Extracting the first contiguous values without looping

Hi everyone,

I need to make a decision based values in a matrix and would like to avoid nested loops. I used arrayfun to produce a matrix of boolean values, but am only interested in the first set of contiguous truths . So, for example, if:

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

B = [ 1 1 0  0 0 0; % first two bits retained
      0 0 1  1 1 0; % unch
      1 0 0  0 0 0; % only first bit retained
      0 0 1  1 1 1] % unch

The options I've come up with are a bit caddy-whompus and are row-based, so they're no good. These involved (1) casting / strfind or (2) find (first 1 in B, then subsequent 0 in ~B).

Any suggestions?

Regards,

Noel

Answer 1

[Azzi Abdelmalek]

A = [ 1 1 0  0 1 1;
    0 0 1  1 1 0;
    1 0 1  1 0 0;
    0 0 1  1 1 1]
B=diff(A,[],2)
[n,m]=size(A);
idx=arrayfun(@(x) min([find(B(x,:)==-1,1)+1 m+1]),1:n)
for k=1:n
    A(k,idx(k):end)=0
end

%Or

A = [ 1 1 0  0 1 1;
    0 0 1  1 1 0;
    1 0 1  1 0 0;
    0 0 1  1 1 1]
B=diff(A,[],2)
[n,m]=size(A);
idx=arrayfun(@(x) min([find(B(x,:)==-1,1)+1 m+1]),1:n)
ii=cell2mat(arrayfun(@(x) [idx(x):m;x*ones(1,m-idx(x)+1)],1:n,'un',0))
jj=sub2ind(size(A),ii(2,:),ii(1,:))
A(jj)=0

[Noel Khan]

Thank you for the prompt reply!

[Noel Khan]

Azzi's code has a lot of gems -- it took a few rounds between the docs and tinkering to understand what it was doing. For those working on a similar problem here's the code (largely Azzi's) I'm running with:

A =

       1     1     0     0     1     1
       0     0     1     1     1     0
       1     0     1     1     0     0
       0     0     1     1     1     1

B=diff(A,[],2);

[m,n]=size(A);

idx=arrayfun(@(x) min([find(B(x,:)==-1,1) n]), 1:m);

cell2mat(arrayfun(@(x) [A(x,1:idx(x)),zeros(1,n-idx(x))], 1:m, 'UniformOutput', 0)')

ans =

       1     1     0     0     0     0
       0     0     1     1     1     0
       1     0     0     0     0     0
       0     0     1     1     1     1

Thanks again Azzi!

Regards,

Noel