Problem 640. Getting logical indexes

This is a basic MATLAB operation. It is for instructional purposes.

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Logical indexing works like this.

thresh = 4;
vec    = [1 2 3 4 5 6 7 8];

vi     = (vec > thresh)

vi =

       0     0     0     0     1     1     1     1

Once you have this TRUE FALSE vector (I call it vi: Valid Indices)

It can be used to get the values out:

big = vec(vi)

big =

       5     6     7     8

Given a vector, vec, and a value, v, return a binary vector that represents the indices where vector, vec, is equal to scalar, v.

Note, this works just as well with scalars and matrices.

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To get the indices where this comparison is true, see this Cody problem.