C strncmp() Function

The strncmp() function in C is a standard library function that compares up to a specified number of characters from two strings. It is part of the C standard library (string.h). This function is useful for comparing the beginning parts of two strings, ensuring a maximum number of characters are compared.

Table of Contents

1. Introduction
2. strncmp() Function Syntax
3. Understanding strncmp() Function
4. Examples
   • Comparing the Beginning of Two Identical Strings
   • Comparing Different Strings
5. Real-World Use Case
6. Conclusion

Introduction

The strncmp() function compares up to n characters from two null-terminated strings lexicographically. It returns an integer less than, equal to, or greater than zero if the first string is found to be less than, equal to, or greater than the second string, respectively, based on the first n characters.

strncmp() Function Syntax

The syntax for the strncmp() function is as follows:

int strncmp(const char *str1, const char *str2, size_t n);

Parameters:

• str1: A pointer to the first null-terminated string to be compared.
• str2: A pointer to the second null-terminated string to be compared.
• n: The maximum number of characters to compare.

Returns:

• The function returns an integer:
  • Less than zero if str1 is less than str2.
  • Zero if str1 is equal to str2.
  • Greater than zero if str1 is greater than str2.

Understanding strncmp() Function

The strncmp() function performs a lexicographical comparison of up to n characters of the two strings, which means it compares the strings character by character using the ASCII values of the characters. The comparison stops when a difference is found, n characters are compared, or the end of one of the strings is reached.

Examples

Comparing the Beginning of Two Identical Strings

To demonstrate how to use strncmp() to compare the beginning of two identical strings, we will write a simple program.

Example

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

int main() {
    char str1[] = "Hello, World!";
    char str2[] = "Hello, World!";

    // Compare the first 5 characters of the strings using strncmp
    int result = strncmp(str1, str2, 5);

    // Print the result
    printf("Comparison result: %d\n", result);

    return 0;
}

Output:

Comparison result: 0

Comparing Different Strings

This example shows how to use strncmp() to compare different strings.

Example

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

int main() {
    char str1[] = "Hello, World!";
    char str2[] = "Hello, Ramesh!";

    // Compare the first 7 characters of the strings using strncmp
    int result = strncmp(str1, str2, 7);

    // Print the result
    if (result < 0) {
        printf("\"%s\" is less than \"%s\" (first 7 characters)\n", str1, str2);
    } else if (result > 0) {
        printf("\"%s\" is greater than \"%s\" (first 7 characters)\n", str1, str2);
    } else {
        printf("\"%s\" is equal to \"%s\" (first 7 characters)\n", str1, str2);
    }

    return 0;
}

Output:

"Hello, World!" is greater than "Hello, Ramesh!" (first 7 characters)

Real-World Use Case

Comparing Version Numbers

In real-world applications, the strncmp() function can be used to compare version numbers or other prefix-based comparisons.

Example: Comparing Version Numbers

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

int main() {
    char version1[] = "1.2.3";
    char version2[] = "1.2.4";

    // Compare the first 3 characters of the version numbers using strncmp
    int result = strncmp(version1, version2, 3);

    // Print the result
    if (result < 0) {
        printf("Version %s is less than version %s (first 3 characters)\n", version1, version2);
    } else if (result > 0) {
        printf("Version %s is greater than version %s (first 3 characters)\n", version1, version2);
    } else {
        printf("Version %s is equal to version %s (first 3 characters)\n", version1, version2);
    }

    return 0;
}

Output:

Version 1.2.3 is equal to version 1.2.4 (first 3 characters)

Conclusion

The strncmp() function is used for comparing the first n characters of two strings in C. By understanding and using this function correctly, you can efficiently perform partial string comparisons in your programs. This is particularly helpful in applications that involve prefix-based comparisons, such as version numbers or partially matching strings. Always ensure that the strings are null-terminated to prevent undefined behavior.