Use lambdas as predicates with the algorithm library

Some functions in the algorithm library require the use of a predicate function. A predicate is a function (or functor or lambda) that tests a condition and returns a Boolean true/false response.

How to do it…

For this recipe, we will experiment with the count_if() algorithm using different types of predicates:

• First, let's create a function for use as a predicate. A predicate takes a certain number of arguments and returns a bool. A predicate for count_if() takes one argument:

  bool is_div4(int i) {
      return i % 4 == 0;
  }

This predicate checks whether an int value is divisible by 4.

• In main(), we'll define a vector of int values, and use it to test our predicate function with count_if():

  int main() {
      const vector<int> v{ 1, 7, 4, 9, 4, 8, 12, 10, 20 };
      int count = count_if(v.begin(), v.end(), is_div4);
    ...