Replace a value if a condition is met

Hi, I got a little problem and I hoped someone can help me out.

When I have a matrix consisting of 3 columns, I want to replace a value in the third column if a row in the first column has a certain value. For example, I have the following matrix:

A =

     3     6     9
     2     6     8
     3     6     8

Lets call the first column x, the second y and the third z. Now what I want is that when x == 3, than z must become 20.

How can I do this?

Answer 1

[Akira Agata]

Please try the following:

idx = A(:,1) == 3;
A(idx,3) = 20;

[Danny Helwegen]

Dear Akira Agata,

thx for the fast response, is it in this way also possible to say if x == 3 and y == 3 than z must become 40?

[Image Analyst]

Of course.

rowsToChange = A(:, 1) == 3 & A(:, 2) == 3;
A(rowsToChange, 3) = 40;

You should really learn how logical indexing works. Since you don't know (yet), you're missing out on one of the most powerful capabilities of MATLAB.

[Danny Helwegen]

This works, thank you very much and I will use the tip

[Akira Agata]

> Image Analyst

Thank you for your comment!