count, count_if

Algorithm

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• Data Type and Member Function Indexes
• Synopsis
• Description
• Complexity
• Example
• Warnings

Count the number of elements in a container that satisfy a given condition.

None

#include <algorithm>
template<class InputIterator, class T>
 iterator_trait<InputIterator>::distance_type
 count(InputIterator first, InputIterator last, 
       const T& value);

template <class InputIterator, class T, class Size>
 void count(InputIterator first, InputIterator last,
            const T& value, Size& n);

template<class InputIterator, class Predicate>
 iterator_trait<InputIterator>::distance_type
 count_if(InputIterator first, InputIterator last, 
          Predicate pred);

template <class InputIterator, class Predicate, class Size>
 void count_if(InputIterator first, InputIterator last,
               Predicate pred, Size& n);

The count algorithm compares value to elements in the sequence defined by iterators first and last. The first version of count return the number of matches. The second version increments a counting value n each time it finds a match. i.e., count returns (or adds to n) the number of iterators i in the range [first, last) for which the following condition holds:

*i == value

The count_if algorithm lets you specify a predicate, and returns the number of times an element in the sequence satisfies the predicate (or increments n that number of times). That is, count_if returns (or adds to n) the number of iterators i in the range [first, last) for which the following condition holds:

pred(*i) == true.

Both count and count_if perform exactly last-first applications of the corresponding predicate.

//
// count.cpp
// 
// Does not demonstrate the partial specialization versions 
// of count and count_if
//
 #include <vector>
 #include <algorithm>
 #include <iostream.h>

 int main()
 {
   int sequence[10] = {1,2,3,4,5,5,7,8,9,10};
   int i=0,j=0,k=0;
   //
   // Set up a vector
   //
   vector<int> v(sequence,sequence + 10);

   count(v.begin(),v.end(),5,i);  // Count fives 
   count(v.begin(),v.end(),6,j);  // Count sixes 
   //
   // Count all less than 8 
   // I=2, j=0
   //
   count_if(v.begin(),v.end(),bind2nd(less<int>(),8),k);
   // k = 7

   cout << i << " " << j << " " << k << endl;
   return 0;
 }

Output :  2 0 7

If your compiler does not support partial specialization then the first version of both count and count_if (the one that returns the count) will not be available.

If your compiler does not support default template parameters then you need to always supply the Allocator template argument. For instance, you'll have to write:

vector <int, allocator<int> >

instead of:

vector <int>

©Copyright 1996, Rogue Wave Software, Inc.