Increment map value associated with a key in C++

Given an ordered or an unordered map and a key in C++, increment the map value associated with that key.

The idea is to use the unordered_map::find function that searches the container for a key and returns an iterator to it, or the unordered_map::end if the key is not found.

Once the key is found, we can increment its value using the ++ operator on the second member function of the pair pointed by the iterator. If the key is not found, we can insert it into the map with value 1 using unordered_map::insert with std::make_pair.

#include <iostream>

#include <unordered_map>

#include <iterator>

int main()

{

std::string s = "ABCBADCA";

std::unordered_map<char, int> freq;

for (const char &c: s)

{

// check if key `c` exists in the map or not

std::unordered_map<char, int>::iterator it = freq.find(c);

// key already present on the map

if (it != freq.end()) {

it->second++; // increment map's value for key `c`

}

// key not found

else {

freq.insert(std::make_pair(c, 1));

}

for (auto &e: freq) {

std::cout << '{' << e.first << ", " << e.second << '}' << std::endl;

}

return 0;

}

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Output:

{D, 1}
{C, 2}
{B, 2}
{A, 3}

We can also use operator[], which makes this very easy. It returns a reference to the mapped value for the specified key. If the key does not exist in the map, it inserts a new pair with that key and the default value of 0 (for numeric data types) and returns a reference to it. Once we reference the mapped value of a key, we can use the ++ operator to increment its value.