C concatenate string with int in loop

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;

1st problem temp[5] isn't bug enough to hold ", valk" make it temp[10] or something just to be sure you have enough space. — John3136
– John3136, Commented Nov 1, 2012 at 1:00
The code you've posted is fine (apart from that it doesn't compile). It also has nothing to do with pointers. You'll need to show us your actual code. — aib
– aib, Commented Nov 1, 2012 at 1:10
I'm not sure where it's going wrong, then... edited to add more context if that helps. — user812786
– user812786, Commented Nov 1, 2012 at 1:21

Brendan · Accepted Answer · 2012-11-01 01:40:19Z

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);
}

antak · Accepted Answer · 2012-11-01 01:12:08Z

3

Works for me:

$ gcc -xc - && ./a.out
int main(void) {
        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);
        }
        printf("[%s]\n", tagstr);
}
[, val0, val1, val2, val3, val4]

Unless you're saying the problem is with the initial ", "..

answered Nov 1, 2012 at 1:12

antak

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1 Comment

Jonathan Leffler Over a year ago

Give or take #include <string.h> and #include <stdio.h>, and I used << and >> in place of square brackets, that's the result I got too.

Gianluca Ghettini · Accepted Answer · 2012-11-01 01:04:38Z

2

your temp array is too short! use

char temp[16];

answered Nov 1, 2012 at 1:04

Gianluca Ghettini

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user812786 Over a year ago

Edited to fix this - it still has the same problem, though.

Jonathan Leffler · Accepted Answer · 2012-11-01 01:21:44Z

2

If you decide you don't want the leading comma and blank, you can use a simple variation on the code you showed:

#include <stdio.h>
#include <string.h>

int main(void)
{
    char tagstr[60] = "";
    const char *pad = "";
    int k;
    int n = 5;
    for (k = 0; k < n; k++)
    {
        char temp[10]  = "";
        snprintf(temp, sizeof(temp), "%sval%d", pad, k);
        strcat(tagstr, temp);
        pad = ", ";
    }
    printf("tagstr <<%s>>\n", tagstr);
    return 0;
}

The output from the program was:

tagstr <<val0, val1, val2, val3, val4>>

However, your code works correctly for me, albeit with the leading comma and blank.

answered Nov 1, 2012 at 1:21

Jonathan Leffler

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Comments

aib · Accepted Answer · 2012-11-01 01:02:21Z

0

temp isn't long enough to hold the result of your sprintf. This is exactly why you should use snprintf, strncat and other variants of the string functions that take a size parameter, whenever you can.

answered Nov 1, 2012 at 1:02

aib

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1 Comment

user812786 Over a year ago

Edited to fix this - still has the same problem.

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