Write a Program to Copy One String to Another in C

Write a Program to Copy One String to Another

Strings are an integral part of programming, used to handle text data. In C, copying one string to another is a common operation, especially when dealing with text manipulation and processing. In this blog post, we’ll guide you on how to write a C program to copy one string into another string, using a manual approach and also using a standard library function. By the end of this post, you'll have a better understanding of how strings work in C and how to implement this operation effectively.

1. Introduction to Strings in C

In C, strings are represented as arrays of characters, terminated by a null character (`'\0'`). Since C doesn’t provide a built-in string type, we rely on arrays and pointers for string operations. Copying one string to another means transferring all characters, including the null terminator, from the source string to the destination string.

2. Why Copying Strings is Important

Copying strings has many practical uses:

• Data Duplication: Sometimes, we need to create an exact copy of a string for backup purposes or for further manipulation.
• Manipulating Text: Many text processing algorithms require copying strings to temporary variables before making changes.
• Dynamic Memory Allocation: When dealing with dynamically allocated memory, string copying becomes essential for memory management.

3. Writing the Program

Let’s implement a simple C program that copies one string to another.

Code Example: Copying Strings Manually

#include <stdio.h>

void copyString(char source[], char destination[]) {
    int i = 0;
    
    // Copy each character from source to destination
    while (source[i] != '\0') {
        destination[i] = source[i];
        i++;
    }
    // Add null terminator to the end of the destination string
    destination[i] = '\0';
}

int main() {
    char source[100], destination[100];
    
    // Prompt user to enter the source string
    printf("Enter a string: ");
    fgets(source, sizeof(source), stdin);

    // Copy the string
    copyString(source, destination);

    // Output the copied string
    printf("Copied string: %s\n", destination);

    return 0;
}

Explanation of the Code

In this program:

• We include the stdio.h header file for input and output operations.
• We define a function copyString() that takes two character arrays (the source and destination) as arguments. This function copies each character from the source string to the destination string until the null terminator (`'\0'`) is encountered.
• The while loop runs as long as the current character in the source string is not the null terminator. Inside the loop, the current character is copied from the source to the destination, and the index i is incremented.
• Once all characters are copied, we add the null terminator (`'\0'`) to the destination string to mark the end.
• In the main() function, we prompt the user to input the source string using fgets(). We then call copyString() to perform the copy operation and finally print the copied string.

4. Using the strcpy() Function

The C standard library also provides a built-in function called strcpy() to copy strings. Let’s see how to use it.

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

int main() {
    char source[100], destination[100];
    
    // Prompt user to enter the source string
    printf("Enter a string: ");
    fgets(source, sizeof(source), stdin);

    // Use strcpy() to copy the string
    strcpy(destination, source);

    // Output the copied string
    printf("Copied string: %s\n", destination);

    return 0;
}

Explanation of strcpy()

In this program, we use the standard library function strcpy() from the string.h header file to copy the string. This function works similarly to our manual copyString() function but is more efficient and commonly used in real-world programs. The rest of the program remains the same.

5. Example Output

Here’s an example of how the program works:

Enter a string: Hello, World!
Copied string: Hello, World!

As you can see, the source string "Hello, World!" is successfully copied to the destination string, and the result is displayed.

6. Common Mistakes to Avoid

When copying strings, there are a few common mistakes you should avoid:

• Array Size Mismatch: Ensure that the destination array has enough space to hold the source string, including the null terminator. Failing to allocate enough space can lead to buffer overflows.
• Forgetting the Null Terminator: Always make sure that the destination string ends with a null character (`'\0'`), as C strings are null-terminated.
• Unintended Modifications: Be careful when copying strings to avoid inadvertently modifying the source string or copying to the wrong destination.

7. Expanding the Program

Here are a few ways you can expand this program:

• Write a function that copies only part of the string, such as the first N characters, instead of the whole string.
• Modify the program to concatenate two strings after copying one of them.
• Handle cases where the user enters an empty string or spaces.

8. Conclusion

In this post, we learned how to write a C program to copy one string to another, both using a manual approach and the strcpy() function from the standard library. Copying strings is a fundamental operation in C programming, and mastering it will help you handle text manipulation efficiently.

As you continue to explore string operations in C, you’ll find that working with strings is both powerful and challenging. By practicing and expanding your programs, you will improve your problem-solving skills and become a better programmer.