prepend char to a string

schneidz · 05-25-2005, 03:02 PM

i have this array:
char* col1[500].
i want to put a char in front of it.

i guess the easiest way is to use strcat(). what's the quickest way to reverse the array? so i could stick a '0' at the end of it and reverse it back?
i'm doing this in c.

thanks,

jim mcnamara · 05-25-2005, 03:19 PM

Some C libraries have strrev().

This reverses a char array

Code:

char *str_rev(char *s){
    char *p=s;
    char *q =s;
    char swap;
    if (*s)
    {
        q=strchr(q,'\0');
        while (--q > p)
        {
            swap = *q;
            *q = *p;
            *p = swap;
            ++p;
        }
    }
    return s;
}

towlie · 05-25-2005, 03:20 PM

You know that ahead of time?

Obviously you could just start building it at [1], but you
probably know that (i.e. if you're getting the array from
somewhere else):

Code:

char final[501];
final[0]= '0';
myFunc( &final[1] );

Quickest way to reverse is to just start at last index and
decrement your count. If you want a completely new
array that is reversed, just allocate another and start
from end and work your way to the beginning.

Allocating another 512 bytes isn't a big deal is it?

EDIT: fixed error in code

schneidz · 05-25-2005, 03:31 PM

thanks guys i'll work on it. unfortunately my aix build doesn't have strrev (thanks for posting it).
i'll start working on your suggestions.

eddiebaby1023 · 05-29-2005, 08:12 PM

You could use memmove(3) to shift the array more efficiently.

kryptonite0110 · 05-30-2005, 12:25 PM

wouldn't it be quicker just to shift the entire string to the right on byte and then sticking a char at the front? eg:

void put_front(char *str, char in)
{
for (size_t i = strlen(str) - 1; i != 1; i--)
{
str[i] = str[i - 1];
}
str[0] = in;
}

just a thought...

eddiebaby1023 · 06-01-2005, 08:53 AM

Quote:

Originally posted by kryptonite0110
[B]wouldn't it be quicker just to shift the entire string to the right on byte and then sticking a char at the front? eg:

That's what I was (implicitly) suggesting in my memmove() post above, move the array and load the character at the start.