How to make conditional calculation?

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Snoopy
Snoopy 2018-2-24
评论： Image Analyst 2018-2-25
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

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Image Analyst
Image Analyst 2018-2-24
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 个评论
Snoopy
Snoopy 2018-2-24
This the logical indexing Walter above mentions right? I now use it. Thanks a lot.
Image Analyst
Image Analyst 2018-2-25
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.]

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更多回答（1 个）

Walter Roberson
Walter Roberson 2018-2-24
if M(:,1) < 0
means the same thing as
if all(M(:,1) < 0)
which is probably not true.
Look up "logical indexing"
  2 个评论
Snoopy
Snoopy 2018-2-24
Thanks. I now use the find function which returns the row numbers (or the row index I shall say?) where the condition for vector one is satisfied. Then I use these row numbers as the row subscript in vector two, and carry out the operation on this subset of the entries of vector two. Is this perhaps the correct approach?

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