C Program to Compare Two Strings Without Using Library Function

Introduction

Comparing two strings means checking whether the strings are identical (i.e., they contain the same sequence of characters) or determining their lexicographical order. This guide will show you how to write a C program to compare two strings without using any library functions like strcmp.

Problem Statement

Create a C program that:

• Takes two strings as input from the user.
• Compares the two strings manually character by character.
• Displays whether the strings are identical or which one is lexicographically greater.

Example:

• Input:

  • String1 = "Hello"
  • String2 = "Hello"

• Output: "Strings are identical."

• Input:

  • String1 = "Apple"
  • String2 = "Banana"

• Output: "String1 is less than String2."

Solution Steps

1. Include the Standard Input-Output Library: Use #include <stdio.h> for standard input-output functions.
2. Write the Main Function: Define the main function, which is the entry point of every C program.
3. Declare Variables: Declare variables to store the two input strings and an integer variable for comparison.
4. Input the Strings: Use gets or scanf to take input from the user for both strings.
5. Compare the Strings Manually: Use a loop to iterate through the characters of the two strings and compare them one by one.
6. Display the Result: Use printf to display the result of the comparison.

C Program to Compare Two Strings Without Using Library Function

#include <stdio.h>

int main() {
    // Step 1: Declare variables to hold the two input strings
    char str1[100], str2[100];
    int i, result = 0;

    // Step 2: Prompt the user to enter the first string
    printf("Enter the first string: ");
    gets(str1);  // Using gets to read the string including spaces

    // Step 3: Prompt the user to enter the second string
    printf("Enter the second string: ");
    gets(str2);  // Using gets to read the string including spaces

    // Step 4: Manually compare the strings character by character
    for (i = 0; str1[i] != '\0' && str2[i] != '\0'; i++) {
        if (str1[i] != str2[i]) {
            result = str1[i] - str2[i];
            break;
        }
    }

    // If strings are identical till now but one string has more characters
    if (result == 0 && (str1[i] != '\0' || str2[i] != '\0')) {
        result = str1[i] - str2[i];
    }

    // Step 5: Display the result
    if (result == 0) {
        printf("Strings are identical.\n");
    } else if (result < 0) {
        printf("String1 is less than String2.\n");
    } else {
        printf("String1 is greater than String2.\n");
    }

    return 0;  // Step 6: Return 0 to indicate successful execution
}

Explanation

Step 1: Declare Variables

• The str1 and str2 arrays are declared to store the input strings. The result variable is used to store the comparison result, and i is used as a loop counter.

Step 2: Input the Strings

• The program prompts the user to enter the first and second strings using printf. The gets function is used to read the strings, allowing them to include spaces.

Step 3: Manually Compare the Strings

• The program uses a for loop to iterate through each character of the two strings:
  • If a mismatch is found, the comparison stops, and the difference between the characters determines the result.
  • If no mismatch is found, but the strings are of different lengths, the longer string is considered greater.

Step 4: Display the Result

• The program checks the value of result and displays whether the strings are identical, or which one is lexicographically greater using the printf function.

Step 5: Return 0

• The return 0; statement indicates that the program executed successfully.

Output Example

Example 1:

Enter the first string: Hello
Enter the second string: Hello
Strings are identical.

Example 2:

Enter the first string: Apple
Enter the second string: Banana
String1 is less than String2.

Example 3:

Enter the first string: Cat
Enter the second string: Car
String1 is greater than String2.

Conclusion

This C program demonstrates how to compare two strings manually by iterating through each character and comparing them. It covers basic concepts such as string manipulation, loops, and conditional statements, making it a useful example for beginners learning C programming.