MATLAB Staying in bounds of Complex matrix

Question

I have a complex matrix cdata , that is 2144x2048 . I am getting elements from cdata that are greater than a specified threshold by doing the following:

[row, col] = find(abs(cdata) > threshold);

row and col can have multiple values. Then, I take the row and col values, I perform a calculation to get N samples of the real x-data, and 33 samples of the y-data as follows:

xdata = real(cdata(row(i),col(i)-bw:col(i)+bw))
ydata = real(cdata(row(i)-bw:row(i)+bw,col(i)-bw:col(i)+bw))

where bw is a constant value that determines the number of samples I need to get. During this calculation, specifically the column portion of cdata for the xdata and the row portion of cdata for the ydata , I exceed the bounds of the matrix and MATLAB throws the following error:

??? Subscript indices must either be real positive integers or logicals

How can I ensure that I don't exceed the bounds? I'm ok with having to skip a row/col pair if it is going to exceed the bounds.

Answer 1

The reason you're having problems is because you're not restricting your search to closer then bw from the edge of the matrix. This means its possible to find values above the threshold near the edges of the matrix. When you add or subtract bw from these indices you end up out of bounds. You can restrict your search like this.

[row, col] = find(abs(cdata(bw+1:end-bw,bw+1:end-bw)) > threshold);
row = row + bw;
col = col + bw;

This guarantees your row and column indices are within the bounds so when you grab a region surrounding them you won't go out of bounds.

On a side note. The ydata variable in your code is indexing an entire square region of the matrix and the xdata is only indexing a section of a row. Should your ydata actually be ydata = real(cdata(row(i)-bw:row(i)+bw, col(i))) ?