Changing Matrix Elements Automatically

[RB_10520285]

I have a large matrix representing an image, 4000 columns, 2000 rows. I would like to change the values of some of the matrix elements and these elements are defined in a pair of vectors. Trying to automate the process because of the extreme size and complexity. I have attached a small example of what I am doing (and doing wrong) in which I have set up a FOR loop. This loop changes all of the elements, not just the ones I want. Help? See attached example.

[terryhendicott · Accepted solution]

[RB_10520285]

Thank you! Very cool. Learned a lot, lots to learn.

[LucMeekes]

For this purpose the built-in, but undocumented, function update2() comes in handy:

Success!
Luc