Thread: adding a number at the end of a char

Originally Posted by Salem

Use sprintf.

Code:

sprintf(c111, num1);

?

There are programming languages, like Python, where it doesn't matter so much what the "type" of a piece of data is. In those languages, you can concatenate numbers and text strings and it will "just work." Some of them might require you to convert the number to a string, doing something like str(num) before you paste it on the end, but that's about it.

C is not one of those programming languages.

In C, you absolutely have to worry about types. If two pieces of data are of different types, there's no way to get them together. So you have to convert one into the other. And in many cases, you have to do the conversion in some specific way.

Consider your case: you want to append a number to a string. What do you want to do with the result? Do you even care about the result, or do you just want to "appear" to append a number to a string, perhaps by printing them side-by-side.

If that is true, just print them side-by-side:

Code:

printf("%s%d\n", "abc", 123);

On the other hand, maybe you need to make an identifier to name a text file. So you want to "have" a string that is the result of appending a numeric string to a non-numeric string. If so, the easy way to do it is as @Salem suggested: use sprintf to write characters into a buffer.

Code:

// log2(10) is somewhere between 3 and 4. So assume 3. 
// 64 bits divided by 3 is 21-ish. Assume 21 is the max # of
// output digits. Add 1 for "sanity".  Add 1 for trailing NUL.

#define MAX_NUM_DIGITS 23

char buffer[sizeof(c111) + MAX_NUM_DIGITS];

sprintf(buffer, sizeof buffer, "%s%d", c111, num1);    // buffer should contain "aa10"

Finally, maybe you need to have lots of these strings laying around. Maybe you're making database keys or something, and there will be millions of entries, maybe billions. You're very concerned about performance.

First: don't. There are very few problems that you will encounter that need that kind of concern. Just use sprintf or snprintf along with maybe strdup if you want to save a copy.

Second: you can use lower-level code to generate your string. Do your own int-to-string conversion. Welcome to C!

Code:

#define MAX_BUF_LEN (sizeof c111 + MAX_NUM_DIGITS)

char *
make_identifier(
        const char * prefix,
        int sequence_number)
{
        char buf[MAX_BUF_LEN];

        size_t plen = strlen(prefix);
        if (plen + MAX_NUM_DIGITS >= sizeof buf)
                FATAL_ERROR("prefix string too long for internal buffer: %s", prefix);

        // Convert number to string at the back end of the buffer, working backwards
        char * wip = buffer + sizeof buf - 1;
        *wip-- = 0;

        int sn = sequence_number;  // work with a temporary value so we can print the original if needed
        do {
                *wip-- = '0' + (sn % 10);  // ASCII/UTF-8 digits are in order, starting at '0', up through '9'. For this.
                sn /= 10;
        } while (sn != 0);

        // If you have a negative number, you'll get an identifier with a minus in the middle: "aa-12"
        if (sequence_number < 0)
                *wip-- = '-';

        assert(wip >= buf + plen);  // Reminder: We checked this at the top of the function.
        strcpy(buf, prefix);
        memmove(buf + plen, wip + 1, buf + sizeof buf - wip);
        return strdup(buf);  // Make a "forever copy" the caller can save.
}

aghast:

The OP does not understand the language at all. The OP will not begin to understand what you presented.

Originally Posted by rstanley

aghast:

The OP does not understand the language at all. The OP will not begin to understand what you presented.

The fact they don't understand it today has no bearing on whether they will understand it tomorrow. Plus, I got to practice writing some C, and become a tiny fraction of a percent better as a tutor and coder. Why else would I even be here?