Write a C Program to count the frequency of each character

In C programming, counting the frequency of each character in a string is a common task that involves scanning through the string and tracking how many times each character appears.

Syntax

int frequency[26];  // For lowercase letters a-z
frequency[character - 'a']++;  // Increment count for character

Algorithm

Step 1: Define maximum string size
Step 2: Declare string and frequency array
Step 3: Read input string 
Step 4: Initialize frequency array to 0
Step 5: Iterate through string and count characters:
   - For lowercase: frequency[string[i] - 'a']++
   - For uppercase: frequency[string[i] - 'A']++
Step 6: Print non-zero frequencies

Example

The following C program counts the frequency of each character in a string −

#include <stdio.h>
#include <string.h>
#define MAX 100

int main() {
    char string[MAX];
    int i, length;
    int frequency[26] = {0};  // Initialize all to 0
    
    /* Input string from user */
    printf("Enter the string: ");
    fgets(string, sizeof(string), stdin);
    
    length = strlen(string);
    
    /* Count frequency of each character */
    for(i = 0; i < length; i++) {
        /* Convert to lowercase and count */
        if(string[i] >= 'a' && string[i] <= 'z') {
            frequency[string[i] - 'a']++;
        }
        else if(string[i] >= 'A' && string[i] <= 'Z') {
            frequency[string[i] - 'A']++;
        }
    }
    
    /* Print frequency of all characters */
    printf("\nCharacter frequencies:
");
    for(i = 0; i < 26; i++) {
        if(frequency[i] != 0) {
            printf("'%c' = %d
", (i + 'a'), frequency[i]);
        }
    }
    
    return 0;
}

Enter the string: Tutorials Point
Character frequencies:
'a' = 1
'i' = 2
'l' = 1
'n' = 1
'o' = 2
'p' = 1
'r' = 1
's' = 1
't' = 3

How It Works

• We use an array of size 26 to store frequencies for letters a-z
• ASCII value manipulation: 'a' - 'a' = 0, 'b' - 'a' = 1, etc.
• Both uppercase and lowercase letters are mapped to the same index
• Non-alphabetic characters are ignored

Conclusion

Character frequency counting in C uses array indexing based on ASCII values. This approach efficiently handles both uppercase and lowercase letters while ignoring non-alphabetic characters.