Extracting the first contiguous values without looping

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Noel Khan
Noel Khan 2014-3-24
评论： Noel Khan 2014-3-25
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

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采纳的回答

Azzi Abdelmalek
Azzi Abdelmalek 2014-3-24
编辑：Azzi Abdelmalek 2014-3-24
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
  2 个评论
Noel Khan
Noel Khan 2014-3-25
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

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