count_if (STL Samples)

Illustrates how to use the count_if Standard Template Library (STL) function in Visual C++.

template<class InputIterator, class Predicate> inline 
   size_t count_if(
      InputIterator First,
      InputIterator Last,
      Predicate P
   )

Remarks

注意

The class/parameter names in the prototype do not match the version in the header file. Some have been modified to improve readability.

The count_if algorithm counts the number of elements in the range [First, Last) that cause the predicate to return true and returns the number of elements for which the predicate was true.

Example

// countif.cpp
// compile with: /EHsc
//
// Functions:
//
//   count_if  - Count items in a range that satisfy a predicate.
//
//   begin     - Returns an iterator that points to the first element in
//               a sequence.
//
//   end       - Returns an iterator that points one past the end of a
//               sequence.

// disable warning C4786: symbol greater than 255 character,
// okay to ignore
#pragma warning(disable: 4786)

#include <iostream>
#include <algorithm>
#include <functional>
#include <string>
#include <vector>

using namespace std;


// Return true if string str starts with letter 'S'
int MatchFirstChar( const string& str)
{
    string s("S") ;
    return s == str.substr(0,1) ;
}

int main()
{
    const int VECTOR_SIZE = 8 ;

    // Define a template class vector of strings
    typedef vector<string > StringVector ;

    //Define an iterator for template class vector of strings
    typedef StringVector::iterator StringVectorIt ;

    StringVector NamesVect(VECTOR_SIZE) ;   //vector containing names

    StringVectorIt start, end, it ;

    ptrdiff_t result = 0 ;   // stores count of elements
                       // that match value.

    // Initialize vector NamesVect
    NamesVect[0] = "She" ;
    NamesVect[1] = "Sells" ;
    NamesVect[2] = "Sea" ;
    NamesVect[3] = "Shells" ;
    NamesVect[4] = "by" ;
    NamesVect[5] = "the" ;
    NamesVect[6] = "Sea" ;
    NamesVect[7] = "Shore" ;

    start = NamesVect.begin() ;   // location of first
                                  // element of NamesVect

    end = NamesVect.end() ;       // one past the location
                                  // last element of NamesVect

    // print content of NamesVect
    cout << "NamesVect { " ;
    for(it = start; it != end; it++)
        cout << *it << " " ;
    cout << " }\n" << endl ;

    // Count the number of elements in the range [first, last +1)
    // that start with letter 'S'
    result = count_if(start, end, MatchFirstChar) ;

    // print the count of elements that start with letter 'S'
    cout << "Number of elements that start with letter \"S\" = "
        << result << endl  ;
}

Output

NamesVect { She Sells Sea Shells by the Sea Shore  }

Number of elements that start with letter "S" = 6

Requirements

Header: <algorithm>

See Also

Concepts

Standard Template Library Samples