicksFromPool ByVal PicksArray   As UInteger  ByVal PicksIndex As UInteger  ByVal PoolArray   As UInteger  ByVal PoolIndex As UInteger      If PicksIndex  lt  PicksArray Length Then         For i As Integer   PoolIndex To PoolArray Length - PicksArray Length   PicksIndex             PicksArray PicksIndex    PoolArray i              CreateAllCombinationsOfPicksFromPool PicksArray  PicksIndex   1  PoolArray  i   1          Next     Else           completed combination  build your collections using PicksArray      End If End Sub          Dim PoolArray   As UInteger   Array ConvertAll  8 10 20 33 41 44 47  Split       Function u  UInteger Parse u           Dim nPicks as UInteger   5         Dim Picks nPicks - 1  As UInteger         CreateAllCombinationsOfPicksFromPool Picks  0  PoolArray  0

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Here you have a lazy evaluated version of that algorithm coded in C        static bool nextCombination int   num  int n  int k                bool finished  changed           changed   finished   false           if  k  gt  0                        for  int i   k - 1   finished  amp  amp   changed  i--                                if  num i   lt   n - 1  -  k - 1    i                                        num i                         if  i  lt  k - 1                                                for  int j   i   1  j  lt  k  j                                                          num j    num j - 1    1                                                                      changed   true                                    finished    i    0                                    return changed             static IEnumerable Combinations lt T gt  IEnumerable lt T gt  elements  int k                T   elem   elements ToArray            int size   elem Length           if  k  lt   size                        int   numbers   new int k               for  int i   0  i  lt  k  i                                  numbers i    i                             do                               yield return numbers Select n   gt  elem n                              while  nextCombination numbers  size  k                      And test part       static void Main string   args                int k   3          var t   new      dog    cat    mouse    zebra             foreach  IEnumerable lt string gt  i in Combinations t  k                         Console WriteLine string Join      i                      Hope this help you

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My implementation in c c     include  lt unistd h gt   include  lt stdio h gt   include  lt iconv h gt   include  lt string h gt   include  lt errno h gt   include  lt stdlib h gt   int main int argc  char   argv        int opt   -1  min len   0  max len   0      char ofile 256   fchar 2   tchar 2       ofile 0    0      fchar 0    0      tchar 0    0      while  opt   getopt argc  argv   o f t l L        -1                    switch opt                                    case  o                       strncpy ofile  optarg  255                       break                      case  f                       strncpy fchar  optarg  1                       break                      case  t                       strncpy tchar  optarg  1                       break                      case  l                       min len   atoi optarg                       break                      case  L                       max len   atoi optarg                       break                      default                      printf  usage   s -oftlL n t-o output file n t-f from char n t-t to char n t-l min seq len n t-L max seq len   argv 0                        if max len  lt  1        printf  error  length must be more than 0 n        return 1    if min len  gt  max len        printf  error  max length must be greater or equal min length n        return 1    if  int fchar 0   gt   int tchar 0         printf  error  invalid range specified n        return 1    FILE  out   fopen ofile   w    if  out        printf  failed to open input file with error   s n   strerror errno        return 1    int cur len   min len  while cur len  lt   max len        char buf cur len       for int i   0  i  lt  cur len  i            buf i    fchar 0       fwrite buf  cur len  1  out       fwrite   n   1  1  out       while buf 0      tchar 0  1                 while buf cur len-1   lt  tchar 0                          int buf cur len-1                 fwrite buf  cur len  1  out               fwrite   n   1  1  out                     if cur len  lt  2              break          if buf 0     tchar 0                         bool stop   true              for int i   1  i  lt  cur len  i                                  if buf i     tchar 0                                         stop   false                      break                                              if stop                  break                    int u   cur len-2          for   u gt  0  amp  amp  buf u   gt   tchar 0   u--                         int buf u             for int i   u 1  i  lt  cur len  i                buf i    fchar 0           fwrite buf  cur len  1  out           fwrite   n   1  1  out             cur len      fclose out   return 0      here my implementation in c    it write all combinations to specified files  but behaviour can be changed  i made in to generate various dictionaries  it accept min and max length and character range  currently only ansi supported  it enough for my needs

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C  simple algorithm   I m posting it since I ve tried to use the one you guys uploaded  but for some reason I couldn t compile it - extending a class  so I wrote my own one just in case someone else is facing the same problem I did   I m not much into c  more than basic programming by the way  but this one works fine   public static List lt List lt int gt  gt  GetSubsetsOfSizeK List lt int gt  lInputSet  int k                        List lt List lt int gt  gt  lSubsets   new List lt List lt int gt  gt                 GetSubsetsOfSizeK rec lInputSet  k  0  new List lt int gt     lSubsets               return lSubsets             public static void GetSubsetsOfSizeK rec List lt int gt  lInputSet  int k  int i  List lt int gt  lCurrSet  List lt List lt int gt  gt  lSubsets                        if  lCurrSet Count    k                                lSubsets Add lCurrSet                   return                             if  i  gt   lInputSet Count                  return               List lt int gt  lWith   new List lt int gt  lCurrSet               List lt int gt  lWithout   new List lt int gt  lCurrSet               lWith Add lInputSet i                   GetSubsetsOfSizeK rec lInputSet  k  i  lWith  lSubsets               GetSubsetsOfSizeK rec lInputSet  k  i  lWithout  lSubsets               USAGE  GetSubsetsOfSizeK set of type List lt int gt   integer k   You can modify it to iterate over whatever you are working with   Good luck

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In C    public static IEnumerable lt IEnumerable lt T gt  gt  Combinations lt T gt  this IEnumerable lt T gt  elements  int k      return k    0   new     new T 0          elements SelectMany  e  i    gt        elements Skip i   1  Combinations k - 1  Select c   gt   new    e   Concat c         Usage   var result   Combinations new     1  2  3  4  5    3     Result   123 124 125 134 135 145 234 235 245 345

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May I present my recursive Python solution to this problem    def choose iter elements  length       for i in xrange len elements            if length    1              yield  elements i            else              for next in choose iter elements i 1 len elements    length-1                   yield  elements i      next def choose l  k       return list choose iter l  k     Example usage    gt  gt  gt  len list choose iter  abcdefgh  3    56   I like it for its simplicity

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Here s some simple code that prints all the C n m  combinations  It works by initializing and moving a set of array indices that point to next valid combination  The indices are initialized to point to the lowest m indices  lexicographically the smallest combination   Then on  starting with the m-th index  we try to move the indices forward  if an index has reached its limit  we try the previous index  all the way down to index 1   If we can move an index forward  then we reset all greater indices   m  rand   n  1     m will vary from 1 to n  for  i 0 i lt n i    a i  i 1      we want to print all possible C n m  combinations of selecting m objects out of n printf  Printing C  d  d  possible combinations     n   n m       This is an adhoc algo that keeps m pointers to the next valid combination for  i 0 i lt m i    p i  i     the p    contain indices to the a vector whose elements constitute next combination  done false  while   done           print combination     for  i 0 i lt m i    printf   2d    a p i         printf   n            update combination        method  start with p m-1   try to increment it  if it is already at the end  then try moving p m-2  ahead         if this is possible  then reset p m-1  to 1 more than  the new  p m-2          if p m-2  can not also be moved  then try p m-3   move that ahead  then reset p m-2  and p m-1          repeat all the way down to p 0   if p 0  can not also be moved  then we have generated all combinations      j m-1      i 1      move found false      while   j gt  0   amp  amp   move found                if  p j  lt  n-i                          move found true              p j        point p j  to next index             for  k j 1 k lt m k                                  p k  p j   k-j                                   else                       j--              i                        if   move found  done true

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short python code  yielding index positions  def yield combos n k         n is set size  k is combo size      i   0     a    0  k      while i  gt  -1          for j in range i 1  k               a j    a j-1  1         i j         yield a         while a i     i   n - k              i -  1         a i     1

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C code for Algorithm L  Lexicographic combinations  in Section 7 2 1 3 of The Art of Computer Programming  Volume 4A  Combinatorial Algorithms  Part 1     include  lt stdio h gt   include  lt stdlib h gt   void visit int  c  int t          for  int j   1  j  lt   t  j      for  int j   t  j  gt  0  j--      printf   d    c j      printf   n       int  initialize int n  int t          c 0  not used   int  c    int   malloc  t   3    sizeof int       for  int j   1  j  lt   t  j        c j    j - 1    c t 1    n    c t 2    0    return c     void comb int n  int t       int  c   initialize n  t     int j     for            visit c  t       j   1      while  c j  1    c j 1           c j    j - 1          j            if  j  gt  t         return        c j         free c      int main int argc  char  argv        comb 5  3     return 0

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I have written a class to handle common functions for working with the binomial coefficient  which is the type of problem that your problem falls under   It performs the following tasks    Outputs all the K-indexes in a nice format for any N choose K to a file   The K-indexes can be substituted with more descriptive strings or letters   This method makes solving this type of problem quite trivial  Converts the K-indexes to the proper index of an entry in the sorted binomial coefficient table   This technique is much faster than older published techniques that rely on iteration   It does this by using a mathematical property inherent in Pascal s Triangle   My paper talks about this   I believe I am the first to discover and publish this technique  but I could be wrong  Converts the index in a sorted binomial coefficient table to the corresponding K-indexes  Uses Mark Dominus method to calculate the binomial coefficient  which is much less likely to overflow and works with larger numbers  The class is written in  NET C  and provides a way to manage the objects related to the problem  if any  by using a generic list   The constructor of this class takes a bool value called InitTable that when true will create a generic list to hold the objects to be managed   If this value is false  then it will not create the table   The table does not need to be created in order to perform the 4 above methods   Accessor methods are provided to access the table  There is an associated test class which shows how to use the class and its methods   It has been extensively tested with 2 cases and there are no known bugs    To read about this class and download the code  see Tablizing The Binomial Coeffieicent   It should not be hard to convert this class to C

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A Lisp macro generates the code for all values r  taken-at-a-time    defmacro txaat  some-list taken-at-a-time     let    vars  reverse  truncate-list   a b c d e f g h i j  taken-at-a-time                     loop for i below taken-at-a-time             for j in vars             with nested   nil             finally  return nested              do               setf                nested                  loop for  j from                       if   lt  i  1-  length vars                               1    nth  1  i  vars                            0                    below  -  length  some-list   i                         if  equal i 0                               collect                                  list                                    loop for k from  1- taken-at-a-time  downto 0                                      append    nth   nth k vars   some-list                                  append  nested           So   CL-USER gt   macroexpand-1   txaat   a b c d  1    LOOP FOR A FROM 0 TO  -  LENGTH   A B C D   1      COLLECT  LIST  NTH A   A B C D     T CL-USER gt   macroexpand-1   txaat   a b c d  2    LOOP FOR A FROM 0 TO  -  LENGTH   A B C D   2        APPEND  LOOP FOR B FROM  1  A  TO  -  LENGTH   A B C D   1                     COLLECT  LIST  NTH A   A B C D    NTH B   A B C D      T CL-USER gt   macroexpand-1   txaat   a b c d  3    LOOP FOR A FROM 0 TO  -  LENGTH   A B C D   3        APPEND  LOOP FOR B FROM  1  A  TO  -  LENGTH   A B C D   2                     APPEND  LOOP FOR C FROM  1  B  TO  -  LENGTH   A B C D   1                                  COLLECT  LIST  NTH A   A B C D                                                  NTH B   A B C D                                                  NTH C   A B C D       T  CL-USER gt     And   CL-USER gt   txaat   a b c d  1    A   B   C   D   CL-USER gt   txaat   a b c d  2    A B   A C   A D   B C   B D   C D   CL-USER gt   txaat   a b c d  3    A B C   A B D   A C D   B C D   CL-USER gt   txaat   a b c d  4    A B C D   CL-USER gt   txaat   a b c d  5  NIL CL-USER gt   txaat   a b c d  0  NIL CL-USER gt

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Here is a simple JS solution    x000D   x000D  function getAllCombinations n  k  f1    x000D   indexes   Array k   x000D    for  let i  0  i lt  k  i      x000D     indexes i    i  x000D      x000D    var total   1  x000D    f1 indexes   x000D    while  indexes 0      n-k    x000D     total    x000D    getNext n  indexes   x000D      f1 indexes   x000D      x000D    return  total   x000D    x000D   x000D  function getNext n  vec    x000D   const k   vec length  x000D    vec k-1     x000D   for  var i 0  i lt k  i      x000D     var currentIndex   k-i-1  x000D      if  vec currentIndex      n - i    x000D      var nextIndex   k-i-2  x000D        vec nextIndex     x000D        vec currentIndex    vec nextIndex    1  x000D        x000D      x000D   x000D   for  var i 1  i lt k  i      x000D      if  vec i      n -  k-i - 1     x000D        vec i    vec i-1    1  x000D        x000D      x000D   return vec  x000D     x000D   x000D   x000D   x000D  let start   new Date    x000D  let result   getAllCombinations 10  3  indexes   gt  console log indexes     x000D  let runTime   new Date   - start   x000D   x000D  console log   x000D  result  runTime x000D      x000D   x000D   x000D

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Short fast C  implementation  public static IEnumerable lt IEnumerable lt T gt  gt  Combinations lt T gt  IEnumerable lt T gt  elements  int k        return Combinations elements Count    k  Select p   gt  p Select q   gt  elements ElementAt q                              public static List lt int   gt  Combinations int setLenght  int subSetLenght    5  3       var result   new List lt int   gt          var lastIndex   subSetLenght - 1      var dif   setLenght - subSetLenght      var prevSubSet   new int subSetLenght       var lastSubSet   new int subSetLenght       for  int i   0  i  lt  subSetLenght  i                  prevSubSet i    i          lastSubSet i    i   dif             while true                  add subSet ad result set         var n   new int subSetLenght           for  int i   0  i  lt  subSetLenght  i                n i    prevSubSet i            result Add n            if  prevSubSet 0   gt   lastSubSet 0               break             start at index 1 because index 0 is checked and breaking in the current loop         int j   1          for    j  lt  subSetLenght  j                          if  prevSubSet j   gt   lastSubSet j                                 prevSubSet j - 1                      for  int p   j  p  lt  subSetLenght  p                        prevSubSet p    prevSubSet p - 1    1                   break                                   if  j  gt  lastIndex              prevSubSet lastIndex                return result

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I had a permutation algorithm I used for project euler  in python   def missing miss src        Returns the list of items in src not present in miss      return  i for i in src if i not in miss    def permutation gen n l        Generates all the permutations of n items of the l list      for i in l          if n lt  1  yield  i          r    i          for j in permutation gen n-1 missing  i  l     yield r j   If   n lt len l     you should have all combination you need without repetition  do you need it   It is a generator  so you use it in something like this   for comb in permutation gen 3 list  ABCDEFGH         print comb

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Short php algorithm to return all combinations of k elements from n  binomial coefficent  based on java solution    array   array 1 2 3 4 5     array result   NULL    array general   NULL   function combinations  array   len   start position   result array   result len   amp  general array        if  len    0                 general array      result array          return             for   i    start position   i  lt   count  array  -  len   i                   result array  result len -  len     array  i           combinations  array   len-1   i 1   result array   result len   general array             combinations  array  3  0   array result  3   array general    echo   lt pre gt    print r  array general   echo   lt  pre gt      The same solution but in javascript   var newArray    1  2  3  4  5   var arrayResult       var arrayGeneral        function combinations newArray  len  startPosition  resultArray  resultLen  arrayGeneral        if len     0            var tempArray               resultArray forEach value   gt  tempArray push value            arrayGeneral push tempArray           return            for  var i   startPosition  i  lt   newArray length - len  i              resultArray resultLen - len    newArray i           combinations newArray  len-1  i 1  resultArray  resultLen  arrayGeneral             combinations newArray  3  0  arrayResult  3  arrayGeneral    console log arrayGeneral

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All said and and done here comes the O caml code for that  Algorithm is evident from the code    let combi n lst       let rec comb l c           if  List length c   n  then  c  else         match l with            - gt                h  t  - gt   combi t  h  c    combi t c      in         combi lst

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Recursively  a very simple answer  combo  in Free Pascal       procedure combinata  n  k  integer  producer  oneintproc            procedure combo  ndx  nbr  len  lnd  integer           begin             for nbr    nbr to len do begin                 productarray ndx     nbr                  if len  lt  lnd then                     combo ndx 1 nbr 1 len 1 lnd                  else                     producer k               end          end       begin         combo  0  0  n-k  n-1       end     producer  disposes of the productarray made for each combination

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I made a general class for combinations in C    It is used like this   char ar      0ABCDEFGH   nCr ncr 8  3   while ncr next          for int i 0  i lt ncr size    i    cout  lt  lt  ar ncr i        cout  lt  lt           My library ncr i  returns from 1  not from 0   That s why there is 0 in the array   If you want to consider order  just chage nCr class to nPr  Usage is identical   Result  ABC ABD ABE ABF ABG ABH ACD ACE ACF ACG ACH ADE ADF ADG ADH AEF AEG AEH AFG AFH AGH BCD BCE BCF BCG BCH BDE BDF BDG BDH BEF BEG BEH BFG BFH BGH CDE CDF CDG CDH CEF CEG CEH CFG CFH CGH DEF DEG DEH DFG DFH DGH EFG EFH EGH FGH  Here goes the header file    pragma once  include  lt exception gt   class NRexception   public std  exception   public      virtual const char  what   const throw             return  Combination   N  R should be positive integer               class Combination   public      Combination int n  int r       virtual  Combination     delete    ar       int amp  operator   unsigned i   return ar i        bool next        int size    return r       static int factorial int n    protected      int  ar      int n  r      class nCr   public Combination   public       nCr int n  int r       bool next        int count   const      class nTr   public Combination   public      nTr int n  int r       bool next        int count   const      class nHr   public nTr   public      nHr int n  int r    nTr n r         bool next        int count   const      class nPr   public Combination   public      nPr int n  int r       virtual  nPr    delete    on       bool next        void rewind        int count   const   private      bool  on      void inc ar int i        And the implementation    include  combi h   include  lt set gt   include lt cmath gt   Combination  Combination int n  int r          if n  lt  1    r  lt  1  throw NRexception        ar   new int r       this- gt n   n      this- gt r   r     int Combination  factorial int n         return n    1   n   n   factorial n-1      int nPr  count   const       return factorial n  factorial n-r      int nCr  count   const       return factorial n  factorial n-r  factorial r      int nTr  count   const       return pow n  r      int nHr  count   const       return factorial n r-1  factorial n-1  factorial r      nCr  nCr int n  int r    Combination n  r        if r    0  return      for int i 0  i lt r-1  i    ar i    i   1      ar r-1    r-1     nTr  nTr int n  int r    Combination n  r        for int i 0  i lt r-1  i    ar i    1      ar r-1    0     bool nCr  next         if r    0  return false      ar r-1         int i   r-1      while ar i     n-r 2 i            if --i    -1  return false          ar i               while i  lt  r-1  ar i 1    ar i      1      return true     bool nTr  next         ar r-1         int i   r-1      while ar i     n 1            ar i    1          if --i    -1  return false          ar i               return true     bool nHr  next         ar r-1         int i   r-1      while ar i     n 1            if --i    -1  return false          ar i               while i  lt  r-1  ar i 1    ar i         return true     nPr  nPr int n  int r    Combination n  r        on   new bool n 2       for int i 0  i lt n 2  i    on i    false      for int i 0  i lt r  i              ar i    i   1          on i    true            ar r-1    0     void nPr  rewind         for int i 0  i lt r  i              ar i    i   1          on i    true            ar r-1    0     bool nPr  next            inc ar r-1        int i   r-1      while ar i     n 1            if --i    -1  return false          inc ar i             while i  lt  r-1            ar   i    0          inc ar i             return true     void nPr  inc ar int i        on ar i     false      while on   ar i         if ar i     n 1  on ar i     true

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In C   the following routine will produce all combinations of length distance first k  between the range   first last     include  lt algorithm gt   template  lt typename Iterator gt  bool next combination const Iterator first  Iterator k  const Iterator last          Credits  Mark Nelson http   marknelson us       if   first    last      first    k      last    k         return false     Iterator i1   first     Iterator i2   last       i1     if  last    i1        return false     i1   last     --i1     i1   k     --i2     while  first    i1             if   --i1  lt   i2                   Iterator j   k           while     i1  lt   j     j           std  iter swap i1 j              i1             j           i2   k           std  rotate i1 j last            while  last    j                           j                i2                      std  rotate k i2 last            return true                  std  rotate first k last      return false      It can be used like this    include  lt string gt   include  lt iostream gt   int main         std  string s    12345       std  size t comb size   3      do               std  cout  lt  lt  std  string s begin    s begin     comb size   lt  lt  std  endl        while  next combination s begin    s begin     comb size  s end           return 0      This will print the following   123 124 125 134 135 145 234 235 245 345

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Another C  version with lazy generation of the combination indices  This version maintains a single array of indices to define a mapping between the list of all values and the values for the current combination  i e  constantly uses O k  additional space during the entire runtime  The code generates individual combinations  including the first one  in O k  time     public static IEnumerable lt T   gt  Combinations lt T gt  this T   values  int k        if  k  lt  0    values Length  lt  k          yield break     invalid parameters  no combinations possible         generate the initial combination indices     var combIndices   new int k       for  var i   0  i  lt  k  i                  combIndices i    i             while  true                   return next combination         var combination   new T k           for  var i   0  i  lt  k  i                          combination i    values combIndices i                      yield return combination              find first index to update         var indexToUpdate   k - 1          while  indexToUpdate  gt   0  amp  amp  combIndices indexToUpdate   gt   values Length - k   indexToUpdate                        indexToUpdate--                     if  indexToUpdate  lt  0              yield break     done             update combination indices         for  var combIndex   combIndices indexToUpdate    1  indexToUpdate  lt  k  indexToUpdate    combIndex                          combIndices indexToUpdate    combIndex                      Test code   foreach  var combination in new     a    b    c    d    e   Combinations 3         System Console WriteLine String Join      combination        Output   a b c a b d a b e a c d a c e a d e b c d b c e b d e c d e

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There is no need for collection manipulations  The problem is almost the same as cycling over K nested loops but you have to be careful with the indexes and bounds  ignoring Java and OOP stuff     public class CombinationsGen       private final int n      private final int k      private int   buf       public CombinationsGen int n  int k            this n   n          this k   k             public void combine Consumer lt int   gt  consumer            buf   new int k           rec 0  0  consumer              private void rec int index  int next  Consumer lt int   gt  consumer            int max   n - index           if  index    k - 1                for  int i   0  i  lt  max  amp  amp  next  lt  n  i                      buf index    next                  next                    consumer accept buf                           else               for  int i   0  i  lt  max  amp  amp  next   index  lt  n  i                      buf index    next                  next                    rec index   1  next  consumer                                     Use like so    CombinationsGen gen   new CombinationsGen 5  2     AtomicInteger total   new AtomicInteger     gen combine arr - gt         System out println Arrays toString arr         total incrementAndGet          System out println total     Get expected results    0  1   0  2   0  3   0  4   1  2   1  3   1  4   2  3   2  4   3  4  10   Finally  map the indexes to whatever set of data you may have

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This is a recursive program that generates combinations for nCk Elements in collection are assumed to be from 1 to n   include lt stdio h gt   include lt stdlib h gt   int nCk int n int loopno int ini int  a int k        static int count 0      int i      loopno--      if loopno lt 0                a k-1  ini          for i 0 i lt k i                          printf   d   a i                      printf   n            count            return 0            for i ini i lt  n-loopno-1 i                  a k-1-loopno  i 1          nCk n loopno i 1 a k             if ini  0      return count      else     return 0     void main         int n k  a count      printf  Enter the value of n and k n        scanf   d  d   amp n  amp k       a  int  malloc k sizeof int        count nCk n k 0 a k       printf  No of combinations  d n  count

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In Python like Andrea Ambu  but not hardcoded for choosing three   def combinations list  k          Choose combinations of list  choosing k elements no repeats         if len list   lt  k          return        else          seq    i for i in range k           while seq              print  list index  for index in seq              seq   get next combination len list   k  seq   def get next combination num elements  k  seq           index to move   find index to move num elements  seq          if index to move    None              return None         else              seq index to move     1               for every element past this sequence  move it down             for i  elem in enumerate seq  index to move 1                      seq i   1   index to move    seq index to move    i   1              return seq  def find index to move num elements  seq              Tells which index should be moved            for rev index  elem in enumerate reversed seq                if elem  lt   num elements - rev index - 1                   return len seq  - rev index - 1         return None

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https   gist github com 3118596  There is an implementation for JavaScript  It has functions to get k-combinations and all combinations of an array of any objects  Examples   k combinations  1 2 3   2  - gt    1 2    1 3    2 3    combinations  1 2 3   - gt    1   2   3   1 2   1 3   2 3   1 2 3

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Here is a method which gives you all combinations of specified size from a random length string  Similar to quinmars  solution  but works for varied input and k   The code can be changed to wrap around  ie  dab  from input  abcd  w k 3   public void run String data  int howMany       choose data  howMany  new StringBuffer    0         n choose k private void choose String data  int k  StringBuffer result  int startIndex       if  result length    k           System out println result toString             return             for  int i startIndex  i lt data length    i             result append data charAt i            choose data k result  i 1           result setLength result length  -1             Output for  abcde        abc abd abe acd ace ade bcd bce bde cde

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I created a solution in SQL Server 2005 for this  and posted it on my website  http   www jessemclain com downloads code sql fn GetMChooseNCombos sql htm  Here is an example to show usage   SELECT   FROM dbo fn GetMChooseNCombos  ABCD   2        results   Word ---- AB AC AD BC BD CD   6 row s  affected

[algorithm] Algorithm to return all combinations of k elements from n

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