Rotating bits of any integer in C

Question

Pass a integer 2 to this function and then return a integer which is 4

x = 2;
x = rotateInt('L', x, 1); 

(left shift the bits by 1)

Example: 00000010 -> rotate left by 1 -> 00000100

but if I pass this:

x = rotateInt('R', x, 3); 

it will return 64, 01000000

Here is the code, can someone correct the error... thanks

int rotateInt(char direction, unsigned int x, int y)
{

unsigned int mask = 0;
int num = 0, result = 0;
int i;

for(i = 0; i < y; i++)
{     
if(direction == 'R')
{
if((x & 1) == 1)     
x = (x ^ 129);
else    
x = x >> 1;
}
else if(direction == 'L')
{
if((x & 128) == 1)  
x = (x ^ 129);   
else
x = x << 1;
}
}

result = (result ^ x);

return result;   

}

Answer 1

Have a look at the bitwise shift operators:

http://en.wikipedia.org/wiki/Bitwise_operators#Shifts_in_C.2C_C.2B.2B_and_Java

Answer 2

So, I'll assume you know what right and left shifts are. And that you know the difference between arithmetic and logical shifts.

C only has arithmetic shifts. It doesn't do logical shifts, nor does it do rotates. okay I lied, C does logical shifts on unsigned ints.

A rotate does, well, exactly that: it's the same as a logical shift, except when you shift past the end of the number, the digits "wrap around" to the other side. For example

0010 right-rotated is 0001. If you right-rotate again, you get 1000. See, the 1 wrapped around, or rotated, to the other side of the integer.

The left rotate is similar: 0100 left rotate 1000 left rotate 0001 left rotate 0010 etc.

Note that rotates don't keep the sign bit as an arithmetic right-shift would.

So, C only has arithmetic shifts. So you have to implement the "rotate" part manually. So, take a left-rotate. You would want to:

1. Capture the value of the left-most bit. (is it a 0 or 1?)
2. Do a left-shift
3. Set the right-most bit - which is the bit we talked about in step 1 (which needs to be rotated around) to the correct value, based on what we captured from step 1.

You should be able to figure out a similar method for right rotates.

good luck!

Answer 3

It seems like your rotate to the right is RIGHT. 1 fell of the side and returned back again from the left?

Anyway, here are your ingredients:

http://tigcc.ticalc.org/doc/keywords.html#if - for determining if it is 'L' or 'R'

http://tigcc.ticalc.org/doc/keywords.html#for - for counting number of times to shift

and

http://msdn.microsoft.com/en-us/library/f96c63ed(VS.80).aspx - to actually shift it

Go, play with it. It WILL work eventually!

Answer 4

I recommend using an unsigned int.

#define DIR_LEFT 0
#define DIR_RIGHT 1

unsigned int rotateInt(unsigned int in, int amount, byte dir)
{
    return(dir == DIR_RIGHT ? in >> amount : in << amount);
}

Answer 5

Since no one told you how to implement this, you can use intrinsics, for visual studio they are _rotl, _rotl64, _rotr, _rotr64.

Oh, but rotation and shifts are 2 different things!