## Problem

## 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.

- First we need to get the Yellow region extracted from X
- Second we need to get blue region from Y
- We need to shift the blue region so that it fits between two yellow region
- 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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Keep blogging..!!

ReplyDeleteBig fan of your blog posts.