Essential Books

std count if

suggest change 
template <class InputIterator, class UnaryPredicate>
typename iterator_traits<InputIterator>::difference_type
count_if (InputIterator first, InputIterator last, UnaryPredicate red);

Effects

Counts the number of elements in a range for which a specified predicate function is true

Parameters

first => iterator pointing to the beginning of the range last => iterator pointing to the end of the range red => predicate function(returns true or false)

Return

The number of elements within the specified range for which the predicate function returned true.

Example 

#include <iostream>
#include <vector>
#include <algorithm>

using namespace std;

/*
    Define a few functions to use as predicates
*/

//return true if number is odd
bool isOdd(int i){
  return i%2 == 1;
}

//functor that returns true if number is greater than the value of the constructor parameter provided
class Greater {
  int _than;
public:
  Greater(int th): _than(th){}
  bool operator()(int i){
    return i > _than;
  }
};

int main(int argc, const char * argv[]) {
  
  //create a vector
  vector<int> myvec = {1,5,8,0,7,6,4,5,2,1,5,0,6,9,7};

  //using a lambda function to count even numbers
  size_t evenCount = count_if(myvec.begin(), myvec.end(), [](int i){return i % 2 == 0;}); // >= C++11
  
  //using function pointer to count odd number in the first half of the vector
  size_t oddCount = count_if(myvec.begin(), myvec.end()- myvec.size()/2, isOdd);
  
  //using a functor to count numbers greater than 5
  size_t greaterCount = count_if(myvec.begin(), myvec.end(), Greater(5));

  cout << "vector size: " << myvec.size() << endl;
  cout << "even numbers: " << evenCount << " found" << endl;
  cout << "odd numbers: " << oddCount << " found" << endl;
  cout << "numbers > 5: " << greaterCount << " found"<< endl;
  
  return 0;
}

Output 

vector size: 15
even numbers: 7 found
odd numbers: 4 found
numbers > 5: 6 found
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Standard library algorithms:
* Standard Library Algorithms
* std next permutation
* std for each
* std accumulate
* std find
* std count
* std count if
* std find if
* std min element
* Using std nth element To Find The Median Or Other Quantiles
Table Of Contents
0 Getting started
1 Literals
2 Basic type keywords
3 Operator precedence
4 Floating point arithmetic
5 Bit operators
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28 std::set and std::multiset
29 std::any
30 std::variant
31 std::optional
32 std::integer_sequence
33 std::function
34 std::forward_list
35 std::iomanip
36 Iterators
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42 Returning multiple values from a function
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92 Trailing return type
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94 Common compile linker errors
95 Design patterns
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98 Type traits
99 One definition rule
100 Unspecified behavio
101 Argument dependent name lookup
102 Attributes
103 Internationalization
104 Profiling
105 Return type covariance
106 Non-static member functions
107 Recursion
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110 C++ vs. C++ 11 vs C++ 17
111 Inline functions
112 Client server examples
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128 Curiusly recurring template pattern
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131 Layout of object types
132 C incompatibilities
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136 Debugging
137 Futures and promises
138 More undefined behaviors
139 Mutexes
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141 Unit testing
142 decltype
143 Digit separators
144 C++ Containers
145 Arithmetic meta-programming
146 Contributors