Print a number converted in base 2 recursively

Question

So I've been trying to do this but I just can't think of a solution for this. I've got this bit of code but it outputs it backwards(if the answer is 11110 I get 01111):

#include <stdio.h>

int base(int n)
{
    if(n==0)
        return 0;
    else
    {
        printf("%d",n%2);
    }
    return base(n/2);


}
int main() {
    int n;
    scanf("%d",&n);
    base(n);
    return 0;
}

Is there any trick for this problem or do I need to analyze this deeper?

Rather than printing the digits in the order they're processed (right to left), pass a buffer around where you can fill in those digits in the correct order. Then print at the end. — Jeff Mercado, Mar 07 '21 at 19:37
I would use a mask in order to avoid having a buffer, see my answer — Antonin GAVREL, Mar 07 '21 at 19:53
thanks for the useful tips, also the one with printing after the call will be very useful @chux-ReinstateMonica I didn't really care about the case when ```n<0```, and when ```n==0``` it should print just ```0``` I guess since that is how you represent it in base 2 — Rares Amza, Mar 08 '21 at 13:24
@RaresAmza With "I didn't really care about the case when n<0, and when n==0 it should print just 0", accepted answer `base(-11)` as `"-10-1-1"` is OK and `base(0)` prints nothing. Interesting. UV for explaining your goals yet disagree with them. — chux - Reinstate Monica, Mar 08 '21 at 13:39
@chux-ReinstateMonica I will revisit this soon to try and make it work 100% — Rares Amza, Mar 09 '21 at 09:19

score 2 · Accepted Answer · edited Mar 07 '21 at 19:59

2

As @rici stated, a very simple fix is to print after the recursive call:

#include <stdio.h>

void base(int n){
    if(n==0)
        return;
    base(n/2);
    printf("%d",n%2);
}

int main() {
    int n;
    scanf("%d",&n);
    base(n);
    return 0;
}

edited Mar 07 '21 at 19:59

dreamcrash

47,137
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answered Mar 07 '21 at 19:57

1

what about the line return? – Antonin GAVREL Mar 07 '21 at 20:03
mm I guess a simple wrapper could fix that! – Mar 07 '21 at 20:07
1

Note: `base(0)` prints nothing. – chux - Reinstate Monica Mar 07 '21 at 21:25
`base(-7)` prints "-1-1-1" – chux - Reinstate Monica Mar 07 '21 at 21:31

score -2 · Answer 2 · edited Mar 07 '21 at 20:00

-2

I would use a mask:

#include <stdio.h>

int base(int n, int mask){
    if(!mask) {
        printf("\n"); // we reach the end, print a line return
        return 0;
    }
    printf("%d",  !!(n & mask)); // if mask and n match, print '1', else print '0'. !! convert any value into 1, and 0 remains 0.
    return base(n, mask >> 1); // divide mask by 2, check the next bit on the right
}

int main() {
    int n;
    scanf("%d",&n);
    base(n, 1 << (31 - __builtin_clz(n))); // call the function with a mask initialized at the same level than the most important bit of n. 
    return 0;
}

edited Mar 07 '21 at 20:00

dreamcrash

47,137
25
94
117

answered Mar 07 '21 at 19:53

Antonin GAVREL

9,682
8
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1

`__builtin_clz(n)` is not part of C. This answer assumes select compilers and `int` range. A shift of `1 << 31` is a problem too. – chux - Reinstate Monica Mar 07 '21 at 21:28

Print a number converted in base 2 recursively

2 Answers2