C++ Tutorial/STL Algorithms Non modifying sequence operations/count if

count_if(): A unary predicate that determines if number is divisible by 3

<source lang="cpp">#include <iostream>

1. include <vector>
2. include <algorithm>

using namespace std;

bool dividesBy3(int i) {

 if((i%3) == 0) return true;
  
 return false;

}

int main() {

 vector<int> v;
 int i;
  
 for(i=1; i < 20; i++) v.push_back(i);
  
 for(i=0; i<v.size(); i++)
   cout << v[i] << endl;
  
 i = count_if(v.begin(), v.end(), dividesBy3);
 cout << i << " numbers are divisible by 3.\n";
  
 return 0;

}</source>

Use std::count_if to count number of elements in vector that are greater than 9

<source lang="cpp">#include <iostream> using std::cout; using std::endl;

1. include <algorithm>
2. include <numeric>
3. include <vector>
4. include <iterator>

bool greater9( int ); int main() {

  std::ostream_iterator< int > output( cout, " " );
  int a2[ 10 ] = { 100, 2, 8, 1, 50, 3, 8, 8, 9, 10 };
  std::vector< int > v2( a2, a2 + 10 ); // copy of a2
  cout << "\n\nVector v2 contains: ";
  std::copy( v2.begin(), v2.end(), output );
  // count number of elements in v2 that are greater than 9
  int result = std::count_if( v2.begin(), v2.end(), greater9 );
  cout << "\nNumber of elements greater than 9: " << result;
  cout << endl;
  return 0;

} bool greater9( int value ) {

  return value > 9;

}</source>

Vector v2 contains: 100 2 8 1 50 3 8 8 9 10
Number of elements greater than 9: 3