Slicing Multidimensional Array by a variable

Question

I am writing a method which accepts a two dimensional array of doubles and an int row number as parameters and returns the highest value of the elements in the given row.

it looks like this:

function getHighestInRow(A, i)
        return(maximum(A[:i,:]))
end

the issue i am having is when i slice the array with

A[:i,:]

I get an argument error because the :i makes i get treated differently.

the code works in the other direction with

A[:,i,:]

Is there a way to escape the colon? so that i gets treated as a variable after a colon?

For better performance it might be better to use a view or to code it up explicitly with a for loop — Dan Getz
– Dan Getz, Commented Oct 17, 2017 at 15:00
Sure but sometimes small views aren't fast either. Let me try it out. — Michael K. Borregaard
– Michael K. Borregaard, Commented Oct 17, 2017 at 15:02
You're right, the view is always faster. I can never work out what the actual rules are when slicing is faster, I thought that was for small arrays. — Michael K. Borregaard
– Michael K. Borregaard, Commented Oct 17, 2017 at 15:06

Michael K. Borregaard · Accepted Answer · 2017-10-17 15:09:52Z

3

You're doing something strange with the colon. In this case you're using the symbol :i not the value of i. Just getHighestInRow(A,i) = maximum(A[i,:]) should work.

Edit: As Dan Getz said in the comment on the question, getHighestInRow(A,i) = maximum(@view A[i,:]) is more efficient, though, as the slicing will allocate a temporary unnecessary array.

edited Oct 17, 2017 at 15:09

answered Oct 17, 2017 at 15:00

Michael K. Borregaard

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