How to Compare Strings in C: A Comprehensive Guide

In C programming, strings are fundamental data types used to represent text. Comparing strings is a common operation in various applications, from sorting lists of names to validating user input. C provides the strcmp() function, a powerful tool for lexicographically comparing two strings. This article will delve into the strcmp() function, explaining its syntax, how it works, and illustrating its usage with practical examples. Whether you are a beginner learning C or an experienced developer, understanding strcmp() is crucial for effective string manipulation.

Understanding the Syntax of strcmp()

The strcmp() function is part of the standard C library and is declared in the <string.h> header file. Before using strcmp(), ensure you include this header in your C program.

#include <string.h>

The syntax of the strcmp() function is as follows:

strcmp(s1, s2);

Parameters:

• s1: A pointer to the first string to be compared. This is typically a char* or a character array.
• s2: A pointer to the second string to be compared, also a char* or a character array.

Return Value:

The strcmp() function returns an integer value based on the lexicographical comparison of the two input strings:

• 0 (Zero): Returned if the string s1 is identical to the string s2. This means both strings have the same sequence of characters.
• Greater than 0 (Positive): Returned if the string s1 is lexicographically greater than the string s2. This indicates that s1 would come after s2 in dictionary order.
• Less than 0 (Negative): Returned if the string s1 is lexicographically less than the string s2. This indicates that s1 would come before s2 in dictionary order.

How strcmp() Function Works: Lexicographical Comparison

The strcmp() function performs a lexicographical comparison, which means it compares strings character by character based on their ASCII values. The comparison process can be broken down into these steps:

1. Character-by-Character Comparison: strcmp() starts by comparing the first character of s1 with the first character of s2.

2. ASCII Value Evaluation: It compares the ASCII values of these characters.

3. Continuing Comparison:

   • If the characters are the same, strcmp() proceeds to compare the next characters in both strings.
   • This process continues until one of the following conditions is met:
     • Characters Differ: If strcmp() encounters characters at the same position in s1 and s2 that have different ASCII values, the comparison stops. The function then determines the return value based on which character has a higher ASCII value.
     • Null Terminator Reached: If strcmp() reaches the null terminator () in both strings simultaneously, it means the strings are identical, and the function returns 0.

4. Determining the Return Value:

   • Strings are Equal: If all characters are the same up to the null terminator, strcmp() returns 0.
   • s1 is Lexicographically Greater: If strcmp() finds a character in s1 that has a higher ASCII value than the corresponding character in s2, or if s1 is longer and matches s2 up to the length of s2, it returns a positive value.
   • s1 is Lexicographically Smaller: If strcmp() finds a character in s1 that has a lower ASCII value than the corresponding character in s2, or if s1 is shorter and is a prefix of s2, it returns a negative value.

Practical Examples of strcmp() in C

Let’s explore several examples to understand how strcmp() works in different scenarios.

Example 1: Comparing Identical Strings

This example demonstrates comparing two strings that are exactly the same.

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

int main() {
    char str1[] = "Compare";
    char str2[] = "Compare";
    int result = strcmp(str1, str2);

    if (result == 0) {
        printf("Strings are identical.n");
    } else {
        printf("Strings are not identical.n");
    }
    return 0;
}

Output:

Strings are identical.

Explanation: strcmp() compares “Compare” and “Compare”. Since they are identical, it returns 0, and the program prints “Strings are identical.”

Example 2: Finding Lexicographically Greater String

In this example, we compare two strings where the first string is lexicographically greater than the second.

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

int main() {
    char string1[] = "Orange";
    char string2[] = "Apple";
    int result = strcmp(string1, string2);

    if (result > 0) {
        printf(""%s" is lexicographically greater than "%s".n", string1, string2);
    } else {
        printf(""%s" is not lexicographically greater than "%s".n", string1, string2);
    }
    return 0;
}

Output:

"Orange" is lexicographically greater than "Apple".

Explanation: strcmp() compares “Orange” and “Apple”. ‘O’ has a higher ASCII value than ‘A’, so strcmp() returns a positive value. The program correctly identifies “Orange” as lexicographically greater.

Example 3: Finding Lexicographically Smaller String

This example shows a case where the first string is lexicographically smaller than the second string.

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

int main() {
    char first_string[] = "Banana";
    char second_string[] = "Cherry";
    int result = strcmp(first_string, second_string);

    if (result < 0) {
        printf(""%s" is lexicographically smaller than "%s".n", first_string, second_string);
    } else {
        printf(""%s" is not lexicographically smaller than "%s".n", first_string, second_string);
    }
    return 0;
}

Output:

"Banana" is lexicographically smaller than "Cherry".

Explanation: strcmp() compares “Banana” and “Cherry”. ‘B’ has a lower ASCII value than ‘C’, so strcmp() returns a negative value, indicating “Banana” is lexicographically smaller.

Example 4: Using strcmp() to Sort an Array of Strings

strcmp() is particularly useful as a comparison function for sorting strings. Here’s how you can use it with qsort() to sort an array of strings alphabetically.

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

int compareStrings(const void *a, const void *b) {
    return strcmp(*(const char **)a, *(const char **)b);
}

int main() {
    const char *names[] = {"Charlie", "Alice", "Bob", "David"};
    int n = sizeof(names) / sizeof(names[0]);

    qsort((void *)names, n, sizeof(names[0]), compareStrings);

    printf("Sorted names:n");
    for (int i = 0; i < n; i++) {
        printf("%sn", names[i]);
    }
    return 0;
}

Output:

Sorted names:
Alice
Bob
Charlie
David

Explanation:

• compareStrings function uses strcmp() to compare two strings, which is required by qsort.
• qsort() sorts the names array using compareStrings as the comparison function.
• The output shows the strings sorted in lexicographical (alphabetical) order.

Key Considerations when Using strcmp()

• Case Sensitivity: strcmp() is case-sensitive. “Apple” and “apple” are considered different strings. If you need case-insensitive comparison, consider using strcasecmp() (non-standard, but available in many systems) or converting strings to the same case before comparison.
• Binary Comparison: strcmp() performs a binary comparison based on ASCII values. This might not align with dictionary order in all languages, especially those with accented characters or different sorting rules. For locale-aware string comparison, explore functions like strcoll().
• Null Termination: strcmp() relies on null-terminated strings. Ensure your strings are properly null-terminated; otherwise, strcmp() might read beyond the intended string boundaries, leading to undefined behavior.

C strcmp() – Frequently Asked Questions

When does strcmp() return zero?

strcmp() returns zero when the two strings being compared are exactly identical, character by character, including case.

What does a positive return value from strcmp() signify?

A positive return value from strcmp(s1, s2) means that s1 is lexicographically greater than s2. In dictionary order, s1 would appear after s2.

What does a negative return value from strcmp() indicate?

A negative return value from strcmp(s1, s2) means that s1 is lexicographically smaller than s2. In dictionary order, s1 would appear before s2.

Can strcmp() be used to compare parts of strings?

No, strcmp() compares entire strings from the beginning to the null terminator. To compare substrings, you would need to use functions like strncmp() which allows you to specify the number of characters to compare.

Is strcmp() efficient for very long strings?

strcmp() is generally efficient for most use cases. However, for extremely long strings, the character-by-character comparison might become a performance bottleneck in very performance-critical applications. In such scenarios, consider if there are alternative approaches or if optimization is genuinely necessary.

Conclusion

The strcmp() function is an essential tool in C for comparing strings. Understanding its lexicographical comparison mechanism, return values, and usage, as demonstrated in the examples, will enable you to effectively work with strings in your C programs. From simple string equality checks to complex sorting routines, strcmp() is a fundamental function in C string manipulation. By mastering strcmp(), you gain a solid foundation for more advanced string processing tasks in C programming.