std::count

Learn counting with std::count and std::count_if.

std::count

std::count counts how many times a value appears in a range. With int n = std::count(v.begin(), v.end(), 2); you find how many 2s there are. The result is an integer.

Its sibling std::count_if counts elements matching a condition. The condition is given as a lambda or functor: std::count_if(v.begin(), v.end(), [](int x) { return x % 2 == 0; }). You will study lambda expressions in detail in the next group. For now it is enough to know that std::count counts a specific value. Both live in <algorithm>.

Counting example

To find how many times a name appears in a class list, you use std::count. It works on std::vector<std::string> too: std::count(names.begin(), names.end(), "Ayşe") gives the number of people named Ayşe.

std::count runs linearly: it checks each element once. The result is of type iterator::difference_type, usually long or ptrdiff_t. You can assign it to an int; for small counts this is fine. If the value is not found it returns 0.

Exercise Print the count of a value

For v = {1, 2, 2, 3, 2, 4}, use std::count to find how many 2s there are and print it with std::cout.

Show example solution

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

int main() {
    std::vector<int> v = {1, 2, 2, 3, 2, 4};
    int n = std::count(v.begin(), v.end(), 2);
    std::cout << n;
    return 0;
}

Test yourself

Match each algorithm to its purpose

std::count

Counts a value
std::count_if

Counts by condition
std::find

Searches a value
std::sort

Sorts

Complete the count call

int n = std::____(v.begin(), v.end(), 2);

Show answer

int n = std::count(v.begin(), v.end(), 2);

What does std::count(v.begin(), v.end(), 2) return?
```
std::vector<int> v = {1, 2, 2, 3, 2};
auto n = std::count(v.begin(), v.end(), 2);
```
- The index of the first 2
- The count of 2s
- The size of the vector
- The position of the last 2
Show answer

The count of 2s — std::count returns the count of 2s (3).

Next lesson: std::transform

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