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Comparing strings in C

PPooja Rao5 min read
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Comparing Strings in C

In C, strings are represented as null-terminated character arrays. Because they are not first-class data types, you cannot compare them directly using standard relational operators like == or >. Instead, string comparison is performed character by character based on their ASCII values.

When comparing two strings, s1 and s2, the comparison yields one of three outcomes:

  1. s1 is equal to s2: Every character matches exactly, including the length.
  2. s1 > s2: The ASCII value of the first unmatched character in s1 is greater than that in s2. Lexicographically (alphabetically), s1 comes after s2.
  3. s1 < s2: The ASCII value of the first unmatched character in s1 is smaller than that in s2. Lexicographically, s1 comes before s2.

In this tutorial, we will explore three different approaches to compare strings in C: using built-in library functions, using custom array indexing, and using pointers.

Approach 1: Using the Built-In strcmp() Function

The simplest and most common way to compare strings in C is by using the standard library function strcmp(), which is defined in the <string.h> header.

Syntax

int strcmp(const char *str1, const char *str2);

The strcmp() function compares two strings character by character. The comparison continues until a mismatch is found or the null terminator (\0) is encountered.

Return Values of strcmp()

| Return Value | Description | | :--- | :--- | | 0 | Both strings are identical. | | Negative Integer | The ASCII value of the first unmatched character in str1 is less than that in str2. | | Positive Integer | The ASCII value of the first unmatched character in str1 is greater than that in str2. |

Note on Case Sensitivity:
ASCII comparisons are case-sensitive. For example, the uppercase letter 'H' has an ASCII value of 72, while the lowercase letter 'h' has an ASCII value of 104. Therefore, "Hello" and "hello" are not equal.

Code Implementation

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

int main() {
    char *str1, *str2;
    size_t size = 100;

    printf("Enter first string\n");
    str1 = (char*)malloc(size);
    getline(&str1, &size, stdin);

    printf("Enter second string\n");
    str2 = (char*)malloc(size);
    getline(&str2, &size, stdin);

    if (strcmp(str1, str2) == 0) {
        printf("The strings are equal.\n");
    } else {
        printf("The strings are not equal.\n");
    }

    free(str1);
    free(str2);
    return 0;
}

Sample Outputs

Case 1: Strings are equal

Enter first string
hello!
Enter second string
hello!
The strings are equal.

Case 2: Strings are unequal (Case Sensitivity)

Enter first string
Hello!
Enter second string
hello!
The strings are not equal.

Approach 2: Custom Comparison (Without Built-In Functions)

If you cannot use <string.h>, you can implement your own comparison logic.

Algorithm

  1. Compare Lengths First: As an optimization, check if the lengths of the two strings match. If they do not, they cannot be equal.
  2. Character-by-Character Iteration: Initialize an index counter c to traverse both strings simultaneously.
  3. Check for Termination: Loop until a character mismatch is found or the null terminator (\0) is reached.
  4. Determine Result: If both strings reach the null terminator at the same index, they are equal. Otherwise, compare the mismatched characters to determine which string is lexicographically greater.

Code Implementation

#include <stdio.h>
#include <stdlib.h>

int string_compare(char s1[], char s2[]) {
    int c = 0;
    int flag = 0;

    while (s1[c] == s2[c]) {
        if (s1[c] == '\0' || s2[c] == '\0') {
            break;
        } else {
            c++;
        }
    }

    if (s1[c] == '\0' && s2[c] == '\0') {
        flag = 1;
        return 0;
    } else {
        if (flag == 0) {
            if (s1[c] > s2[c]) {
                printf("String 1 is greater than string 2\n");
            } else {
                printf("String 2 is greater than string 1\n");
            }
        }
        return -1;
    }
}

int main() {
    char *str1, *str2;
    size_t size = 100;
    ssize_t l1, l2;

    printf("Enter first string\n");
    str1 = (char*)malloc(size);
    l1 = getline(&str1, &size, stdin); // Storing number of characters read

    printf("Enter second string\n");
    str2 = (char*)malloc(size);
    l2 = getline(&str2, &size, stdin);

    // Strip the trailing newline character ('\n') added by getline
    l1--;
    l2--;

    if (l1 == l2) {
        if (string_compare(str1, str2) == 0) {
            printf("The strings are equal.\n");
        } else {
            printf("The strings are not equal.\n");
        }
    } else {
        printf("The length of strings are not equal.\n");
    }

    free(str1);
    free(str2);
    return 0;
}

Sample Outputs

Case 1: Strings are equal

Enter first string
why?
Enter second string
why?
The strings are equal.

Case 2: Strings are not equal because of length

Enter first string
where are you ?
Enter second string
where are you
The length of strings are not equal.

Case 3: Strings are not equal because of character mismatch

Enter first string
WHY
Enter second string
WHy
String 2 is greater than string 1
The strings are not equal.

Note: The lowercase 'y' in the second string has a higher ASCII value (121) than the uppercase 'Y' (89) in the first string.

Approach 3: Comparing Strings Using Pointers

Pointers offer a highly efficient and idiomatic way to traverse and compare strings in C. Instead of using array indexing (like s1[c]), we can directly dereference the pointers (*str1) and increment them (str1++) to move to the next character.

Code Implementation

#include <stdio.h>
#include <stdlib.h>

int string_compare(char *str1, char *str2) {
    while (*str1 == *str2) {
        if (*str1 == '\0' || *str2 == '\0') {
            break;
        }
        str1++;
        str2++;
    }

    if (*str1 == '\0' && *str2 == '\0') {
        return 0;
    } else {
        return -1;
    }
}

int main() {
    char *str1, *str2;
    size_t size = 100;
    ssize_t l1, l2;

    printf("Enter first string\n");
    str1 = (char*)malloc(size);
    l1 = getline(&str1, &size, stdin);

    printf("Enter second string\n");
    str2 = (char*)malloc(size);
    l2 = getline(&str2, &size, stdin);

    // Strip the trailing newline character ('\n')
    l1--;
    l2--;

    if (l1 == l2) {
        if (string_compare(str1, str2) == 0) {
            printf("The strings are equal.\n");
        } else {
            printf("The strings are not equal.\n");
        }
    } else {
        printf("The length of strings are not equal.\n");
    }

    free(str1);
    free(str2);
    return 0;
}

Sample Outputs

Case 1: Strings are equal

Enter first string
help
Enter second string
help
The strings are equal.

Case 2: Strings are not equal because of length

Enter first string
help
Enter second string
helping
The length of strings are not equal.

Case 3: Strings are not equal because of character mismatch

Enter first string
Hello!
Enter second string
Hellp!
The strings are not equal.

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