C: How to append/concatenate 'x' spaces to a string

Question

I want to add a variable number of spaces to a string in C, and wanted to know if there is a standard way to do it, before I implement it by myself.

Until now I used some ugly ways to do it:

Please assume that before I called any of the below functions, I took care to have enough memory allocated for the spaces I want to concatenate

This is one way I used:

add_spaces(char *dest, int num_of_spaces) {
    int i;
    for (i = 0 ; i < num_of_spaces ; i++) {
        strcat(dest, " ");
    }
}

This one is a better in performance, but also doesn't look standard:

add_spaces(char *dest, int num_of_spaces) {
    int i;
    int len = strlen(dest);
    for (i = 0 ; i < num_of_spaces ; i++) {
        dest[len + i] = ' ';
    }
    dest[len + num_of_spaces] = '\0';
}

So, do you have any standard solution for me, so I don't reinvent the wheel?

Have you looked at memset? I just ask you to take care with buffer overflows. — oblitum, Feb 18 '14 at 13:53
Take a look [here](http://stackoverflow.com/questions/2488563/strcat-implementation). All tries can convert to straightforward algorithms and you can boost the them performance with assumptions and optimizations. The main issue is too grant `dest` enough space to receive new characters. — wesley.mesquita, Feb 18 '14 at 14:00
I assume that there is enough spaces for the characters in the destination. The question is whether there is a standard way to generate such string of spaces in one command — SomethingSomething, Feb 18 '14 at 14:05
@user3322273 you first init your `dest` array by `memset` like `memset( dest, ' ', sizeof(dest));`. So after that you not even need to add space or your implementation also optimized. — Jayesh Bhoi, Feb 18 '14 at 14:13

Ingo Leonhardt · Accepted Answer · 2014-02-18T13:59:58.170

10

I would do

add_spaces(char *dest, int num_of_spaces) {
    int len = strlen(dest);
    memset( dest+len, ' ', num_of_spaces );   
    dest[len + num_of_spaces] = '\0';
}

But as @self stated, a function that also gets the maximum size of dest (including the '\0' in that example) is safer:

add_spaces(char *dest, int size, int num_of_spaces) {
    int len = strlen(dest);
    // for the check i still assume dest tto contain a valid '\0' terminated string, so len will be smaller than size
    if( len + num_of_spaces >= size ) {
        num_of_spaces = size - len - 1;
    }  
    memset( dest+len, ' ', num_of_spaces );   
    dest[len + num_of_spaces] = '\0';
}

edited Feb 18 '14 at 13:59

answered Feb 18 '14 at 13:52

Ingo Leonhardt

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Unsafe, total size of dest is missing. – this Feb 18 '14 at 13:53
2

@self any version of `add_spaces()` that only takes these two arguments depends on the caller to check for appropriate sizes. I will add a solutiion that also taks the maximum size. – Ingo Leonhardt Feb 18 '14 at 13:55

score 4 · Answer 2 · answered Feb 18 '14 at 14:17

4

void add_spaces(char *dest, int num_of_spaces) {
    sprintf(dest, "%s%*s", dest, num_of_spaces, "");
}

answered Feb 18 '14 at 14:17

BLUEPIXY

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could you comment on what `%*s` does? – Nathan Fellman Feb 18 '14 at 14:30
@NathanFellman Take a width that is specified in the argument. – BLUEPIXY Feb 18 '14 at 14:31
2

It may be undefined behavior this is in violation of the restrict of a C99 to be exact, but probably okay. – BLUEPIXY Feb 18 '14 at 14:34
Is there also a way to do it with any other character? E.g. I want to create a string of 'x' dashes in one command. – SomethingSomething Feb 18 '14 at 15:47
@user3322273 Function to do it in standard function probably does not exist. – BLUEPIXY Feb 18 '14 at 16:02

score 1 · Answer 3 · answered Feb 18 '14 at 14:20

1

Please assume that before I called any of the below functions, I took care to have enough memory allocated for the spaces I want to concatenate

So in main suppose you declared your array like char dest[100] then initialize your string with speces.

like

char dest[100];
memset( dest,' ',sizeof(dest));

So you not need even add_spaces(char *dest, int num_of_spaces).

answered Feb 18 '14 at 14:20

Jayesh Bhoi

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A `0` set after the spaces would be nice. Also, what does your answer bring that isn't in the answer from @Ingo above, posted 28 min before yours? – Déjà vu Feb 18 '14 at 14:25

C: How to append/concatenate 'x' spaces to a string

3 Answers3