Anagram Program in C – Check if Two Strings are Anagrams

An anagram is a word or phrase formed by rearranging all the characters of another. In C, the standard way to check if two strings are anagrams is to count character frequencies: if both strings have exactly the same character counts, they are anagrams — regardless of order.

This page shows a complete anagram program in C using the frequency-counting method, with step-by-step explanation, sample output, and time complexity analysis.

How to Check for Anagrams — Step by Step

1. Check that both strings have the same length. If not, they cannot be anagrams.
2. Create an integer array of size 256 (one slot per ASCII character), initialized to zero.
3. Loop through the first string: increment the frequency of each character.
4. Loop through the second string: decrement the frequency of each character.
5. If all 256 entries are still zero, both strings have identical character counts — they are anagrams.

Example: “listen” and “silent” — both have the same characters (e, i, l, n, s, t), just in different order. After step 4, every count returns to zero. ✓ Anagram.

Anagram Program in C

#include <stdio.h>
#include <string.h>

#define CHARS 256

int isAnagram(const char *s1, const char *s2)
{
    int freq[CHARS] = {0};
    int i;

    for (i = 0; s1[i] != '\0'; i++)
        freq[(unsigned char)s1[i]]++;

    for (i = 0; s2[i] != '\0'; i++)
        freq[(unsigned char)s2[i]]--;

    for (i = 0; i < CHARS; i++)
        if (freq[i] != 0)
            return 0;

    return 1;
}

int main(void)
{
    char s1[100], s2[100];

    printf("Enter first string: ");
    fgets(s1, sizeof s1, stdin);
    s1[strcspn(s1, "\n")] = '\0';

    printf("Enter second string: ");
    fgets(s2, sizeof s2, stdin);
    s2[strcspn(s2, "\n")] = '\0';

    if (isAnagram(s1, s2))
        printf("\"%s\" and \"%s\" are anagrams.\n", s1, s2);
    else
        printf("\"%s\" and \"%s\" are not anagrams.\n", s1, s2);

    return 0;
}

How to Compile and Run

gcc -ansi -Wall -Wextra -o anagram anagram.c
./anagram

Sample Input and Output

Enter first string: listen
Enter second string: silent
"listen" and "silent" are anagrams.

Enter first string: hello
Enter second string: world
"hello" and "world" are not anagrams.

Code Explanation

• freq[CHARS]: One integer per ASCII value (0–255). The cast (unsigned char) prevents undefined behavior on platforms where char is signed and the character value is negative.
• Two-pass approach: Increment on the first string, decrement on the second. This avoids the need for a second array and handles length mismatches automatically — a longer string will leave non-zero entries.
• fgets + strcspn: Safe input reading. fgets limits input to buffer size; strcspn(s, "\n") strips the trailing newline that fgets leaves in the buffer.
• Final loop: If any freq[i] != 0, the strings differ in at least one character — not anagrams.

Time and Space Complexity

This is optimal: you must read every character at least once (Ω(n)), and the space is constant regardless of input length.

What This Program Teaches

• Using a frequency/histogram array to compare character distributions
• Safe string input with fgets() instead of the dangerous gets()
• The (unsigned char) cast pattern — important when indexing arrays by character value
• Why single-array increment/decrement is cleaner than two separate count arrays

Related Programs

• C Program to Sort a String
• C Program to Find All Permutations of a String
• C Program to Count Occurrences of a Word in a String
• C Program to Reverse a String Using Pointers

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Further Reading

Strings, arrays, and standard library functions like strcspn are all covered thoroughly in The C Programming Language by K&R — the definitive reference for C. Available on Amazon.com as well.