For loop in Julia. Syntax confusion

Question

I am a complete noob in Julia and its syntax. I am trying to follow this article on semi-definite-programming on Julia.

I would apprecieate if someone can help me figure out what the for loop in In[4] actually does:

for i in 1:m
A[:, (i-1)*n+1:i*n] .= random_mat_create(n)
b[i] = tr(A[:, (i-1)*n+1:i*n]*X_test)
end

To my understanding it should create a vector of matrices A (m of those) as well as an m-dimensional vector b. I am totally confused though on the indexing of A and the indexing of b.

I would like an explanation of the :, (i-1)*n+1:i*n part of this code. The reason I ask here is because I also dont know what to Google or what to search for in Julia documentation.

Answer 1

(i-1)*n+1:i*n creates a range from (i-1)*n + 1 to i*n. For example, if i=2 and n=10, this range becomes 11:20, so A[:, (i-1)*n+1:i*n] will grab all the rows of A (that's what : does), and columns 11-20.