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

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!