MATLAB: Extraction of subrows of different length

Question

Hi,

I have a matrix of boolean n*m say:

A =[0 1 1 1 0; 0 1 1 0 1; 0 1 0 1 1; 0 0 1 0 1]

I have 2 vectors of size n of starting and ending indexes corresponding to the columns of A, say:

iStart = [2 2 4 5];
iEnd = [4 5 5 4];

I would like to check whether, for each row of A, all the booleans for the columns in between iStart and iEnd are true. The loop version would be:

     bool = false(size(A,1))
     for i = 1:size(A,1)
           if cumprod( A(i, iStart(i):iEnd(i)) )
              bool(i) = true; 
           end
     end

In the above example, we would have:

bool = [1 0 1 0]

So I was wondering whether there is a smarter kind of vectorized version of this that would speed up the code ? I am usually not bad for vectorizing stuff but the different sizes are not very nice for Matlab and give me troubles.

Thanks a lot 🙂

Answer 1

One solution

>> arrayfun(@(i1,i2,i) all(A(i,i1:i2)),iStart,iEnd,[1:size(A,1)])
ans =
1×4 logical array
 1   0   1   1
>>

altho your loop may be just as fast or faster...

for i=1:size(A,1)
  B(i)=all(A(i,iStart(i):iEnd(i)));
end