CODEWITHSUNDEEP
matlabmatrix
Christina
Christina

Reputation: 935

Test if a coordinate (x,y) is inside a matrix

I have a matrix D as follows:

  D=[ 
    1 2  
    3 4  
    5 6
       ]

What I want to do is to test if my coordinate at position (ii,jj) belongs to a one line in D.

In other terms, I have a for loop, and inside it, I want to increase a variable by 1 if the condition that verifies "(ii & jj) belongs to a line of D" is true.

For example, we can do this manually:

var = 0;
for ii = 1: 20
    for jj = 1:30
        if((ii==1 && jj==2) || (ii==3 && jj==4) || (ii==5 && jj==6))
var = var + 1;
end
    end
        end

This is straightforward since the matrix D contains only 3 lines. But in my work, I have of about 1000 lines, so D is of size 1000*2. In this case, I have to find a method which can do the same job as I have written above but automatically and in a very fast manner. But how?

In fact one can think to use ismember, for example:

var = 0;
for ii = 1 : 20
    for jj = 1 : 30
        if(ismember(ii, D(:,1)) && ismember(jj, D(:,2)))
var = var + 1;
end
    end
        end

But this is not correct, since for example, the code just above can find ii=1 and jj=6, but in this case (ii,jj) will be (1,6) and which does not belong to any of (1,2), (3,4) and (5,6).

Please, any help will be very appreciated!

Upvotes: 0

Views: 100

Answers (1)

Suever
Suever

Reputation: 65430

You can use the 'rows' input to ismember to check if your array is a row within the matrix

tf = ismember([ii,jj], D, 'rows');

You can also check multiple values of ii and jj at once

% Create all permutations of ii and jj
[ii,jj] = ndgrid(1:20, 1:30);

% Test all of these permutations to see which ones are in D
tf = ismember([ii(:), jj(:)], D, 'rows');

And then to compute var

var = sum(tf);

If you are dealing with non-integers, you should use ismembertol instead to deal with possible floating point errors.

Upvotes: 3

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