/* dec2bin.c */
/* print a decimal number in binary */

#include <stdio.h>

void dec2bin (int);

int main()
{
int n;

printf("Enter a natural number: ");
if (scanf("%d", &n) != 1) {
printf("Input error.\n");
return 1;
}
else if (n<0) {
printf("dec2bin is undefined for negative integers.\n");
return -1;
}
else {
dec2bin(n);
}
return 0;
}

void dec2bin(int n)
{
if (0 == n) {
return;
} else {
dec2bin(n/2);
putchar((n % 2) ? '1' : '0');
}
}

//--------------------------------------------------------------------
examples for bin2dec function:
//--------------------------------------------------------------------

while (*s)
r = (r << 1) | (*s++ - '0');

//--------------------------------------------------------------------
long bin2dec(char *s)
{
long r;
for (r=0; *s; r = (r<<1) | (*s++ - '0'));
return r;
}

//--------------------------------------------------------------------
long bin2dec (char *s)
{
long r;

for (r = 0; *s != 0; s++)
{
r = (r << 1) | (*s - '0')
}
return r;
}