Assign values to elements of a matrix with indices less than those specified in an array
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Hi,
I'm not really sure how to word my question (hence the confusing title!). What I'm trying to do is take an array of values, like A=[2,4,1,3]
and use it to make a matrix B that looks like;
1 1 1 1
1 1 0 1
0 1 0 1
0 1 0 0
i.e. there are ones for rows lower than the value in the column from A. I can see how I'd do this using a for loop, but we'd like to speed up our (currently very slow) program, so I wondered if anybody had an idea how I could make this vectorised?
Cheers
4 Comments
dpb
on 20 Oct 2013
Just, forgot the length argument in B...
B=zeros(max(A)); % preallocate
for i=1:length(B)
B(1:A(i),i)=ones(A(i),1);
end
Accepted Answer
Azzi Abdelmalek
on 20 Oct 2013
Not the best solution, but it should work
A=[2,4,1,3]
B=zeros(max(A),numel(A))
idx=cell2mat(arrayfun(@(x) sub2ind(size(B),(1:A(x)),ones(1,A(x))*x),1:numel(A),'un',0))
B(idx)=1
3 Comments
Azzi Abdelmalek
on 20 Oct 2013
Or simply
A=[2,4,1,3];
n=max(A);
m=numel(A);
B=zeros(n,m);
for k=1:m
B(1:A(k),k)=1;
end
disp(B)
dpb
on 20 Oct 2013
Yep...simpler is often better. I just forgot to write the explicit length on the fly in original response.
More Answers (1)
Andrei Bobrov
on 20 Oct 2013
n = max(A);
nn = numel(A);
p = zeros(n,nn);
p(A + n*(0:nn-1))=1;
out = flipud(cumsum(flipud(p)));
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