Thread: binary to decimal

Things like %d perform a conversion for you. So the only thing you can do for simple input is use %s

char buff[100];
scanf("%s",buff);

Now you have "101" or "11001" or "11100011001101" typed in by the user

Try converting "1234" in decimal
base = 10
result = 0
loop
- result = result * base
- result = result + numeric value of char
end loop

You can rotate the bits and test for 1 or 0 using a mask if you AND 1 with the rotated number it will give you the bit then use a conditional statement to print either a 0 or a 1 to the screen.

Code:

int number=3;
int number_copy=number;
_rotr(number,1);  //rotates number right _rotl() goes left

if(number & 1)  //if number & 1 is true
{
putchar('1');
}
putchar('0');

you have to work with it a bit because if you shift it the wrong direction it prints it out in reverse but you get the idea

[edit]
OOPS wrong direction you wanted binary to decimal

You could just do the reverse. For each 1 or 0 you can use the mask to set the bits and then rotate.

>_rotr(number,1); //rotates number right _rotl() goes left
My compiler must be broken. I just get undefined references concerning _rotr and _rotl.

_rotl() and _rotr() are supposed to mimic the assembly language instructions for rotating bits.

I suppose you could write your own using the bitwise shift operator>> and <<. Normally when you shift the bits left or right it just discards the bit and you eventually end up with 0. If you create a mask using the number for the bit you want to use (ie to test or set the 32nd bit set the mask=2^31) then you can use the following algorithm:

for rotating right
mask_lowbit=1; //2^0 the first bit set to 1
mask_highbit=21474836848; //2^31 the 32nd bit set to 1

test lowbit for 1 or 0 by ANDing (mask_lowbit & number)
if result is 0 then simply shift number to the right
if true then shift numbers to the right and set the 32nd bit by ORing(number | mask_highestbit)

just reverse the process for shifting left

>_rotl() and _rotr() are supposed to mimic the assembly language instructions for rotating bits.
I'm well aware of what they're supposed to do. My point was that the two functions/macros are not standard C. Maybe I missed the OP stating a platform and compiler where your suggestion would work, but you appear to be making far too many assumptions.

Your right. I just copied and paste one of my examples without thinking. I need to pay more attention to standards and compiler/op. Thank you. Anyway here is code example of home made functions.

Code:

unsigned int bit_rotate_right(unsigned int num, unsigned int numbits)
{
    unsigned int lowbitmask=1;          // mask for 1st bit
    unsigned int highbitmask=2147483648;    //mask for 32nd bit 2^31

	for(int i=0;i<numbits; i++)
	    {
		if(num & lowbitmask)
		    {
			num=num>>1;
			num=(num|highbitmask); 
		    }
		else
		    {
			num=num>>1;
		    }
	    }
return num;
}

unsigned int bit_rotate_left(unsigned int num, unsigned int numbits)
{
    unsigned int lowbitmask=1;          // mask for 1st bit
    unsigned int highbitmask=2147483648;     //mask for 32nd bit 2^31

	for(int i=0;i<numbits; i++)
	    {
		if(num &highbitmask )
		    {
			num=num<<1;
			num=(num|lowbitmask); 
		    }
		else
		    {
			num=num<<1;
		    }
	    }
return num;
}

Code:

unsigned int highbitmask=2147483648;

For the high bit of an unsigned int, consider the following.

Code:

unsigned int highbitmask = (-1U >> 1) + 1;

Also,

Code:

for(int i=0;i<numbits; i++)

it may be preferable to conform to C89 until C99 implementations become more prevalent. Otherwise the distinction between C and C++ can be blurred.