Indexing a matrix in matlab according to conditions set on other matrices

Question

I am trying to index my matrix based on two conditions, I'll explain.

Let's say I have two matrices:

a = [7 3 4; 5 6 7; 4 8 0];
b = [1 9 8; 2 4 6; 6 1 6];

And a third matrix to index:

c = [1 2 3; 4 5 6; 7 8 9];

My aim is to index c in a way that I get a 3x3 matrix in which only the values of c are copied over for whose indexes the following conditions are met and the rest are zeros.

a <= 5, b >= 6

Resulting matrix:

result = [0 2 3; 0 0 0; 7 0 9]

I hope I was able to explain my problem.

score 1 · Accepted Answer · answered Jun 21 '20 at 17:15

1

Given

a = [7 3 4; 5 6 7; 4 8 0];
b = [1 9 8; 2 4 6; 6 1 6];
c = [1 2 3; 4 5 6; 7 8 9];
result = zeros(size(c);

Using logical indexing,

>> d = (a <= 5) & (b >= 6)
d =
  0  1  1
  0  0  0
  1  0  1

>> result(d) = c(d)
result =
   0   2   3
   0   0   0
   7   0   9

answered Jun 21 '20 at 17:15

beaker

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This is exactly what I was looking for, thank you so much! – Nikit Parakh Jun 21 '20 at 17:19
1

@NikitParakh You might be interested in [this Q&A](https://stackoverflow.com/questions/32379805/linear-indexing-logical-indexing-and-all-that). – beaker Jun 21 '20 at 17:22

score 0 · Answer 2 · edited Jun 21 '20 at 19:53

0

Loop throw rows and columns and set the result.

for row=1:size(a,1)
    for col=1:size(a,2)
          if(a(row,col)> b(row,col))
              result(row,col) = 0
          else
               result(row,col) = c(row,col)
          end  
    end
end

edited Jun 21 '20 at 19:53

Paolo

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answered Jun 21 '20 at 17:07

vishnu sandhireddy

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What if the conditions are a<=3, b>=8? – Nikit Parakh Jun 21 '20 at 17:12
And is there a way to do this without loops? Because this may work for smaller matrices, but with larger ones, it may take a lot of time. – Nikit Parakh Jun 21 '20 at 17:14
1

MATLAB doesn't run in JavaScript snippets. It would be better to use normal code formatting. – beaker Jun 21 '20 at 17:15
If you are going to use a loop, you should at least preallocate `result` to avoid slowing down due to memory reallocation – Luis Mendo Jun 21 '20 at 19:06

Indexing a matrix in matlab according to conditions set on other matrices

2 Answers2