How to count the number of ones in a byte without a loop in c? [duplicate]

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Closed 14 years ago.

Possible Duplicate:
Best algorithm to count the number of set bits in a 32-bit integer?

Hello,

Is there a more compact way of counting the number of ones in a byte without using a loop? I don't want to do the following if I don't have to. Thanks.

char myValue = 0x0F;
int counter = 0;
while (myValue > 0)
{
 if (myValue & 0x01)
 {
 counter ++;
 }
 myValue = myValue >> 1;
}

• vitaut: Possible duplicate of stackoverflow.com/questions/109023/…
• wheaties: Are you asking for a solution in C? C++? objective-c? These are all very different languages. The solution is going to be slightly different for each one.
• Paul Nathan: @wheaties: I'm pretty sure C and C++ don't differ here.
• Paul Nathan: @vitaut: not a duplicate question, but there's definitely a duplicate answer.
• wheaties: @Paul Nathan in C++ I'd just put it into a std::bitset and then call the bitset::count function. In C, that doesn't exist.

Answer 1

 ((i>>3)&1)+((i>>2)&1)+((i>>1)&1)+(i&1)

Or use assembly (SSE/MMX). http://gurmeet.net/puzzles/fast-bit-counting-routines/

• Ivan Kuckir: Your code is a sum of four brackets, each bracket can be either 0 or 1. So the result can be at most four, but there can be 8 ones in a Byte.

Answer 2

This works if you initialize the table correctly.

static const unsigned char one_bits_in_byte[] = { 0, 1, 1, 2, 1, ... };
int counter = one_bits_in_byte[myValue & 0xFF];

Of course, you'd write a program with a loop in it to generate the table so that your final program doesn't have a loop in it. If you are feeling cost conscious, you can encode just the data for 0..15 and process the two halves (nybbles) of the value with shifts. But with 24 GB of physical memory, this is unlikely to be a major problem. And you could also simply compute the value on demand with masking operations, etc (like VGE suggests, but extended to a whole byte).

• Steve Jessop: If you're going to be cost-conscious, presumably it won't be about your 2/4GB of physical memory, it will be about your L1 cache. RAM is just L4 cache for the page file anyway ;-)
• R.. GitHub STOP HELPING ICE: @Steve: +1 your "L4" comment made me laugh! :-)