C : 0 & 1 combinations using recursion

Question

I want to list combinations of o & 1 recursively using c depending on variables number (number)

the output I want is

000
001
010
011
100
101
110
111

I've tried many algorithms the last one is :

void permute(unsigned number)    {

    if(number == 0) {
        printf("\n");
        return;
    }

    permute(number - 1);
        printf("0");
    permute(number - 1);
        printf("1");


}   //permute ends here


void permuteN(unsigned number) {

    unsigned i;

    for(i = 0; i < number + 1; i++){
        permute(i);
    }
}   //permuteN ends here

I think it gives me the answer but not ordered because I don't know where to put \\n;

need your help!

Answer 1

If you are indeed just looking for combinations of 1 's and 0 's I'd suggest you just count up the numbers and list them in binary.

Take the numerals 0...7 in binary and taking only the last 3 bits (apply mask maybe), and you end up with the same set you specified:

000
001
...
...
111

For n-digit combinations, you need to do 0..2^n - 1

---

Based off this answer , for one specific case of 3-bits (Credit to @ChrisLutz and @dirkgently)

#include <stdio.h>
int main(){
int numdigits = 3, j;
    for(j=1; j<8; j++)
        printbits(j);
}

void printbits(unsigned char v) {
  int i;
  for(i = 2; i >= 0; i--) putchar('0' + ((v >> i) & 1));
  printf("\n");
}

Output:

000
001
010
011
100
101
110
111

Answer 2

All you are actually doing is converting a number to binary.... A simple loop does this without any library calls (aside from printf )...

const unsigned int numbits = 3;
unsigned int bit;

for( bit = 1U << (numbits-1); bit != 0; bit >>= 1 ) {
    printf( number&bit ? "1" : "0" );
}
printf( "\n" );

---

Edited, since you seem to want recursion. You need to have some way to specify how many bits you require. You need to pass this into your recursive routine:

#include <stdio.h>

void permute(unsigned number, unsigned bits)
{
    if( bits == 0 ) return;
    permute(number / 2, bits-1);
    printf( "%d", number % 2 );
}   //permute ends here


void permuteN(unsigned number, unsigned bits ) {

    unsigned i;

    for(i = 0; i < number + 1; i++){
        permute(i, bits);
        printf("\n");
    }
}   //permuteN ends here

int main(void)
{
    permuteN(7, 3);
    return 0;   
}

To get the output in the order you require, you can't know when to write the newline. So in this case, you write it afterwards.

Answer 3

@paddy has a nice answer; only adding a bit (as of my toughs by your reply on my comment - was a bit late to the game). This rely on pow() , (and log10 for some nicety in print), tho so; if using gcc compile with -lm :

base might be a bit confusing here - but guess you get the meaning.

gcc -Wall -Wextra -pedantic -o combo combo.c -lm

/* gcc - Wall -Wextra -pedantic  -o combo combo.c -lm */
#include <stdio.h>
#include <stdlib.h>
#include <math.h>

static void prnt_combo(unsigned number, unsigned bits, int base)
{
    if (!bits)
        return;
    prnt_combo(number / base, --bits, base);
    printf("%d", number % base);
}


void prnt_combos(int bits, int base)
{
    int i;
    int n = pow(base, bits);
    int wp = log10(n) + 1;

    fprintf(stderr,
        "Printing all combinations of 0 to %d by width of %d numbers. "
        "Total %d.\n",
        base - 1, bits, n
    );

    for (i = 0; i < n; i++) {
        fprintf(stderr, "%*d : ", wp, i);
        prnt_combo(i, bits, base);
        printf("\n");
    }
}


/* Usage: ./combo [<bits> [<base>]]
 *        Defaults to ./combo 3 2
 * */
int main(int argc, char *argv[])
{
    int bits = argc > 1 ? strtol(argv[1], NULL, 10) : 3;
    int base = argc > 2 ? strtol(argv[2], NULL, 10) : 2;

    prnt_combos(bits, base);
    return 0;
}

Sample:

$ ./combo 4 2
Printing all combinations of 0 to 1 by width of 4 numbers. Total 16.
 0 : 0000
 1 : 0001
 2 : 0010
 3 : 0011
 4 : 0100
 5 : 0101
 6 : 0110
 7 : 0111
 8 : 1000
 9 : 1001
10 : 1010
11 : 1011
12 : 1100
13 : 1101
14 : 1110
15 : 1111

Or clean output:

$ ./combo 3 2 >&2-
000
001
010
011
100
101
110
111

You might like to add something like:

if (base > 10)
    printf("%x", number % base);
else
    printf("%d", number % base);

in prnt_combo() . This way you get ie by 2 16:

    0 : 00
    1 : 01
    2 : 02
    3 : 03
    4 : 04
  ...
  250 : fa
  251 : fb
  252 : fc
  253 : fd
  254 : fe
  255 : ff