RCU 21BCA1C1P · Lab Program 14

Reverse a String Without Library Functions in C

Read a line of text and reverse it in place, without using any string library function.

Algorithm

Read a line with fgets and remove the trailing newline.
Find the length by counting characters until the terminator.
Swap characters at positions i and length - 1 - i, moving i from the start towards the middle.
Print the reversed string.

C program

#include <stdio.h>

int main(void) {
    char text[200];
    int length = 0, i;
    char temp;

    printf("Enter a string:\n");
    if (fgets(text, sizeof text, stdin) == NULL) {
        printf("Error: no input\n");
        return 1;
    }

    while (text[length] != '\n' && text[length] != '\0') {
        length++;
    }
    text[length] = '\0';

    for (i = 0; i < length / 2; i++) {
        temp = text[i];
        text[i] = text[length - 1 - i];
        text[length - 1 - i] = temp;
    }

    printf("Reversed: %s\n", text);
    return 0;
}

Sample input and output

A word with an even number of letters

Input

program

Output

Enter a string:
Reversed: margorp

Edge case: a single character

Input

Output

Enter a string:
Reversed: a

How the program works

Swapping from both ends works in place, so no second array is needed. The loop runs only half the length, because each swap handles two characters.

The length is found by hand with a loop that stops at the newline or the terminator, which is exactly the job strlen does in the library.

Run this code

Edit the program and press Run. It is free and needs no sign-in.

Program input (what the program reads with scanf)

Copy a sample and paste it into the STDIN box in the editor before you press Run.

A word with an even number of letters
```
program
```
Expected output
```
Enter a string:
Reversed: margorp
```
Edge case: a single character
```
a
```
Expected output
```
Enter a string:
Reversed: a
```

Viva questions

Why does the loop run only length / 2 times?

Each iteration swaps a pair, so going past the middle would swap the pairs back.

What is the index of the last character?

length - 1, because the characters occupy positions 0 to length - 1 and the terminator follows.

Why is the terminator set after the length is found?

fgets stores the newline; overwriting it with the terminator makes the printed string end cleanly.

Why is the temporary variable of type char?

It holds one character during the swap.

What is the time complexity?

O(n): each character is visited at most once during the swaps.

Common mistakes

Looping over the whole length, which swaps each pair twice and leaves the string unchanged.
Forgetting to remove the newline, so the reversal starts with a newline and the output breaks.

Back to all RCU 21BCA1C1P lab programs.

C program

#include <stdio.h> int main(void) { char text[200]; int length = 0, i; char temp; printf("Enter a string:\n"); if (fgets(text, sizeof text, stdin) == NULL) { printf("Error: no input\n"); return 1; } while (text[length] != '\n' && text[length] != '\0') { length++; } text[length] = '\0'; for (i = 0; i < length / 2; i++) { temp = text[i]; text[i] = text[length - 1 - i]; text[length - 1 - i] = temp; } printf("Reversed: %s\n", text); return 0; }

Viva questions

Why does the loop run only length / 2 times?

Each iteration swaps a pair, so going past the middle would swap the pairs back.

What is the index of the last character?

length - 1, because the characters occupy positions 0 to length - 1 and the terminator follows.

Why is the terminator set after the length is found?

fgets stores the newline; overwriting it with the terminator makes the printed string end cleanly.

Why is the temporary variable of type char?

It holds one character during the swap.

What is the time complexity?

O(n): each character is visited at most once during the swaps.