How do I find an element position in std vector

Question

I need to find an element position in an std  vector to use it for referencing an element in another vector   int find  const vector lt type gt  amp  where  int searchParameter         for  int i   0  i  lt  where size    i              if  conditionMet  where i   searchParameter                  return i                     return -1       caller  const int position   find  firstVector  parameter    if  position    -1         doAction  secondVector position         however vector  size   returns size t which corresponds to an unsigned integral type that can t directly store -1  How do I signal that the element is not found in a vector when using size t instead of int as an index

User · Answer

std  vector has random-access iterators  You can do pointer arithmetic with them  In particular  this my vec begin     my vec size      my vec end   always holds  So you could do   const vector lt type gt   const iterator pos   std  find if  firstVector begin                                                          firstVector end                                                          some predicate parameter     if  position    firstVector end           const vector lt type gt   size type idx   pos-firstVector begin        doAction  secondVector idx         As an alternative  there s always std  numeric limits lt vector lt type gt   size type gt   max   to be used as an invalid value

User · Answer

You probably should not use your own function here  Use find   from STL   Example   list L  L push back 3   L push back 1   L push back 7    list  iterator result   find L begin    L end    7   assert result    L end       result    7

User · Answer

Get rid of the notion of vector entirely template lt  typename IT  typename VT gt  int index of IT begin  IT end  const VT amp  val        int index   0      for    begin    end    begin                if   begin    val  return index            return -1     This will allow you more flexibility and let you use constructs like int squid      5 2 7 4 1 6 3 0   int sponge      4 2 4 2 4 6 2 6   int squidlen   sizeof squid  sizeof squid 0    int position   index of  amp squid 0    amp squid squidlen   3   if  position  gt   0    std  cout  lt  lt  sponge position   lt  lt  std  endl     You could also search any other container sequentially as well

User · Answer

Take a vector of integer and a key  that we find in vector      Now we are traversing the vector until found the key value or last index otherwise      If we found key then print the position   otherwise print  -1          include  lt bits stdc   h gt         using namespace std          int main                         vector lt int gt str              int flag temp key   len num              flag 0             cin gt  gt len              for int i 1  i lt  len  i                                  cin gt  gt key                 v push back key                            cin gt  gt num              for int i 1  i lt  len  i                                  if str i   num                                     flag                      temp i-1                  break                                          if flag  0     cout lt  lt temp lt  lt endl           else           cout lt  lt  -1  lt  lt endl              str clear                 return 0

User · Answer

Something like this  I think  find if counted hpp    ifndef FIND IF COUNTED HPP  define FIND IF COUNTED HPP   include  lt algorithm gt   namespace find if counted impl       template  lt typename Func gt      struct func counter               explicit func counter Func amp  func  unsigned  amp count             func func            count count                               template  lt typename T gt          bool operator   const T amp  t                           count               return  func t                  private          Func amp   func          unsigned amp   count               generic find if counted     returns the index of the found element  otherwise returns find if not found const size t find if not found   static cast lt size t gt  -1    template  lt typename InputIterator  typename Func gt  size t find if counted InputIterator start  InputIterator finish  Func func        unsigned count   0      find if counted impl  func counter lt Func gt  f func  count        InputIterator result   find if start  finish  f        if  result    finish                return find if not found            else               return count - 1            endif   Example    include  find if counted hpp   include  lt cstdlib gt   include  lt iostream gt   include  lt vector gt   typedef std  vector lt int gt  container   int rand number void        return rand      20     bool is even int i        return i   2    0     int main void        container vec1 10       container vec2 10        std  generate vec1 begin    vec1 end    rand number       std  generate vec2 begin    vec2 end    rand number        unsigned index   find if counted vec1 begin    vec1 end    is even        if  index    find if not found                std  cout  lt  lt   vec1 has no even numbers    lt  lt  std  endl            else               std  cout  lt  lt   vec1 had an even number at index     lt  lt  index  lt  lt                vec2 s corresponding number is     lt  lt  vec2 index   lt  lt  std  endl            Though I feel like I m doing something silly     X Any corrections are welcome  of course

User · Answer

Take a look at the answers provided for this question  Invalid value for size t   Also you can use std  find if with std  distance to get the index   std  vector lt type gt   iterator iter   std  find if vec begin    vec end    comparisonFunc   size t index   std  distance vec begin    iter   if index    vec size             invalid

User · Answer

If a vector has N elements  there are N 1 possible answers for find   std  find and std  find if return an iterator to the found element OR end   if no element is found   To change the code as little as possible  your find function should return the equivalent position   size t find  const vector lt type gt  amp  where  int searchParameter        for  size t i   0  i  lt  where size    i              if  conditionMet  where i   searchParameter                  return i                     return where size         caller  const int position   find  firstVector  parameter    if  position    secondVector size           doAction  secondVector position         I would still use std  find if  though

User · Answer

First of all  do you really need to store indices like this  Have you looked into std  map  enabling you to store key    value pairs   Secondly  if you used iterators instead  you would be able to return std  vector end   to indicate an invalid result  To convert an iterator to an index you simply use  size t i   it - myvector begin

User · Answer

You could use std  numeric limits lt size t gt   max   for elements that was not found  It is a valid value  but it is impossible to create container with such max index  If std  vector has size equal to std  numeric limits lt size t gt   max    then maximum allowed index will be  std  numeric limits lt size t gt   max  -1   since elements counted from 0

User · Answer

In this case  it is safe to cast away the unsigned portion unless your vector can get REALLY big   I would pull out the where size   to a local variable since it won t change during the call   Something like this   int find  const vector lt type gt  amp  where  int searchParameter        int size   static cast lt int gt  where size         for  int i   0  i  lt  size  i              if  conditionMet  where i   searchParameter                  return i                     return -1

[c++] How do I find an element position in std::vector?

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