count_if (STL Samples)
Viene illustrato come utilizzare la funzione della libreria di modelli (STL) standard di count_if in Visual C++.
template<class InputIterator, class Predicate> inline
size_t count_if(
InputIterator First,
InputIterator Last,
Predicate P
)
Note
[!NOTA]
La classe/nomi di parametro nel prototipo non corrisponde alla versione nel file di intestazione.alcuni sono stati modificati per migliorare la leggibilità.
L'algoritmo di count_if viene calcolato il numero di elementi nell'intervallo [First, Last) che causa il predicato per restituire true e restituisce il numero di elementi per cui il predicato è true.
Esempio
// 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
Requisiti
intestazione: <algoritmo>