Replace Only Certain Values within a Matrix Based on Indices from Another

Hi all!

I have a 200 X 500 matrix. Within that matrix, only a handful of elements have actual numerical values. I have identified the index position of these numerical values (ind), and I will use the indices to look at another 200 x 500 matrix and find the values at the specified index position. (For example, if the first matrix had a numerical value at the 150th index position, I will look at the 150th index position of the second matrix and keep that value.) However, I want to keep ONLY the values from the second matrix at the index positions I identified in the first matrix, while all of the other values in the matrix are turned to zeros or NaNs. (Whereas the first matrix only has a handful of elements with actual numerical values, the second matrix has a numerical value for all elements.) So, I'm trying to keep the same matrix size (200 x 500), but I want to keep only the values at the index position I identified in the first matrix. Any idea how to do this?

Here's what I have so far...

x ***first matrix (only a few elements have numerical values)
y ***second matrix (all elements have numerical values)
ind = find(x~=0)
y(ind) ***this only gives me the values at the index position but I would like to keep values within matrix structure

Answer 1

[Michelle De Luna]

Got it! Thanks for the help! :)

x
y
ind = find(x~=0)

index=setdiff(1:numel(y),ind);
y(index)=0
y

Answer 2

[Image Analyst]

See if this gives you what you want:

y = y .* ind;

If not, then attach your x and y in a .mat file and tell us what you expect as an output.

[Michelle De Luna]

Image Analyst, thank you for responding so quickly! I tried running your code, but the error message states the matrix dimensions must agree (i.e. the "ind" variable is of size 200x1 while the "y" variable is a matrix of size 200x500). What I hope my output will look like is a 200x500 matrix mostly filled with zeros, with the exception of numerical values at the "ind" positions. Let me try to upload a .mat file (it's my first time uploading, so let me see how it goes).

[Image Analyst]

You should have used

ind = x ~= 0;  % Don't use find()