Exercise 2.8-returns the value of the integer x rotated to the right by n positions

Question

Write a function rightrot(x,n) that returns the value of the integer x rotated to the right by n positions.

/* Write a function rightrot(x,n) that returns the value of the integer x
 * rotated to rightby n bit positions */

#include <stdio.h>

unsigned rightrot(unsigned x, int n);

int main(void) {
    printf("%u", (unsigned)rightrot((unsigned)8, (int)1));

    return 0;
}

/* rightrot: rotate x to right by n bit positions */

unsigned rightrot(unsigned x, int n) {
    int wordlength(void);
    unsigned rbit; /* rightmost bit */

    rbit = x << (wordlength() - n);

    x = x >> n;
    x = x | rbit;

    return x;
}

int wordlength(void) {
    int i;

    unsigned v = (unsigned)~0;

    for (i = 1; (v = v >> 1) > 0; i++)
        ;

    return i;
}

Explanation

We need to get the right most bit of the number provided.

First we get the right n bits at one time.

rbit = x << (wordlength() - n);

Once we get the right n bits, in order to rotate the value of x, we right shift x for n bits and then OR the result of x with the rbit determined in the previous step.

x = x >> n;
x = x | rbit;

For the same example.

n is between 0 - wordlength()

condition 1.when  (n == 0) or (n == wordlength())

            rightrot(x, 0) == x

condition 2. when  (n > 0) and (n < wordlength())   like n = 3

            x  = 0001 1001
            the right n bits will be 001.
            the right rightrot(x,n)result should be 0010 0011

            x << (wordlength() - n) = 0001 1001 << (8 - 3)
            = 0001 1001 << 5
            = 0010 0000

So we have got the right most n bits set.Now we right x by 1 and OR the rbit with x.

x >> n =  0001 1001 >> 3
=  0000 0011

x | rbit = 0000 0011 | 0010 0000
= 0010 0011

Which is our expected result.

condition 3. when (n > wordlength())        like n = 12

The Compiler will auto transfer “n” to “n % wordlength()”, n will be 3, then see “n” as condition 2. The result should be correct too!

condition 4.    when n < 0  (which is not Often use)

The result will mirror the function,the rightrot(x,n) function will move the left most n(n > 0)bits to the right side ，the function should called leftrot(x,n).

Visualization