Count the number of bits that are "on" in a byte

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Count the number of set bits in an integer
Best algorithm to count the number of set bits in a 32-bit integer?

That's an exam question and that is all I have - "Count the number of bits that are "on" in a byte" "On" means 1, I assume. Do I need to create a BitArray, randomly populate it and then iterate through it or is there a different way?

Also related - [Counting bits set in a .Net BitArray Class](https://stackoverflow.com/q/5063178/465053) — RBT, Jun 22 '18 at 00:02

score 8 · Answer 1 · answered Mar 21 '11 at 19:38

8

Using BitArray might be efficient but you could also do

byte b = ... ;

int count = Convert.ToString(b,2).ToCharArray().Count(c => c=='1');

answered Mar 21 '11 at 19:38

Bala R

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I think that is called a lambda expression, right? BtW, sorry I can not upvote, I am not registered yet. – Max Mar 21 '11 at 19:50
yes `c => c == '1'` is a lambda expression predicate. – Bala R Mar 21 '11 at 19:53

score 1 · Answer 2 · answered Mar 21 '11 at 19:42

1

Is this a interview question?

For a byte the fastest way would be to pre-compute an array such that a[i] = number of bits in i - the memory overhead is negligible.

answered Mar 21 '11 at 19:42

Ophir Yoktan

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http://gurmeet.net/puzzles/fast-bit-counting-routines/ – BadPirate Jan 22 '12 at 21:29
Unless it's for an embedded system. Then, doing it by the nibble is a better tradeoff. – Jamie May 03 '21 at 18:05

Count the number of bits that are "on" in a byte

2 Answers2