Lesson 6.4: Logical indexing in MATLAB 

So many bits in it were funny. I like learning from Mr. Mike Fitzpatrick.

You need to have a vector v for that line to work. Try v = randn(1,20)

You could use a for loop and check every element or use this es.mathworks.com/help/matlab/ref/find.html k = find(X,n) returns the first n indices corresponding to the nonzero elements in X. As an example: a = 2*rand(16,1)-1 indexes = find(a > 0)