Concatenating a char in a recursive function in C

Question

Even though I have more experience with higher level languages, I am having a lot of troubles understanding how memory allocation and how strings really work in C.

I am trying to implement a very simple base converter that works recursively. The only thing is that it should return a char* instead of an int

Here is my code. I already tested the recursive calls and it works if I use integers. So, the problem is definitely with the string part. It gives me an infinite loop.

char* baseConversion(int num, int baseIn, int baseOut){

    //convert num to base ten

    int quotient = num / baseOut;

    int remainder = num % baseOut;

    char rem = (char)(((int)'0') + remainder);

    char *result = malloc(strlen(output) + 1);

    strcpy(result, rem);

    if (quotient == 0)
        return result;
    else
        return strcat(result, baseConversion(quotient, baseIn, baseOut));
}

Many thanks

The second argument of `strcpy` must be a null-terminated string. `rem` is a char, not a string. — Barmar, Oct 25 '14 at 04:09
Sixth line states that there is an infinite loop super cool guy ;) — Luis Lavieri, Oct 25 '14 at 04:10
@CoolGuy: A question doesn't need to end with a question mark sign. — dotNET, Oct 25 '14 at 04:10
The algorithm does not work, even if all the coding errors are fixed. Example, Lets say 'num' started out as 101 in decimal and the baseout = 10. Then the first pass would produce a 1. The first recursion would produce 0, so the result would be 1:0 (10 in printable decimal.) When, the actual value should be 1:0:1 (101 in printable decimal). Also, the final result would be undefined behaviour unless the variable 'output' is >= 3 in length and pre-set to all '\0'. — user3629249, Oct 25 '14 at 09:19
You are forgetting two ones produced from the recursive calls — Luis Lavieri, Oct 25 '14 at 16:51

score 1 · Accepted Answer · answered Oct 25 '14 at 04:12

1

Change:

strcpy(result, rem);

to:

result[0] = rem;
result[1] = 0;

This will create a single-character string containing the character in rem.

You also may need to fix:

malloc(strlen(output)+1)

as there's no variable named output in your function.

answered Oct 25 '14 at 04:12

Barmar

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Vlad from Moscow · Answer 2 · 2014-10-25T05:01:19.423

If I have understood correctly what you are saying about then what you need is the following

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

char * baseConversion( unsigned int x, unsigned int base )
{
    unsigned int digit;
    char *p = NULL;
    size_t n = 2;

    if ( base > 10 || base < 2 ) base = 10;

    digit = x % base;

    if ( x / base ) p = baseConversion( x / base, base );

    if ( p ) n += strlen( p );

    p = realloc( p, n );

    *( p + n  - 2 ) = digit + '0';
    *( p + n  - 1 ) = '\0';

    return p;
}   


int main(void) 
{
    unsigned int x = 255;

    char *p = baseConversion( x, 10 );

    printf( "%s\n", p );

    free( p );

    p = baseConversion( x, 8 );

    printf( "%s\n", p );

    free( p );

    p = baseConversion( x, 2 );

    printf( "%s\n", p );

    free( p );

    return 0;
}

The output is

255
377
11111111

P.S. It is funny when one answer is marked as the best but the code will be used from other answer.:)

My question was about how to correct the infinite loop. @Barmar helped me out with that. I was not asking for the proper algorithm. That is why you don't get the right answer. — Luis Lavieri, Oct 25 '14 at 16:37
@Luis Lavieri My answer is the most helpful because it shows how the program should be written correctly. It teaches how such programs should be written in C. — Vlad from Moscow, Oct 25 '14 at 16:41

Concatenating a char in a recursive function in C

2 Answers2