Question

I’m new to C and am having trouble with strings. What I would like to do is create a string like “val1, val2, val3” in a loop.

Currently my code looks something like:

char tagstr[60] = "";
int k;
int n = 5;
for (k=0; k < n; k++) {
    char temp[10]  = "";
    sprintf(temp, ", val%d", k);
    strcat(tagstr, temp);
}

But the output of tagstr is “, val#”, where # is some long integer value. I’m guessing I’m doing something wrong with pointers here but I’ve tried everything I can think of without success… any help would be much appreciated.

EDIT: more context, if it helps:

int tagsClosed = strlen(pch1) - strcspn(pch1, ")");
do {
    if (curTag.size > 0) {
        // problem section
        char tagstr[60] = "";
        int k;
        for (k = 0; k < 5; k++) {
            char temp[10] = "";
            sprintf(temp, ", val%i", temp, k);
            strcat(tagstr, temp);
        }

        // This prints out something like ", val-890132840" x 5 (same number)
        printf ("String is now: %s\n", tagstr);
    }
    curTag = *(curTag.parent);
    tagsClosed--;
} while (tagsClosed > 0);

curTag is a struct:

typedef struct Tag {
    char * name;
    int size; // number of children
    int tagnum;
    struct Tag* parent;
} Tag;

Answer 1

The problem is that sprintf(temp, ", val%i", temp, k); adds the value of temp (which is actually the address of the first character in the array) to the string, and doesn’t add the value of k to the string at all. This should be sprintf(temp, ", val%i", k);.

You can calculate the amount of space you’d need in advance (including zero terminator):

5+1 + 5+1 + 5+1 + 5+1 + 5+1 + 1 = 31 characters

Also; using strcat is bad (for performance) because you’d be repeatedly searching for the end of the tagstr and then copying the new characters to the end. It would be better to keep track of the current end of tagstr and store then next group of characters directly at the end with no searching, no temporary string and no copying. For example:

void thing(void) {
    char tagstr[60];
    int pos = 0;
    int k;
    int n = 5;

    for (k=0; k < n; k++) {
        pos += sprintf(&tagstr[pos], ", val%d", k);
    }
    printf ("String is now: %s\n", tagstr);
}