Use bitget
:
% generate a random int number
>> n = uint32( randi( intmax('uint32'), 1, 1 ) )
n =
3771981510
>> count = sum(bitget(n,1:32))
count =
18
Alternatively, if you are concern with performance, you can use a lookup table (LUT) to count the bits:
Constructing a LUT for 8-bits ints (only 256 entries):
function lut = countBitsLUT()
for ii = 0:255
lut(ii+1) = sum(bitget(uint8(ii),1:8));
end
You only need to construct the LUT once.
Once you have the LUT, you can count the number of bits using:
count = lut( bitand(n,255)+1 ) + ... % how many set bits in first byte
lut( bitand( bitshift(n,-8), 255 ) + 1 ) + ... % how many set bits in second byte
lut( bitand( bitshift(n,-16), 255 ) + 1 ) + ... % how many set bits in third byte
lut( bitand( bitshift(n,-24), 255 ) + 1 ); % how many set bits in fourth byte
I also did a small "benchmark":
lutCount = @( n ) lut( bitand(n,255)+1 ) + ... % how many set bits in first byte
lut( bitand( bitshift(n,-8), 255 ) + 1 ) + ... % how many set bits in second byte
lut( bitand( bitshift(n,-16), 255 ) + 1 ) + ... % how many set bits in third byte
lut( bitand( bitshift(n,-24), 255 ) + 1 ); % how many set bits in fourth byte
t = [ 0 0 ];
for ii=1:1000
n = uint32( randi( intmax('uint32'), 1, 1 ) );
tic;
c1 = sum(bitget(n,1:32));
t(1) = t(1) + toc;
tic;
c2 = lutCount( n );
t(2) = t(2) + toc;
assert( c1 == c2 );
end
And the run times are:
t = [0.0115 0.0053]
That is, LUT is ~twice as fast as sum
of bitget
.