Find count of an element in a vector in C++

This post will discuss how to find the count of an element in a vector in C++.

1. Using `std::count`

The standard solution to get the count of an element in a vector is using the std::count function. It returns the total number of elements in the specified range that is equal to the target, as shown below:

#include <iostream>

#include <vector>

#include <algorithm>

int main()

{

std::vector<int> values = {1, 2, 3, 2, 5, 2, 6, 7};

int target = 2;

int count = std::count(values.begin(), values.end(), target);

std::cout << count << std::endl; // 3

return 0;

}

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2. Using `std::count_if`

To count the number of elements in the vector that satisfies a predicate, we can use the std::count_if standard algorithm. A typical implementation of this function would look like this:

#include <iostream>

#include <vector>

#include <algorithm>

int main()

{

std::vector<int> v = {1, 2, 3, 2, 5, 2, 6, 7};

int target = 2;

int count = std::count_if(v.begin(), v.end(), [&target](int &i) {

return i == target;

});

std::cout << count << std::endl; // 3

return 0;

}

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3. Using frequency map

Finally, we can create a frequency map and get the count of the specified target. This can be implemented as follows in C++.

#include <iostream>

#include <vector>

#include <unordered_map>

int main()

{

std::vector<int> values = {1, 2, 3, 2, 5, 2, 6, 7};

int target = 2;

std::unordered_map<int, int> freq;

for (auto &i: values) {

freq[i]++;

}

int count = freq[target];

std::cout << count << std::endl; // 3

return 0;

}

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That’s all about finding the count of an element in a vector in C++.

Find count of an element in a vector in C++

1. Using std::count

2. Using std::count_if

3. Using frequency map

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1. Using `std::count`

2. Using `std::count_if`