RCU 21BCA1C1P · Lab Program 15

Length of a String Without strlen in C

Read a line of text and find its length by counting characters, without calling strlen.

Algorithm

Read a line with fgets.
Start a counter at 0.
Increase the counter for every character until the newline or the terminator is reached.
Print the counter as the length.

C program

#include <stdio.h>

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

    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++;
    }

    printf("Length: %d\n", length);
    return 0;
}

Sample input and output

A phrase with a space

Input

Hello World

Output

Enter a string:
Length: 11

Edge case: an empty line

Input

(no input)

Output

Enter a string:
Length: 0

How the program works

The length is the number of characters before the terminator. The newline that fgets keeps is not part of the text, so the loop stops at it as well.

Spaces are counted, because they are characters of the string. Only the newline and the terminator end the count.

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 phrase with a space
```
Hello World
```
Expected output
```
Enter a string:
Length: 11
```
Edge case: an empty line
Expected output
```
Enter a string:
Length: 0
```

Viva questions

What does strlen return for "Hello World"?

11; the space counts as a character.

Why does the loop check for the newline as well as the terminator?

fgets keeps the newline at the end of the line, which is not part of the string's text.

Is the terminator counted in strlen?

No. strlen counts characters before the '\0', so the terminator is excluded.

What is the buffer size needed for this program?

Any line up to 199 characters plus the terminator, since the buffer holds 200 bytes.

What is the time complexity?

O(n) in the length of the string.

Common mistakes

Counting the newline, which adds one to every length read with fgets.
Stopping at the first space, which would count only the first word.

Back to all RCU 21BCA1C1P lab programs.

C program

#include <stdio.h> int main(void) { char text[200]; int length = 0; 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++; } printf("Length: %d\n", length); return 0; }

Viva questions

What does strlen return for "Hello World"?

11; the space counts as a character.

Why does the loop check for the newline as well as the terminator?

fgets keeps the newline at the end of the line, which is not part of the string's text.

Is the terminator counted in strlen?

No. strlen counts characters before the '\0', so the terminator is excluded.

What is the buffer size needed for this program?

Any line up to 199 characters plus the terminator, since the buffer holds 200 bytes.

What is the time complexity?

O(n) in the length of the string.