Frequency of each character in a string using map in c++

In some competitive programming problems, it is required to check if a character in a string is repeating or not, number of repeating character or frequency of a particular character or all characters in a string.

So, in this C++ tutorial, we are going to discuss how to find the frequency of each character in a string using map.

Frequency of characters in a string in c++ using map

What is map ?




Map are the containers which stores elements in mapping way. There is a key value and a unique map value for it.

It prints the value in the sorted order of the key value.

 Syntax:

map<data_type of key value , data_type of mapping value>alias_name for map;

Ex:

map<char,int>m;

What is iterator?

An iterator is a pointer which points to elements in CPP Standard Template Library (STL). We use iterators in c++ to move through the elements in containers.

Syntax:

container<Data_type 1,Data_type 2>::iterator alias_name

Ex:

map<char,int>::iterator itr;

Iterators reduce the time complexity of a program.

Operations of iterators :-

1. begin() :- begin() is used to return beginning position of container.

2. end() :- end() is used to return after end position of container.

Algorithm to find out the frequency of a character in C++ using map

  1. Declare a map of char to int where key values are the characters of the string and mapped values are its frequencies.
  2. Read the characters from first to last in the string and increment the value in the map while reading each characters.
  3. As map do not contains duplicate keys , so finally, our map stores unique key which is characters in string uniquely and its mapped value which is the frequency of each character.
  4. To print values in the map we declare an iterator , and print the value of itr->first ( characters in a string) and itr->second (their frequencies) till itr does not reach the end of the map.

Code to count frequency of each character in a string using map in CPP

#include<bits/stdc++.h>

 using namespace std;

 int main()
{
    string s;
    
      cin>>s;
      
      map<char , int >m;
      
         map<char , int >::iterator itr;
       
      for(long i=0;i<s.length();i++)
        m[s[i]]++;
        
        for(itr=m.begin();itr!=m.end();itr++)
          
            cout<<itr->first<<" - "<<itr->second<<endl;
}

INPUT

Akarshika

OUTPUT

A - 1
a - 2
h - 1
i - 1
k - 2
r - 1
s - 1

Here, the key values are the characters in the string “Akarshika” and mapping values are its frequencies. Key values are printed in sorted order.

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2 responses to “Frequency of each character in a string using map in c++”

  1. Sukanya says:

    m[s[i]]++;
    what does this mean?

  2. Pritam Hande says:

    In “for(long i=0;i -&lt s.length();i++) { m[s[i]]++; }” we are traversing whole string character by character. In the first iteration, i will be 0 and s[i] will be ‘A’ for the string “Akarshika”. In m[s[i]]++, value of m[s[i]] i.e. m[A] (as s[i]=A for i=0) will increment by 1 (which was initially 0 by default for indices A-Z, a-z ) . So by the end of the first iteration, we will get A-1 pair. similarly all iterations will execute. The reason we will have A-Z, a-z indices because we have used char as the key of our map ‘m’ and we will get the output pairs in alphabetical order. First, we will get pairs of ‘A-Z’ and then we will get pairs of ‘a-z’.

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