K & R : Exercise 2.6 - setbits

Problem

Write a function setbits(x, p, n, y) that returns x with n bits that begin at position p set to the right most n bits of y, leaving the other bits unchanged.

Solution

Here, i feel most complicated thing is understanding the question rather than solving it by programming. Let me explain the question visually by an example.

n bits at position p in x has to replaced with n right most bits of y to get the result.

1. First we need to get the Yellow region extracted from X
2. Second we need to get blue region from Y
3. We need to shift the blue region so that it fits between two yellow region
4. We need to 'bitwise OR' 1 and 3.

Yellow region

We need to create zero mask(means selective bits are zero and rest all are one) of n bits at position p. Expression to that is

((~0 << (p + 1)) | (~(~0 << (p + 1 - n))))

Blue region

Blue region can be easily extracted from the below expression.

(y & ~(~0 << n))

Shifting Blue region

This can be very easily gone by

((y & ~(~0 << n)) << (p + 1 - n)

Putting all together

Now putting all together final program looks like this.

unsigned setbits(unsigned x, unsigned p, unsigned n, unsigned y)
{
  return (x & ((~0 << (p + 1)) | 
          (~(~0 << (p + 1 - n))))) | 
          ((y & ~(~0 << n)) << (p + 1 - n));
}

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