Question

I have a matrix X(1e4,20) which takes on values 0:4.
I’m interested in finding (row by row) the number of times values are ~=0, ==1&2&3 and ==3

Why doesn’t
eg:

X=randi([0 4],1e4,20)

for ii=1:1e4
  onestwosorfours(ii,1)=sum(X(ii,:)==1|2|4)
end

work?

I’ve ended up doing

sum(X(ii,:)==1)+sum(X(ii,:)==2), etc

Answer 1

This expression is wrong:

sum( X(ii,:)==1|2|4 )

You are finding the bitwise or of 1,2 and 4 which is true, because anything other than false or 0 is true. Then you are finding the amount of times that the array equals the number.

Instead, rewrite it as :

sum( X(ii,:)==1 |  X(ii,:)==2 |  X(ii,:)==4 )

Or, even better

numel( X(ii,:)==1 |  X(ii,:)==2 |  X(ii,:)==4 )

Which clarifies what you really meant.