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

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

#include <iostream>
#include <vector>
#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;
}

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

#include <iostream>
using std::cout;
using std::endl;
#include <algorithm>
#include <numeric>
#include <vector>
#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;
}
Vector v2 contains: 100 2 8 1 50 3 8 8 9 10
Number of elements greater than 9: 3