Convert Integer to String using putchar() function and pointers

Question

I have the following working putchar() functions for integers:

void write_uint(unsigned n) {
    if (n / 10) write_uint(n / 10);
    putchar(n % 10 + '0');
}

void write_int(int n) {
    if (n < 0) {
        putchar('-');
        write_uint(-(unsigned)n);
    }
    else write_uint(n);
}

I am trying to modify it to convert the integer to a char array using pointers instead of writing to console as follows:

void write_uint(unsigned n, char *p) {
    if (n / 10) write_uint(n / 10, p);
    *p++ = n % 10 + '0';
}

void write_int(int n, char *p) {
    if (n < 0) {
        *p++ = '-';
        write_uint(-(unsigned)n, p);
    }
    else write_uint(n, p);
}

int n = 123456789;
char *str = malloc(13000), *p = str;

write_int(n, p);
*p++ = '\n';

Then print the string, but all I get is a bunch of empty lines.

Any help is appreciated

Ingo Leonhardt · Accepted Answer · 2014-07-23T15:44:26.533

In write_int() and write_uint() you increment p but because you have passed char *p, that incrementation is not passed back to the caller. So in the last line of your sample code, p is still equal to str and so you get the empty line.

You can modifiy your code in one of these ways:

pass a char ** instead of a char *

void write_uint(unsigned n, char **p) {
    if (n / 10)
        write_uint(n / 10, p);
    **p = n % 10 + '0';
    (*p)++;
}

void write_int(int n, char **p) {
    if (n < 0) {
        **p = '-';
        (*p)++;
        write_uint(-(unsigned)n, p);
    }
    else
        write_uint(n, p);
}

....
write_int( n, &p );
*p++ = '\n';

return the modified pointer

char *write_uint(unsigned n, char *p) {
    if (n / 10)
        p = write_uint(n / 10, p);
    *p++ = n % 10 + '0';
    return p;
}

char *write_int(int n, char *p) {
    if (n < 0) {
        *p++ = '-';
        p = write_uint(-(unsigned)n, p);
    }
    else 
        p = write_uint(n, p);
    return p;
}

....
p = write_int( n, p );
*p++ = '\n';

Of course, as #pm100 has stated, you should add a NUL after the '\n' too

That code is inside a loop and I am doing *p = '\0' at the end. I am still getting empty lines using your second code — ceferrari, Jul 23 '14 at 15:46
it works absolutely fine in my test program .. do you really check `str` (not `p`) ? — Ingo Leonhardt, Jul 23 '14 at 15:49
You got it. I wasn't calling the function the correct way ('p = call' in your code). Thank you! — ceferrari, Jul 23 '14 at 16:14

pm100 · Answer 2 · 2014-07-26T18:40:38.490

-1

you certainly need to zero terminate str

*p++ = 0;

Or if you prefer

*p++ = '\0';

edited Jul 26 '14 at 18:40

answered Jul 23 '14 at 15:33

pm100

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That code is inside a loop and I am doing *p = '\0' at the end – ceferrari Jul 23 '14 at 15:47

Convert Integer to String using putchar() function and pointers

2 Answers2