Binary representation starting with a least significant bit in C - c

Question

I'm trying to make a binary representation of an integer using binary() function and then use reverse() function to get bit reversal as can be seen below:
#include <stdio.h>
unsigned char reverse(unsigned char b) {
b = (b & 0xF0) >> 4 | (b & 0x0F) << 4;
b = (b & 0xCC) >> 2 | (b & 0x33) << 2;
b = (b & 0xAA) >> 1 | (b & 0x55) << 1;
return b;
}
int binary(int n)
{
int b;
if (n == 0)
{
return 0;
}
else
{
b = (n % 2) + 10 * binary(n / 2);
unsigned char i = (unsigned char)b;
return reverse(i);
}
}
int main()
{
int n;
printf("Enter a decimal number: ");
scanf("%d", &n);
printf("The binary equivalent of %d is %d\n", n, binary(n));
return 0;
}
The issue I'm having is that I expect binary() to produce a binary number (for example, a binary representation fo 18 would be 10010), and then reverse() function to take this number and produce a reversed binary representation starting with the least significant bit (LSB) which should be 01001 in this case. However, when I try to run it, I get returned 0.
I'm new to C and I'm thinking maybe the issue is in the unsigned char datatype, but when, why if I convert b into unsigned char it still doesn't work?

Answer 1

When reversing the bits, all you have to do is check the LSB. Add that value to an output variable. Then shift the output variable by one to the left and shift the original number by one to the right. In the mathematical example below, I do a first 0 shift so it become easier on the eye with the formatting.
In your example of 10010:
Round 1: output = (00000 << 1) + (10010 & 1) = 00000
N = 10010 >> 1 = 01001
Round 2: output = (00000 << 1) + (01001 & 1) = 00001
N = 01001 >> 1 = 00100
Round 3: output = (00001 << 1) + (00100 & 1) = 00010
N = 00100 >> 1 = 00010
Round 4: output = (00010 << 1) + (0010 & 1) = 00100
N = 00010 >> 1 = 00001
Round 5: output = (00100 << 1) + (00001 & 1) = 01001
N = 00001 >> 1 = 00000
Example:
#include <stdio.h>
int binary(int n)
{
int output = 0;
if ( n == 0 ) return 0;
// Save one left shift
output += n & 1;
n = n >> 1;
while( n != 0 ){
output = output << 1;
output += n & 1;
n = n >> 1;
printf("%d\n", output);
}
return output;
}
void printBin(int n){
char bin[33] = {0};
for (int i = 31; i > -1; i-- ){
bin[i] = (char)((n & 1) ^ 48);
n = n >> 1;
}
puts(bin);
}
int main()
{
int n; int rb;
printf("Enter a decimal number: ");
scanf("%d", &n);
rb = binary(n);
printf("The binary equivalent of %d is %d\n", n, rb);
printf("%11d:", n);
printBin(n);
printf("%11d:", rb);
printBin(rb);
return 0;
}