C Program to Remove Spaces from String

Use a loop to copy characters from the original string to a new position skipping spaces with code like for (int i = 0, j = 0; str[i] != '\0'; i++) if (str[i] != ' ') str[j++] = str[i]; str[j] = '\0'; to remove spaces from a string in C.

Examples

Inputhello world

Outputhelloworld

Input c programming

Outputcprogramming

Inputnospace

Outputnospace

How to Think About It

To remove spaces from a string, think of reading each character one by one. If the character is not a space, keep it. If it is a space, skip it. This way, you build a new string without spaces by overwriting the original string in place.

Algorithm

1

Get the input string.

2

Create two indexes: one for reading characters, one for writing characters.

3

Loop through each character of the string.

4

If the character is not a space, copy it to the write position and move the write index forward.

5

If the character is a space, skip it and do not move the write index.

6

After the loop, add a null character at the write index to end the string.

7

Return or print the modified string.

Code

c

#include <stdio.h>

int main() {
    char str[100];
    int i, j = 0;

    printf("Enter a string: ");
    fgets(str, sizeof(str), stdin);

    for (i = 0; str[i] != '\0'; i++) {
        if (str[i] != ' ' && str[i] != '\n') {
            str[j++] = str[i];
        }
    }
    str[j] = '\0';

    printf("String without spaces: %s\n", str);
    return 0;
}

Output

Enter a string: hello world
String without spaces: helloworld

Dry Run

Let's trace the input "hello world" through the code.

1

Initial string

str = "hello world\n"; i = 0; j = 0

2

Check character 'h'

str[0] = 'h' (not space), copy to str[0], j=1

3

Check character 'e'

str[1] = 'e' (not space), copy to str[1], j=2

4

Check character 'l'

str[2] = 'l' (not space), copy to str[2], j=3

5

Check character 'l'

str[3] = 'l' (not space), copy to str[3], j=4

6

Check character 'o'

str[4] = 'o' (not space), copy to str[4], j=5

7

Check character ' ' (space)

str[5] = ' ' (space), skip, j=5

8

Check character 'w'

str[6] = 'w' (not space), copy to str[5], j=6

9

Check character 'o'

str[7] = 'o' (not space), copy to str[6], j=7

10

Check character 'r'

str[8] = 'r' (not space), copy to str[7], j=8

11

Check character 'l'

str[9] = 'l' (not space), copy to str[8], j=9

12

Check character 'd'

str[10] = 'd' (not space), copy to str[9], j=10

13

Check character '\n'

str[11] = '\n' (newline), skip, j=10

14

End string

Add '\0' at str[10], final string: "helloworld"

i (read index)	j (write index)	Character read	Action	String state
0	0	h	copy	h
1	1	e	copy	he
2	2	l	copy	hel
3	3	l	copy	hell
4	4	o	copy	hello
5	5		skip	hello
6	5	w	copy	hellow
7	6	o	copy	hellowo
8	7	r	copy	hellowor
9	8	l	copy	helloworl
10	9	d	copy	helloworld
11	10	\n	skip	helloworld

Why This Works

Step 1: Loop through each character

The program reads each character of the string one by one using a loop with index i.

Step 2: Check for spaces

If the character is not a space or newline, it is copied to the position indicated by index j.

Step 3: Overwrite spaces

Spaces are skipped, so the write index j only moves when a non-space character is copied, effectively removing spaces.

Step 4: Terminate the string

After copying, a null character '\0' is added to mark the end of the new string without spaces.

Alternative Approaches

Using a separate output array

c

#include <stdio.h>

int main() {
    char str[100], result[100];
    int i = 0, j = 0;

    printf("Enter a string: ");
    fgets(str, sizeof(str), stdin);

    while (str[i] != '\0') {
        if (str[i] != ' ' && str[i] != '\n') {
            result[j++] = str[i];
        }
        i++;
    }
    result[j] = '\0';

    printf("String without spaces: %s\n", result);
    return 0;
}

This method uses extra memory for a new string but keeps the original string unchanged.

Using standard library function to remove spaces (legacy)

c

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

int main() {
    char str[100];
    int i = 0, j = 0;

    printf("Enter a string: ");
    fgets(str, sizeof(str), stdin);

    for (i = 0; str[i]; i++) {
        if (str[i] != ' ' && str[i] != '\n') {
            str[j++] = str[i];
        }
    }
    str[j] = '\0';

    printf("String without spaces: %s\n", str);
    return 0;
}

This is similar to the main approach but uses <code>strlen</code> or direct loop condition; it is simple and efficient.

Complexity: O(n) time, O(1) space

Time Complexity

The program loops through each character once, so time grows linearly with string length.

Space Complexity

The removal is done in place using the same string, so no extra space is needed beyond variables.

Which Approach is Fastest?

In-place removal is fastest and uses least memory; using a separate array uses more space but keeps original string intact.

Approach	Time	Space	Best For
In-place removal	O(n)	O(1)	Memory efficient, fast
Separate output array	O(n)	O(n)	Preserving original string
Legacy loop with strlen	O(n)	O(1)	Simple and clear code