Find frequency of any element in a vector in C++

This post will discuss how to find the frequency of any element present in the vector in C++.

1. Using `std::count` function

The recommended approach is to use the std::count algorithm that returns the total number of elements in the specified range equal to the specified value.

#include <iostream>

#include <vector>

#include <algorithm>

int main()

{

std::vector<int> v = { 7, 2, 2, 4, 3, 2, 6 };

int val = 2;

int freq = std::count(v.begin(), v.end(), val);

std::cout << "Element " << val << " occurs " << freq << " times";

return 0;

}

Download Run Code

Output:

Element 2 occurs 3 times

2. Using `std::count_if` function

Another solution is to use the std::count_if, which takes a predicate.

1. In C++98/03, we can use a functor.

#include <iostream>

#include <vector>

#include <algorithm>

#include <unordered_map>

struct compare

{

int val;

compare(int const &i): val{i} {}

bool operator()(const int &i) const {

return (i == val);

}

};

int main()

{

std::vector<int> v = { 7, 2, 2, 4, 3, 2, 6 };

int val = 2;

int freq = std::count_if(v.begin(), v.end(), compare(val));

std::cout << "Element " << val << " occurs " << freq << " times";

return 0;

}

Download Run Code

Output:

Element 2 occurs 3 times

2. In C++11 and above, the elegant solution is to use lambdas:

#include <iostream>

#include <vector>

#include <algorithm>

#include <unordered_map>

int main()

{

std::vector<int> v = { 7, 2, 2, 4, 3, 2, 6 };

int val = 2;

int freq = std::count_if(v.begin(),

v.end(),

[&](int const &i) {

return (i == val);

});

std::cout << "Element " << val << " occurs " << freq << " times";

return 0;

}

Download Run Code

Output:

Element 2 occurs 3 times

3. Using frequency map

Finally, if the total number of function calls is more, the efficient solution is to preprocess the vector by creating a frequency map. This will take linear time and some extra space, but now we can count for any element present in the vector in constant time.

#include <iostream>

#include <vector>

#include <unordered_map>

int main()

{

std::vector<int> v = { 7, 2, 2, 4, 3, 2, 6 };

int val = 2;

std::unordered_map<int, int> freq;

for (int const &i: v) {

freq[i]++;

}

std::cout << "Element " << val << " occurs " << freq[val] << " times";

return 0;

}

Download Run Code

Output:

Element 2 occurs 3 times

That’s all about finding the frequency of any element in a vector in C++.

Find frequency of any element in a vector in C++

1. Using std::count function

2. Using std::count_if function

3. Using frequency map

Share this:

1. Using `std::count` function

2. Using `std::count_if` function