If a value in one matrix matches that in another matrix, how do I assign the same index number?

I have a matrix with a list of latitude and longitude coordinates (64,800x3). Each latitude and longitude pair have their own index number.

Ex.

[Index    Lat     Lon;
 10521    -45.5    50.5;
 10522    -45.5    51.5;
 10523    -45.5    52.5]

I have another matrix with a list of latitude and longitude numbers, but no index numbers. Also, this matrix is not the same size as the previous matrix. (250x2; It has Lat and Lon only, no index number yet.)

I want to write a script that will examine the pairs of latitude and longitude coordinates for each matrix. When there is a pair that matches between the matrices, it would assign the same index number to the second matrix from the first matrix.

Answer 1

[Sebastian Castro]

Sure you can. You can use the find function to find the index at which the 2nd and 3rd column match, and then copy the elements over.

Your code could look like this:

origMat =   [10521    -45.5    50.5;
             10522    -45.5    51.5;
             10523    -45.5    52.5];

newMat =   [0    -45.5    52.5;
            0    -45.5    52.5;
            0    -45.5    51.5;
            0    -45.5    50.5];

% Loop through the new matrix
for ii = 1:size(newMat,1)
   % Find the index location where 2nd and 3rd column match
   idxLocation = origMat(:,2)==newMat(ii,2) & ... 
                 origMat(:,3)==newMat(ii,3);  
   % If valid, set the index number (1st column) of the new matrix
   if ~isempty(idxLocation)
        newMat(ii,1) = origMat(idxLocation,1);
   end
end

format short g
newMat

This gives me

newMat =
        10523        -45.5         52.5
        10523        -45.5         52.5
        10522        -45.5         51.5
        10521        -45.5         50.5

[Chris H]

This worked beautifully! Thank you so much!

[Stephen23]

Actually find is not required: simply remove the find and use logical indexing, thus making your code simpler and faster. Beginners often use find when logical indexing would be a simpler solution. The MATLAB editor will also underline this command and recommend removing it.

https://blogs.mathworks.com/steve/2008/01/28/logical-indexing/

https://www.mathworks.com/help/matlab/math/matrix-indexing.html

[Sebastian Castro]

Thanks -- let me edit that in the above post too.

[Stephen23]

@Sebastion Castro: you also might want to replace ~isempty with any.