Return array in a function

Question

I have an array int arr 5  that is passed to a function fillarr int arr      int fillarr int arr          for           return arr       How can I return that array  How will I use it  say I returned a pointer how am I going to access it

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In C  11  you can return std  array    include  lt array gt  using namespace std   array lt int  5 gt  fillarr int arr          array lt int  5 gt  arr2      for int i 0  i lt 5    i            arr2 i  arr i  2            return arr2

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i used static array so that while returning array it should not throw error as you are returning address of local variable    so now you can send any locally created variable from function by making it as static   as it works as global variable      include lt iostream gt  using namespace std   char  func int n          char a 26     if we use this then an error will occur because you are                              returning address of a local variable       static char a 26       char temp  A        for int i 0 i lt n i               a i  temp temp              return a    int main         int n 26      char  p func n       for int i 0 i lt n i         cout lt  lt   p i  lt  lt  quot   quot         or you can also print like this        for int i 0 i lt n i         cout lt  lt p i  lt  lt  quot   quot

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Just define a type    as return value  like           private string   functionReturnValueArray string one  string two                 string   x    one  two             x 0     a           x 1     b            return x                function call   string   y  y   functionReturnValueArray stringOne  stringTwo

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int  fillarr int arr      You can still use the result like  int  returned array   fillarr some other array   if returned array 0     3      do important cool stuff

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the answer may depend a bit on how you plan to use that function   For the simplest answer  lets decide that instead of an array  what you really want is a vector   Vectors are nice because the look for all the world like boring  ordinary values you can store in regular pointers   We ll look at other options and why you want them afterwards    std  vector lt int gt  fillarr  std  vector lt int gt  arr            do something     return arr      This will do exactly what you expect it to do   The upside is that std  vector takes care of making sure everything is handled cleanly   the downside is that this copies a very large amount of data  if your array is large   In fact it copies every element of the array twice   first it copies the vector so that the function can use it as a parameter   then it copies it again to return it to the caller   If you can handle managing the vector yourself  you can do things quite a bit more easily    it may copy it a third time if the caller needs to store it in a variable of some sort to do more calculation   It looks like what you re really trying to do is just populate a collection   if you don t have a specific reason to return a new instance of a collection  then don t   we can do it like this  void fillarr std  vector lt int gt   amp   arr           modify arr        don t return anything     this way you get a reference to the array passed to the function  not a private copy of it   any changes you make to the parameter are seen by the caller   You could return a reference to it if you want  but that s not really a great idea  since it sort of implies that you re getting something different from what you passed     If you really do need a new instance of the collection  but want to avoid having it on the stack  and all the copying that entails   you need to create some kind of contract for how that instance is handled   the easiest way to do that is to use a smart pointer  which keeps the referenced instance around as long as anyone is holding onto it   It goes away cleanly if it goes out of scope   That would look like this   std  auto ptr lt std  vector lt int gt   gt  fillarr  const std  vector lt int gt   amp  arr        std  auto ptr lt std  vector lt int gt   gt  myArr new std  vector lt int gt           do stuff with arr and  myArr     return myArr      For the most part  using  myArr works identically to using a plain vanilla vector   This example also modifies the parameter list by adding the const keyword   Now you get a reference without copying it  but you can t modify it  so the caller knows it ll be the same as before the function got to it     All of this is swell  but idiomatic c   rarely works with collections as a whole   More normally  you will be using iterators over those collections   that would look something more like this  template  lt class Iterator gt  Iterator fillarr Iterator arrStart  Iterator arrEnd        Iterator arrIter   arrStart      for  arrIter  lt   arrEnd  arrIter               do something     return arrStart      Using it looks a bit odd if you re not used to seeing this style     vector lt int gt  arr  vector lt int gt   iterator foo   fillarr arr begin    arr end       foo now  points to  the beginning of the modified arr     What s really nice about this is that it works equally well on vector as on plain C arrays and many other types of collection  for example  int arr 100   int  foo   fillarr arr  arr 100     Which now looks an awful lot like the plain pointer examples given elsewhere in this question

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This is  a fairly old question  but I m going to put in my 2 cents as there are a lot of answers  but none showing all possible methods in a clear and concise manner  not sure about the concise bit  as this got a bit out of hand  TL DR     I m assuming that the OP wanted to return the array that was passed in without copying as some means of directly passing this to the caller to be passed to another function to make the code look prettier   However  to use an array like this is to let it decay into a pointer and have the compiler treat it like an array   This can result in subtle bugs if you pass in an array like  with the function expecting that it will have 5 elements  but your caller actually passes in some other number   There a few ways you can handle this better   Pass in a std  vector or std  array  not sure if std  array was around in 2010 when the question was asked    You can then pass the object as a reference without any copying moving of the object   std  array lt int  5 gt  amp  fillarr std  array lt int  5 gt  amp  arr            before c  11      for auto it   arr begin    it    arr end      it           do stuff              Note the following are for c  11 and higher   They will work for all        the other examples below except for the stuff after the Edit           c  11 and up      for auto it   std  begin arr   it    std  end arr     it           do stuff              range for loop  c  11 and up      for auto amp  element   arr           do stuff           return arr     std  vector lt int gt  amp  fillarr std  vector lt int gt  amp  arr        for auto it   arr begin    it    arr end      it           do stuff          return arr      However  if you insist on playing with C arrays  then use a template which will keep the information of how many items in the array   template  lt size t N gt  int  amp fillarr int  amp arr  N    N           N is easier and cleaner than specifying sizeof arr  sizeof arr 0       for int  it   arr  it    arr   N    it           do stuff          return arr      Except  that looks butt ugly  and super hard to read   I now use something to help with that which wasn t around in 2010  which I also use for function pointers   template  lt typename T gt  using type t   T   template  lt size t N gt  type t lt int  amp   N  gt  fillarr type t lt int  amp   N  gt  arr           N is easier and cleaner than specifying sizeof arr  sizeof arr 0       for int  it   arr  it    arr   N    it           do stuff          return arr      This moves the type where one would expect it to be  making this far more readable   Of course  using a template is superfluous if you are not going to use anything but 5 elements  so you can of course hard code it   type t lt int  amp   5  gt  fillarr type t lt int  amp   5  gt  arr           Prefer using the compiler to figure out how many elements there are        as it reduces the number of locations where you have to change if needed      for int  it   arr  it    arr   sizeof arr  sizeof arr 0      it           do stuff          return arr      As I said  my type t lt  gt  trick wouldn t have worked at the time this question was asked   The best you could have hoped for back then was to use a type in a struct   template lt typename T gt  struct type     typedef T type      typename type lt int  amp   5  gt   type fillarr typename type lt int  amp   5  gt   type arr           Prefer using the compiler to figure out how many elements there are        as it reduces the number of locations where you have to change if needed      for int  it   arr  it    arr   sizeof arr  sizeof arr 0      it           do stuff          return arr      Which starts to look pretty ugly again  but at least is still more readable  though the typename may have been optional back then depending on the compiler  resulting in   type lt int  amp   5  gt   type fillarr type lt int  amp   5  gt   type arr           Prefer using the compiler to figure out how many elements there are        as it reduces the number of locations where you have to change if needed      for int  it   arr  it    arr   sizeof arr  sizeof arr 0      it           do stuff          return arr      And then of course you could have specified a specific type  rather than using my helper   typedef int  amp array5  5    array5 fillarr array5 arr           Prefer using the compiler to figure out how many elements there are        as it reduces the number of locations where you have to change if needed      for int  it   arr  it    arr   sizeof arr  sizeof arr 0      it           do stuff          return arr      Back then  the free functions std  begin   and std  end   didn t exist  though could have been easily implemented   This would have allowed iterating over the array in a safer manner as they make sense on a C array  but not a pointer   As for accessing the array  you could either pass it to another function that takes the same parameter type  or make an alias to it  which wouldn t make much sense as you already have the original in that scope    Accessing a array reference is just like accessing the original array   void other function type t lt int  amp   5  gt  x       do something else       void fn         int array 5       other function fillarr array        or  void fn         int array 5       auto amp  array2   fillarr array      alias  But why bother      int forth entry   array 4       int forth entry2   array2 4      same value as forth entry     To summarize  it is best to not allow an array decay into a pointer if you intend to iterate over it   It is just a bad idea as it keeps the compiler from protecting you from shooting yourself in the foot and makes your code harder to read   Always try and help the compiler help you by keeping the types as long as possible unless you have a very good reason not to do so   Edit  Oh  and for completeness  you can allow it to degrade to a pointer  but this decouples the array from the number of elements it holds   This is done a lot in C C   and is usually mitigated by passing the number of elements in the array   However  the compiler can t help you if you make a mistake and pass in the wrong value to the number of elements      separate size value int  fillarr int  arr  size t size        for int  it   arr  it    arr   size    it           do stuff          return arr      Instead of passing the size  you can pass the end pointer  which will point to one past the end of your array   This is useful as it makes for something that is closer to the std algorithms  which take a begin and and end pointer  but what you return is now only something that you must remember      separate end pointer int  fillarr int  arr  int  end        for int  it   arr  it    end    it           do stuff          return arr      Alternatively  you can document that this function will only take 5 elements and hope that the user of your function doesn t do anything stupid      I document that this function will ONLY take 5 elements and     return the same array of 5 elements   If you pass in anything    else  may nazal demons exit thine nose  int  fillarr int  arr        for int  it   arr  it    arr   5    it           do stuff          return arr      Note that the return value has lost it s original type and is degraded to a pointer   Because of this  you are now on your own to ensure that you are not going to overrun the array   You could pass a std  pair lt int   int  gt   which you can use for begin and end and pass that around  but then it really stops looking like an array   std  pair lt int   int  gt  fillarr std  pair lt int   int  gt  arr        for int  it   arr first  it    arr second    it           do stuff          return arr     if you change arr  then return the original arr value     void fn         int array 5       auto array2   fillarr std  make pair  amp array 0    amp array 5             Can be done  but you have the original array in scope  so why bother      int fourth element   array2 first 4       or  void other function std  pair lt int   int  gt  array           Can be done  but you have the original array in scope  so why bother      int fourth element   array2 first 4      void fn         int array 5       other function fillarr std  make pair  amp array 0    amp array 5          Funny enough  this is very similar to how std  initializer list work  c  11   but they don t work in this context

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the Simplest way to do this  is to return it by reference   even if you don t write  the   amp   symbol   it is automatically returned by reference       void fillarr int arr 5              for

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template lt typename T  size t N gt  using ARR REF   T   amp   N    template  lt typename T  size t N gt  ARR REF lt T N gt  ArraySizeHelper ARR REF lt T N gt  arr     define arraysize arr  sizeof ArraySizeHelper arr

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Actually when you pass an array inside a function  the pointer to the original array is passed in the function parameter and thus the changes made to the array inside that function is actually made on the original array     include  lt iostream gt   using namespace std   int  func int ar          for int i 0 i lt 100 i             ar i  i      int  ptr ar      return ptr      int main         int  p      int y 100   0           p func y        for int i 0 i lt 100 i             cout lt  lt i lt  lt       lt  lt y i  lt  lt   n       Run it and you will see the changes

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In this case  your array variable arr can actually also be treated as a pointer to the beginning of your array s block in memory  by an implicit conversion   This syntax that you re using   int fillarr int arr      Is kind of just syntactic sugar   You could really replace it with this and it would still work   int fillarr int  arr    So in the same sense  what you want to return from your function is actually a pointer to the first element in the array   int  fillarr int arr      And you ll still be able to use it just like you would a normal array   int main       int y 10     int  a   fillarr y     cout  lt  lt  a 0   lt  lt  endl

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Here s a full example of this kind of problem to solve   include  lt bits stdc   h gt  using namespace std  int  solve int brr   int n    sort brr brr n   return brr     int main     int n  cin gt  gt n  int arr n   for int i 0 i lt n i          cin gt  gt arr i     int  a solve arr n   for int i 0 i lt n i          cout lt  lt a i  lt  lt endl     return 0

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Source  https   www tutorialspoint com cplusplus cpp return arrays from functions htm  C   does not allow to return an entire array as an argument to a function  However  you can return a pointer to an array by specifying the array s name without an index    If you want to return a single-dimension array from a function  you would have to declare a function returning a pointer as in the following example     int   myFunction                             C   does not advocate to return the address of a local variable to outside of the function so you would have to define the local variable as static variable    Applying these rules on the current question  we can write the program as follows      include  lt iostream gt   using namespace std   int   fillarr       int main          int  p      p   fillarr        for   int i   0  i  lt  5  i            cout  lt  lt   p    lt  lt  i  lt  lt         lt  lt    p   i   lt  lt  endl       return 0      int   fillarr          static int  arr 5        for  int i   0  i  lt  5    i          arr i    i       return arr        The Output will be    p 0  0 p 1  1 p 2  2 p 3  3 p 4  4

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C   functions can t return C-style arrays by value  The closest thing is to return a pointer  Furthermore  an array type in the argument list is simply converted to a pointer   int  fillarr  int arr          arr  decays  to type int       return arr      You can improve it by using an array references for the argument and return  which prevents the decay   int    amp fillarr  int   amp arr  5      5       no decay  argument must be size 5     return arr      With Boost or C  11  pass-by-reference is only optional and the syntax is less mind-bending   array lt  int  5  gt   amp fillarr  array lt  int  5  gt   amp arr         return arr      array  being boost  array or std  array     The array template simply generates a struct containing a C-style array  so you can apply object-oriented semantics yet retain the array s original simplicity

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This   int fillarr int arr      is actually treated the same as   int fillarr int  arr    Now if you really want to return an array you can change that line to  int   fillarr int arr            do something to arr     return arr      It s not really returning an array  you re returning a pointer to the start of the array address   But remember when you pass in the array  you re only passing in a pointer  So when you modify the array data  you re actually modifying the data that the  pointer is pointing at   Therefore before you passed in the array  you must realise that you already have on the outside the modified result   e g   int fillarr int arr        array 0    10     array 1    5     int main int argc  char  argv        int arr       1 2 3 4 5           arr 0     1       arr 1     2 etc    int result   fillarr arr         arr 0     10       arr 1     5        return 0      I suggest you might want to consider putting a length into your fillarr function like this   int   fillarr int arr    int length    That way you can use length to fill the array to it s length no matter what it is   To actually use it properly   Do something like this   int   fillarr int arr    int length      for  int i   0  i  lt  length    i            arr i         do what you want to do here         return arr        then where you want to use it  int arr 5   int  arr2   arr2   fillarr arr  5       at this point  arr  amp  arr2 are basically the same  just slightly    different types   You can cast arr to a  char   and it ll be the same    If all you re wanting to do is set the array to some default values  consider using the built in memset function   something like      memset  int   amp arr  5  sizeof int     While I m on the topic though  You say you re using C     Have a look at using stl vectors   Your code is likely to be more robust   There are lots of tutorials  Here is one that gives you an idea of how to use them  http   www yolinux com TUTORIALS LinuxTutorialC  STL html

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8 3 5 8 states-   Functions shall not have a return type of type array or function  although they may have a return type of type pointer or reference to such things  There shall be no arrays of functions  although there can be arrays of pointers to functions    int   amp fn1 int   amp arr  5    5          declare fn1 as returning refernce to array    return arr     int  fn2 int arr                     declare fn2 as returning pointer to array    return arr      int main       int buf 5      fn1 buf      fn2 buf

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and what about   int   func          int  f   new int 10   1 2 3        return f     int fa 10      0    auto func2   - gt  int      10        return  amp fa

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As above mentioned paths are correct  But i think if we just return a local array variable of a function sometimes it returns garbage values as its elements    in-order to avoid that i had to create the array dynamically and proceed  Which is something like this   int  func       int  Arr   new int 100     return Arr     int main       int  ArrResult   func      cout  lt  lt  ArrResult 0   lt  lt       lt  lt  ArrResult 1   lt  lt  endl    return 0

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And why don t  quot return quot  the array as a parameter  fillarr int source    size t dimSource  int dest    size t dimDest         if  dimSource  lt   dimDest                for  size t i   0  i  lt  dimSource  i                               some stuff                        else                  some stuff            or  in a simpler way  but you have to know the dimensions      fillarr int source    int dest

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to return an array from a function   let us define that array in a structure  So it looks something like this  struct Marks     int list 5       Now let us create variables of the type structure    typedef struct Marks marks  marks marks list    We can pass array to a function in the following way and assign value to it   void setMarks int marks array        for int i 0 i lt sizeof marks array  sizeof int  i           marks list list i  marks array i       We can also return the array  To return the array   the return type of the function should be of structure type ie marks  This is because in reality we are passing the structure that contains the array  So the final code may look like this   marks getMarks     return marks list

[c++] Return array in a function

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