Efficient Code for Filling Rows of an Array involving Indices Ranges

Hello,

I have an array A of zeros. I need to set a range of column indices for each row of A to ones. These ranges are determined by elements of another vector B, specifying start and end indices for the ranges for each row. For example, this is what I'm trying to figure out:

The first row of B indicates that the first row of A should have ones from columns 1 to 3. The second row of B indicates that the second row of A should have ones from columns 2 to 4 etc.

I could code this with a for-loop that goes through each of the rows and creates indices with the colon operator however, I'm hoping for a smarter, more efficient way to do it than that.

Can anyone help me with this or any suggestions?

Thanks.

Answer 1

[Matt J]

e=1:size(A,2);

A = bsxfun(@ge,e,B(:,1)) & bsxfun(@le,e,B(:,2));

[Mohammad Abouali]

nice

[Matt J]

Thanks. The syntax is brief, but I still think a for-loop will be more efficient.

Answer 2

[Ray]

Hi All,

I implemented the above and acquired the average execution times for each approach:

The for-loop approach is the most efficient.

On a larger matrix (50000x20) the results are similar however, the approach suggested by Guillaume takes a bit longer ~0.2 seconds.

Thanks to everyone for their replies and help.

[Matt J]

This was with a for-loop across columns, correct?

Answer 3

[Guillaume]

I don't think there's a more efficient way of doing it than with a loop. The following would work, but it's arguable that it does not involve loops because of the cellfun, and it's certainly convoluted:

A=A';
bounds = sub2ind(size(A), B', repmat(1:size(B, 1), size(B, 2), 1));
indices = cellfun(@(b) b(1):b(2), num2cell(bounds, 1), 'UniformOutput', false);
A([indices{:}]) = 1;
A=A';

[Matt J]

I don't think there's a more efficient way of doing it than with a loop.

I don't think so either, nor do I expect a loop's performance to be bad.

Answer 4

[Joseph Cheng]

so... it is certainly a brain teaser to do this without a loop. so far i'm up to this.

A=zeros(3,4);
B = [1 3;2 4;1 1];

a=A';
B=B+repmat([0:4:4*(size(B,1)-1)]',1,2)
a(B)=1
a=a'

which leaves just filling in the zeros between ones without looping.

Answer 5

[Matt J]

     [M,N]=size(A);
       e=(1:M).';
      idx1=sub2ind([M,N], e, B(:,1));

       idx2a=B(:,2)<N;   
      idx2b=sub2ind([M,N], e(idx2a), B(idx2a,2)+1);

      A(idx1)=1;
      A(idx2b)=-1;
      A=cumsum(A,2)