Julia: how to iterate row by row in a multidimensional array

Question

Say that I have 4x4 multidimensional array A:

A = collect(reshape(1:16, 4, 4))
4×4 Array{Int64,2}:
 1  5   9  13
 2  6  10  14
 3  7  11  15
 4  8  12  16

and I want to iterate row by row (i.e. [1, 5, 9, 13] first, then [2, 6, 10, 14], then ...).

How do I do it? For now I have come up with the following:

`for row in 1:size(A, 1)
    println(A[row, :])
    # do something
end`

but I was wondering if there was a more "pythonic" way of doing it: kind of for line in A: for element in line: ....

I also know about CartesianRange, but I would like to have an array-like row to work with at each iteration.

This question may be relevant: stackoverflow.com/questions/21778374/… — Alex Riley
– Alex Riley, Commented Oct 30, 2017 at 16:49
Possible duplicate of What is the recommended way to iterate a matrix over rows? — mbauman
– mbauman, Commented Oct 30, 2017 at 16:53
Your PS isn't a dumb question at all — I'd split it out and ask it independently of the iterate-by-rows question. — mbauman
– mbauman, Commented Oct 30, 2017 at 16:54

guest3457823 · Accepted Answer · 2020-01-23 09:16:51Z

11

for visibility: nowadays (julia > 1.1) use "eachrow"

for row in eachrow(A)
    println(row)
end

answered Jan 23, 2020 at 9:16

guest3457823

1111 silver badge3 bronze badges

Sign up to request clarification or add additional context in comments.

Comments

Chris Rackauckas · Accepted Answer · 2017-10-30 18:44:38Z

1

Because arrays in Julia are stored column-major, it may be wiser / more performant to just transpose the matrix (A') and then iterate through it if you want to do a lot of things row-by-row.

answered Oct 30, 2017 at 18:44

Chris Rackauckas

19.2k3 gold badges53 silver badges91 bronze badges

4 Comments

Pigna Over a year ago

Ok, but once it is transposed how do i get the behavior I want? I looked in to the suggested duplicate and the only way is through slicing. I don't get the "line" as in Python

Chris Rackauckas Over a year ago

Once you made the column-major matrix, you might as well use views.

gggg Over a year ago

Here are two issues asking for a function rows to enable for row in rows(A) github.com/JuliaLang/julia/issues/14491 and github.com/JuliaCollections/IterTools.jl/issues/11

Liso Over a year ago

Maybe A.' is better answer. Currently nor A' nor A.' support structural transpose and something like permutedims(A, (2,1)) is necessary if A is for example array of strings. See: github.com/JuliaLang/julia/issues/20978 (there is chance that A.' will do structural transpose in Julia 1.0)

tholy · Accepted Answer · 2017-11-01 16:21:49Z

1

A very nice package for this kind of thing is JuliennedArrays.

answered Nov 1, 2017 at 16:21

tholy

12.3k2 gold badges31 silver badges42 bronze badges

Collectives™ on Stack Overflow

Julia: how to iterate row by row in a multidimensional array

3 Answers 3

Comments

4 Comments

Comments

Your Answer

Linked

Hot Network Questions

Collectives™ on Stack Overflow

3 Answers 3

Comments

4 Comments

Comments

Your Answer

Sign up or log in

Post as a guest

Linked

Related