Question

I am receiving a number N where N is a 4-bit integer and I need to change its LSB to 1 without changing the other 3 bits in the number using C.

Basically, all must read XXX1.

So lets say n = 2, the binary would be 0010. I would change the LSB to 1 making the number 0011.

I am struggling with finding a combination of operations that will do this. I am working with: !, ~, &, |, ^, <<, >>, +, -, =.

This has really been driving me crazy and I have been playing around with >>/<< and ~ and starting out with 0xF.

Answer 1

Try

number |= 1;

This should set the LSB to 1 regardless of what the number is. Why? Because the bitwise OR (|) operator does exactly what its name suggests: it logical ORs the two numbers’ bits. So if you have, say, 1010b and 1b (10 and 1 in decimal), then the operator does this:

   1 0 1 0
OR 0 0 0 1
=  1 0 1 1

And that’s exactly what you want.

For your information, the

number |= 1;

statement is equivalent to

number = number | 1;