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;
        }
    }

Related: https://stackoverflow.com/questions/26378781/c-program-to-extract-different-substrings-from-array-of-strings?rq=1 — reader, Mar 22 '15 at 20:34
You can use [algorithm library](http://www.cplusplus.com/reference/algorithm/) functions. — Abozanona, Mar 22 '15 at 20:39
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, Mar 22 '15 at 20:48
I assume you mean list[**count**] = malloc(1000), didn't you? The *count*th substring? Same with all `i` indices in the loop. — Peter - Reinstate Monica, Mar 22 '15 at 20:48
need to initialize 'start' to 0 need to initialize 'cnt' to 0 — user3629249, Mar 22 '15 at 20:56

score 1 · Answer 1 · answered Mar 22 '15 at 21:16

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)

score 1 · Accepted Answer · answered Mar 22 '15 at 21:17

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

score 0 · Answer 3 · answered Mar 27 '15 at 09:50

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.

C: storing substrings in an array of strings

3 Answers3