C: storing substrings in an array of strings

Question

I have a long string and I want to take specific substrings of it and store it into an array of strings. I tried using malloc() with memcpy() but it doesn't seem to work. how would I go about doing this? code:

for(i = 0; i < strlen(p); i++){
        if(p[i] == ':'){
            cnt++;
            end = i - start;
            list[i] = malloc(1000);
            memcpy( list[i], &p[start], end );
            list[i][end] = '\0';
            //printf("%s\n", list[i]);
            start = i + 1;
        }
    }

What are you trying to do? Do you want to split the string on colons? You use the index i for both the char buffer and the substrings, which is certainly wrong. If you know the length of your substring, there's no need to guess the allocated memory to 1000; use malloc(end + 1). Finally, do you really need copies? There might be an easier way to do what you want, for example strtok. — M Oehm
– M Oehm, Commented Mar 22, 2015 at 20:48
I assume you mean list[count] = malloc(1000), didn't you? The countth substring? Same with all i indices in the loop. — Peter - Reinstate Monica
– Peter - Reinstate Monica, Commented Mar 22, 2015 at 20:48

user3629249 · Accepted Answer · 2015-03-22 21:17:18Z

1

int len   = 0;
int start = 0;
int cnt   = 0;

for( = 0; i < strlen(p); i++)
{       
    if(p[i] == ':')
    { // then, found delimeter
        len = i - start+1;
        list[cnt] = malloc(len);
        memset( list[cnt],'\0', len );
        memcpy( list[cnt], &p[start], len );

        //printf("%s\n", list[i]);
        start = i + 1;
        cnt++;
    } // end if
} // end for

answered Mar 22, 2015 at 21:17

user3629249

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Scis · Accepted Answer · 2015-03-22 21:16:05Z

1

One solution could be to use strtok, strncpy, realloc functions. For example:

int main(void){
    char str[] = "asbd0:sdsd1:ssdwdwd2";
    int tokens = 0;
    int i = 0;
    char **res = NULL;

    char *token = strtok(str, ":");
    for (tokens = 1; token; ++tokens) {
        res = realloc(res, tokens *sizeof(*res));       
        res[tokens - 1] = malloc(strlen(token) + 1);
        strncpy(res[tokens - 1], token, strlen(token) + 1);
        token = strtok(NULL, ":");
    }

    for (i = 0; i<tokens-1; ++i){
        puts(res[i]);
    }
    return 0;
}

This way we expand our "array of strings" as needed every time we find another : delimited string.

(coliru)

answered Mar 22, 2015 at 21:16

Scis

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user4710450 · Accepted Answer · 2015-03-27 09:50:38Z

I suggest you to use strtok or better strtok_r. Use them like this.

char *str = "Hello you new guy";
char *temp;
temp = strtok(str," ");
while(temp!=NULL) {
    printf("%s\n",temp);
    temp = strtok(NULL," ");
}

Be careful!strtok is destructive. Use strtok_r instead :

Just like strtok, this function splits the string into several tokens which can be accessed by successive calls to strtok_r. The difference is that the information about the next token is stored in the space pointed to by the third argument, save_ptr, which is a pointer to a string pointer. Calling strtok_r with a null pointer for newstring and leaving save_ptr between the calls unchanged does the job without hindering reentrancy.

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