Get n LSB from Integer C++

Question

So I'm getting an unsigned int passed into my function. Now I must obtain the n LSB bits of that integer and use it to access a location in an array of size 2^n.

So for example, if my array is of size 1024, n = 10.

I am currently doing this:

unsigned int location = my_unsigned_int << n;

However this doesn't work as location ends up being way too large and out of bounds.

Try to do it on paper first, to see that you get it correctly. And to give you a hint, you need to *mask out* the top-level bits. — Some programmer dude, Feb 12 '17 at 18:51

Scott Hunter · Accepted Answer · 2017-02-12T19:58:14.587

3

You can just mask the bits you want:

unsigned int location = my_unsigned_int & ((1<<n) - 1);

This assumes that your int is at least n+1 bits in size.

edited Feb 12 '17 at 19:58

answered Feb 12 '17 at 18:53

Scott Hunter

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I think you mean only one `&` because the `&&` isn't bitwise – theKidOfArcrania Feb 12 '17 at 18:54
It's giving me an error "suggest parentheses around '-' inside <<" – Alk Feb 12 '17 at 18:57

Get n LSB from Integer C++

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