# How to make conditional calculation?

14 views (last 30 days)
Snoopy on 24 Feb 2018
Commented: Image Analyst on 25 Feb 2018
I have two vectors, contained in matrix M, each with a dimension of 1000 by 1. Each vector contains integer numbers. The values in one vector (call it vector two) is generated in way that it depends on the values in the other vector (call it vector one). I want to calculate the variance of the numbers in vector two if the values are below a certain threshold in vector one. I try to use the code below, but it returns nothing. Where am I making the mistake?
if M(:,1) < 0
var(M(:,2))
end

Image Analyst on 24 Feb 2018
Maybe you mean this:
indexesToConsider = M(:, 1) < 0; % Consider the index if the M value is less than 0.
col2Variance = var(M(indexesToConsider, 2)); % Compute variance of only those rows.
##### 2 CommentsShowHide 1 older comment
Image Analyst on 25 Feb 2018
Yes, it's logical indexing in that it gives values of true and false. If you passed that into find() you'd get "linear indexes" which are the actual index numbers, like [1, 3, 5, 33, 45, etc.] instead of [1 0 1 0 1 0 0 etc.]

Walter Roberson on 24 Feb 2018
if M(:,1) < 0
means the same thing as
if all(M(:,1) < 0)
which is probably not true.
Look up "logical indexing"
##### 2 CommentsShowHide 1 older comment
Walter Roberson on 24 Feb 2018
That is one approach, but it is more efficient to use logical indexing. https://www.mathworks.com/help/matlab/math/matrix-indexing.html#bq7egb6-1