What’s the use of “& 0xff” in programming?

by Oscar
Published: Last Updated on

When I strated out, I was really confused about bit shift and hexadecimal number and binary. This is a good example how confusing it could get sometimes. Occasionally you see this in the code, for exmaple in C++:

byte a = ( b >>  8) & 0xff; // b is an integer

Okay, I know what “>> 8” means, we are shifting variable “b” to the right by 8 bits. But, what the heck is “& 0xff” doing there?

Well, 0xff is the hexadecimal number FF which has a integer value of 255. And the binary representation of FF is 00000000000000000000000011111111 (under the 32-bit integer).

The & operator performs a bitwise AND operation. a & b will give you an integer with a bit pattern that has a 0 in all positions where b has a 0, while in all positions where b has a 1, the corresponding bit value froma is used (this also goes the other way around). For example, the bitwise AND of 10110111 and00001101 is 00000101.

In a nutshell, “& 0xff” effectively masks the variable so it leaves only the value in the last 8 bits, and ignores all the rest of the bits.

It’s seen most in cases like when trying to transform color values from a special format to standard RGB values (which is 8 bits long).

Leave a Comment

By using this form, you agree with the storage and handling of your data by this website. Note that all comments are held for moderation before appearing.

6 comments

Shayan 14th April 2018 - 11:51 pm

I was confused about 0xff. this article helped me alot. thanks for sharing.

Reply
Hoswar Ruby 5th January 2017 - 5:01 am

i think , it’s really wierd …

Reply
Muslim Aswaja 15th March 2016 - 3:58 pm

Thank you, Oscar! It’s really helpfull for me.
:-)

Reply
Simon 18th November 2015 - 9:39 pm

Hi Oscar,

Thanks for your knowledge, it is really helpful to me!

Reply
Abder-Rahman 23rd August 2015 - 5:59 pm

Thanks for the nice tutorial!

Reply
bit_shift 2nd December 2014 - 9:47 pm

Forgive me, but isn’t it >> a right shift, because you know it point right and move the bits to the right, and << is the left shift?

Reply