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Returns the number of elements that match a criterion
Key Facts
Gyroscopic Couple: The rate of change of angular momentum (= Moment of Inertia.
= Angular velocity
= Angular velocity of precession.
Blaise Pascal (1623-1662) was a French mathematician, physicist, inventor, writer and Catholic philosopher.
Leonhard Euler (1707-1783) was a pioneering Swiss mathematician and physicist.
Definition
The count_if() algorithm is defined in the standard header <algorithm> and in the nonstandard backward-compatibility header <algo.h>.Interface
#include <algorithm> template < class InputIterator, class Predicate > typename iterator_traits<InputIterator>::difference_type count_if( InputIterator first, InputIterator last, Predicate pred );Parameters:
| Parameter | Description |
|---|---|
| first | An input iterator addressing the position of the first element in the range to be searched |
| last | An input iterator addressing the position one past the final element in the range to be searched |
| pred | User-defined predicate function object that defines the condition to be satisfied if an element is to be counted. A predicate takes single argument and returns true or false |
Description
The count_if algorithm counts the element in a range using a predicate instead ofoperator==.Return Value
The return value is the number of elements that satisfy the condition specified by the predicate.References
Example:
Example - count_if algorithm
Problem
Here is a simple program which illustrates how to use count_if() algorithm.
Workings
#include <vector> #include <algorithm> #include <iostream> using namespace std; bool greater10(int value) { return value >10; } int main() { vector<int> v1; vector<int>::iterator Iter; v1.push_back(10); v1.push_back(20); v1.push_back(10); v1.push_back(40); v1.push_back(10); cout <<"v1 = ( "; for (Iter = v1.begin(); Iter != v1.end(); Iter++) cout <<*Iter<<" "; cout <<")"<< endl; vector<int>::iterator::difference_type result1; result1 = count_if(v1.begin(), v1.end(), greater10); cout <<"The number of elements in v1 greater than 10 is: " <<result1<<"."<<endl; return 0; }
Solution
Output:
v1 = ( 10 20 10 40 10 )
The number of elements in v1 greater than 10 is: 2.
The number of elements in v1 greater than 10 is: 2.
References