How to replace values in a specific column of a matrix within a defined index range in MATLAB?

Question

I have the following matrix in MATLAB where the first column contains just ones:

How can I replace the values in column 1 (regardless of what the original value is) with a new value (e.g. 99) within a specific index range (e.g. just for rows 9 to 12) to obtain this:

If I use the following command I can replace all the 1 in the first column with 99:

finalMatrix(finalMatrix(:,1) == 1,1) = 99;

To replace just the values in column 1 from index 9 to 12 I tried this

finalMatrix(finalMatrix(9:12,1) == 1,1) = 99;

but it doesn't work.

Any suggestion?

Answer 1

When you do

finalMatrix(finalMatrix(9:12,1) == 1,1) = 99;

you will actually replace the first 4 elements of finalMatrix, assuming that the corresponding values in 9:12 were ==1. The reason is that Matlab does the following steps:

1) generate a 4x1 matrix finalMatrix(9:12,1)
2) figure out which of the four elements is ==1
3) return a boolean array (four elements long) corresponding to those elements
4) perform logical indexing on finalMatrix with these four elements -
   which now reference the first four elements of the first column of finalMatrix

You just need

finalMatrix(9:12,1) = 99;

Or, if you care that the values were 1 to begin with, you could do

indx = find(finalMatrix(9:12,1)==1);
finalMatrix(indx+8,1)=1;