Compare String C: A Comprehensive Guide to strcmp() in C

In the realm of C programming, string manipulation is a fundamental aspect. Among the plethora of string functions available, strcmp() stands out as a crucial tool for Compare String C. This function, part of the standard C library, allows developers to lexicographically compare two strings, determining their order and equality. This article delves deep into the workings of strcmp(), exploring its syntax, functionality, and practical applications with illustrative examples, aiming to provide a comprehensive understanding for both novice and experienced programmers.

Understanding the Syntax of strcmp()

The strcmp() function is declared within the <string.h> header file in C. To utilize it, you must include this header in your C program. The syntax is straightforward:

strcmp(s1, s2);

Parameters:

• s1: A pointer to the first string (character array) to be compared.
• s2: A pointer to the second string (character array) to be compared.

Return Value:

The strcmp() function returns an integer value based on the lexicographical comparison of the two input strings. The return value can be one of the following:

• 0 (Zero): This indicates that both strings s1 and s2 are identical. Every character at each position in both strings is the same, and they are of the same length.
• Greater than 0 (Positive Value): This signifies that the first string s1 is lexicographically greater than the second string s2. This occurs when the first differing character in s1 has a higher ASCII value than the corresponding character in s2, or if s1 is a longer string and s2 is a prefix of s1.
• Less than 0 (Negative Value): This indicates that the first string s1 is lexicographically smaller than the second string s2. This happens when the first differing character in s1 has a lower ASCII value than the corresponding character in s2, or if s2 is a longer string and s1 is a prefix of s2.

How strcmp() Function Works: Lexicographical Comparison

The core functionality of strcmp() lies in its ability to perform a lexicographical comparison. This means it compares strings character by character based on their ASCII values. The process unfolds as follows:

1. Character-by-Character Comparison: strcmp() starts by comparing the first character of both strings (s1[0] and s2[0]). It then proceeds to compare the subsequent characters (s1[1] and s2[1], and so on).

2. ASCII Value Determination: For each character pair, strcmp() examines their ASCII values. ASCII (American Standard Code for Information Interchange) assigns numerical values to characters. For instance, ‘A’ is 65, ‘a’ is 97, ‘0’ is 48, and so forth.

3. Comparison until Difference or Null Terminator: The comparison continues until one of the following conditions is met:

   • A Mismatch is Found: If strcmp() encounters a pair of characters at the same position with different ASCII values, the comparison stops. The function then determines the return value based on which character has a higher ASCII value.
   • Null Terminator is Reached: Strings in C are null-terminated, meaning they end with a special character (null character). If strcmp() reaches the null terminator in both strings simultaneously without finding any mismatches, it means the strings are identical, and it returns 0.

4. Determining the Return Value:

   • Strings are Equal: If the loop completes because both strings reached the null terminator at the same time without mismatches, strcmp() returns 0.

   • s1 is Lexicographically Greater: If a mismatch is found, and the character in s1 has a higher ASCII value, strcmp() returns a positive value (specifically, the difference in ASCII values). Or if s1 is longer and starts with s2.

   • s1 is Lexicographically Smaller: If a mismatch is found, and the character in s1 has a lower ASCII value, strcmp() returns a negative value (specifically, the difference in ASCII values). Or if s2 is longer and starts with s1.

In the scenario depicted above, “Geeks” is compared with “Geeks”. strcmp() iterates through each character, finding them identical until the null terminator. Consequently, it accurately concludes that the strings are equal, returning 0.

Here, “zebra” is compared to “apple”. The comparison begins, and the first characters ‘z’ and ‘a’ are evaluated. Since ‘z’ has a higher ASCII value than ‘a’, strcmp() immediately determines that “zebra” is lexicographically greater and returns a positive value.

In this case, “apple” is compared to “zebra”. Again, the first characters ‘a’ and ‘z’ are compared. ‘a’ has a lower ASCII value than ‘z’, so strcmp() concludes that “apple” is lexicographically smaller and returns a negative value.

Practical Examples of strcmp() in C

Let’s explore several code examples to illustrate the usage and behavior of strcmp() in different scenarios.

Example 1: Identifying Identical Strings

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

int main() {
    char s1[] = "programming";
    char s2[] = "programming";
    int result = strcmp(s1, s2);

    if (result == 0) {
        printf("Strings are Equaln");
    } else {
        printf("Strings are Unequaln");
    }
    return 0;
}

Output:

Strings are Equal

Explanation: This code snippet compares two strings, s1 and s2, both initialized to “programming”. strcmp() correctly identifies them as identical and returns 0. The if condition then evaluates to true, printing “Strings are Equal”.

Example 2: Finding Lexicographically Greater String

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

int main() {
    char str1[] = "banana";
    char str2[] = "apple";
    int res = strcmp(str1, str2);

    if (res == 0) {
        printf("Equaln");
    } else if (res > 0) {
        printf("'%s' is lexicographically greater than '%s'n", str1, str2);
    } else {
        printf("'%s' is lexicographically smaller than '%s'n", str1, str2);
    }
    return 0;
}

Output:

'banana' is lexicographically greater than 'apple'

Explanation: Here, str1 is “banana” and str2 is “apple”. When strcmp() compares them, it finds that ‘b’ in “banana” has a higher ASCII value than ‘a’ in “apple”. Therefore, strcmp() returns a positive value, and the program correctly determines that “banana” is lexicographically greater than “apple”.

Example 3: Sorting an Array of Strings

strcmp() is invaluable when sorting arrays of strings. It can serve as a comparison function for sorting algorithms like qsort().

#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 *stringArray[] = {"zebra", "orange", "apple", "grape"};
    int n = sizeof(stringArray) / sizeof(stringArray[0]);

    qsort(stringArray, n, sizeof(stringArray[0]), compareStrings);

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

Output:

Sorted Array of Strings:
apple
grape
orange
zebra

Explanation: This example demonstrates sorting an array of strings using qsort() and strcmp(). The compareStrings function acts as a comparator, using strcmp() to compare two strings. qsort() then utilizes this comparator to sort the stringArray lexicographically, resulting in the output shown.

FAQs about strcmp() in C

Q1: When does strcmp() return zero?

strcmp() returns zero when the two strings being compared are exactly identical. This means they have the same sequence of characters and the same length.

Q2: What does a positive return value from strcmp() signify?

A positive return value from strcmp() indicates that the first string is lexicographically greater than the second string. This implies that in a dictionary or sorted list, the first string would appear after the second string.

Q3: What does a negative return value from strcmp() mean?

A negative return value from strcmp() means that the first string is lexicographically smaller than the second string. In a dictionary or sorted list, the first string would come before the second string.

Q4: Can strcmp() be used to compare non-string data types in C?

No, strcmp() is specifically designed to compare null-terminated strings (character arrays) in C. It cannot be directly used to compare other data types like integers, floats, or structures. For comparing other data types, you would use different comparison operators or functions.

Conclusion: Mastering String Comparison in C with strcmp()

The strcmp() function is an indispensable tool for any C programmer working with strings. Its ability to efficiently and accurately compare string c lexicographically is crucial for various tasks, including string sorting, searching, and data validation. By understanding its syntax, working mechanism, and return values, as demonstrated through the examples in this guide, you can effectively leverage strcmp() in your C programs to handle string comparisons with confidence.