MATLAB: Assign values to elements of a matrix with indices less than those specified in an array

matrix array assign indexing

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;

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

Best Answer

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

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MATLAB: Distance between elements of two matrices

I think this is what you're after. It's a bit brute-force, but I can't think of a neater, vectorized way.

% Make some matrices
n = 4;
M = magic(n);
A = 1-bsxfun(@eq,M,max(M,[],2))
B = ones(n);
B(n:n-1:end-1) = 0
% Actual code:
f = @(k) abs(find(A(k,:)==0,1,'first')-find(B(k,:)==0,1,'first'));
arrayfun(f,1:size(A,1))

If you don't like arrayfun, feel free to replace it with a for-loop (which is all I'm really doing):

d = zeros(size(A,1),1);
for k = 1:size(A,1)
    d(k) = abs(find(A(k,:)==0,1,'first')-find(B(k,:)==0,1,'first'));
end

MATLAB: Finding the number of rows to the next row containing a 1

It took some time, but here is a solution that should also work for large matrices.

clc,clear
format compact
A = [0 0 0 1 0 1 1 1 0 0 0 0 0 1]';
%    3 2 1 0 1 0 0 0 5 4 3 2 1 0
B=A;
pad=B(end)~=1;
if pad,B(end+1)=1;end %this method requires the last position to be a 1
B=flipud(B);
C=zeros(size(B));
C(B==1)=[0;diff(find(B))];
C=ones(size(B))-C;
out=cumsum(C)-1;
out=flipud(out);
if pad,out(end)=[];end
%only for display:
[A out]

Best Answer

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MATLAB: Distance between elements of two matrices

MATLAB: Finding the number of rows to the next row containing a 1

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