bit operations in c

Question

how do you reverse and rotate hex numbers and return the number in C with bitwise operators?

for example:

0xabcd -> 0xdcba

0xabcd -> 0xdabc

Answer 1

It's hard to know where to begin with this question. Plus I smell homework.

Some points:

• There is no such thing as a "hex number". Hex is just a notation. How do you reverse and rotate decimal numbers and return the number in C? For example:

  1776 -> 6771

  1776 -> 6771?

• To solve this problem, you need a deep understanding of positional notation, whether it's base 10, base 16, base 2, or what have you.

• All you need can be had by adding, subtracting, multiplying, and dividing. Those are operations on numbers. Modulus is also very helpful.

• If you happen to want to multiply or divide by a power of two, I commend to you the C left shift << and right shift >> operators. These work perfectly for numbers that are represented using the C types unsigned or unsigned long.

Answer 2

Hex numbers are numbers, as Norman's answer points out. However, 1 hex digit = 4 bits, so these operations actually make sense as things you could possibly want to do to integer values.

The 2nd one is a bitwise rotation by 4 bits. See Best practices for circular shift (rotate) operations in C++ for best practices for compiler-friendly rotates that guard against C/C++ undefined-behaviour.

If your input isn't 8, 16, 32, or 64 bits, then you might need to shift + mask manually, instead of relying on shift-in-zeroes.

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The first one will require more code: it reverses the order of the nibbles. There is no machine instruction for that, or easy way to build it out of a few bitwise operations on the entire number at once.

I think you'd have to reverse the order of the bytes, and then reverse the order of nibbles within each byte (an 8-bit rotate by 4).