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matlabmatrixmatrix-indexing

Select all BUT certain index pairs in multi-dimensional array Matlab

I am trying to select all BUT certain index pairs in a multi-dimensional array. i.e. I have a set of paired indices (e.g. [1,2] and [4,5]). I want to set all BUT those indexed pairs to 0.

The closest I have come to this is:

A(setdiff(1:length(A(:,1)),lon),setdiff(1:length(A(1,:)),lat)) = 0;

, where A is the matrix and lon and lat are the index pairs I want to keep. However, that also leaves all the intersecting rows and columns of those pairs.

Any ideas?

Here is some example code:

A = ones([5,5])
A =

     1     1     1     1     1
     1     1     1     1     1
     1     1     1     1     1
     1     1     1     1     1
     1     1     1     1     1

lon = [1];

lat = [4];

A(setdiff(1:length(A(:,1)),lon),setdiff(1:length(A(1,:)),lat)) = 0

A =

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

What I want is:

A =

     0     0     0     1     0
     0     0     0     0     0
     0     0     0     0     0
     0     0     0     0     0
     0     0     0     0     0
Solution

  • The easiest thing to do is actually the opposite of what you have tried. First you want to start with a matrix of zeros and then only fill in those pairs that you have stored in lat and lon. Also because you have paired subscripts, you will want to convert those to a linear index using sub2ind

    %// Convert subscripts to a linear index
inds = sub2ind(size(A), lon, lat);

%// Start off with a matrix of zeros
B = zeros(size(A));

%// Fill in the values at the specified lat/lon from A
B(inds) = A(inds);