How can I threshold a matrix and get the coordinates of those values?

I have a matrix, X with dimensions 132X132, with values ranging from -0.1 to 0.4. I'd like to threshold it to values above 0.2, and most importantly, obtain the coordinates to which those values belonged to in the previous matrix.

Thank you.

Answer 1

[MUHAMMED IRFAN]

ind = find(X>.2);

This gives the index of all the points in X greater than 0.2 using linear indexing.

To convert the single indices to subscripts,

[I,J] = ind2sub([132,132],ind);

This gives x and y coordinates of all points with values greater than 0.2

To get the values of all those points,Go

myPoints = X(find(X>0.2));

[Stephen23]

Note that indexing with one index is called linear indexing in the MATLAB documentation:

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

[Guillaume]

If you want 2D coordinates out of find just ask for them rather than getting linear indices and converting them into 2d indices:

[row, column] = find(X > 0.2);  %no need for ind = find(X>0.2) followed by [row, column] = ind2sub(...)

Similarly, find can also give you the values. It's the third output:

[row, column, value] = find(X > 0.2);  %much faster than X(find(X>0.2))

Also, note that as a filter, find is never needed. You can use the logical array input directly as a filter, so:

X(find(X > 0.2))

is a waste of time,

X(X > 0.2)

is faster and simpler.

Finally, it's never a good idea to hardcode the size of the matrices. If you had to use ind2sub,

[row, column] = ind2sub(size(X), ind);  %instead of ind2sub([132 132], ind)

would be a lot safer.