How to iterate over two arrays in parallel

7

votes

I have two arrays that I want to iterate over at the same time.

I'm using this:

julia> xs = [1,2,3];

julia> ys = [4,5,6];

julia> for i in 1:length(xs)
           x = xs[i]
           y = ys[i]
           @show x, y
       end
(x, y) = (1, 4)
(x, y) = (2, 5)
(x, y) = (3, 6)

Is there a better way to iterate over multiple arrays in Julia?

julia

11

votes

Use zip along with tuple destructuring:

julia> xs = [1,2,3];

julia> ys = [4,5,6];

julia> for (x, y) in zip(xs, ys)
           @show x, y
       end
(x, y) = (1, 4)
(x, y) = (2, 5)
(x, y) = (3, 6)

zip will stop iteration at the shortest array:

julia> for (x, y) in zip([1,2], [0,0,0])
           @show x, y
       end
(x, y) = (1, 0)
(x, y) = (2, 0)

This pattern can be generalized to an arbitrary number of lists:

julia> for (x, y, z) in zip([1,2], [3,4], [5,6])
           @show x, y, z
       end
(x, y, z) = (1, 3, 5)
(x, y, z) = (2, 4, 6)

4

votes

One possibility consists in using the eachindex function: if it is given multiple Array-like arguments, it will return a iterable set of indices suitable to iterate on all arguments at once.

This is useful in particular in the following situations:

when you need to use the index itself (for example because you don't only need to access the elements of the collections, but also set some of them), or
when you want to check that both arrays indeed have the same number of elements (this might or might not be a desired property depending on your use case).

Example 1: using the index itself to fill the first array with values coming from the second

julia> x = [1,2,3];

julia> y = [4,5,6];
julia> @inbounds for i in eachindex(x, y)
           x[i] = 2*y[i]
       end
julia> x
3-element Array{Int64,1}:
  8
 10
 12

Example 2: check that the arrays have the same range

julia> x = [1,2];
julia> y = [4,5,6];

julia> @inbounds for i in eachindex(x, y)
           x[i] = 2*y[i]
       end
ERROR: DimensionMismatch("all inputs to eachindex must have the same indices, got [1, 2] and [1, 2, 3]")

Example 3: note that eachindex generalizes well for multi-dimensional arrays too.

julia> x = zeros(2, 3);
julia> y = ones(2, 3);

julia> @inbounds for i in eachindex(x, y)
           x[i] = 2*y[i]
       end
julia> x
2×3 Array{Float64,2}:
 2.0  2.0  2.0
 2.0  2.0  2.0

2

votes

You can iterate over multiple collections using map and foreach. For example, with map:

julia> x, y = 1:3, 4:6;

julia> map(hypot, x, y)
3-element Array{Float64,1}:
 4.123105625617661
 5.385164807134504
 6.708203932499369

For more complicated multi-line anonymous functions, you can use do-block syntax:

julia> xs, ys = 1:4, 10:10:40;

julia> map(xs, ys) do x, y
           if isodd(x)
               x + y
           else
               x * y
           end
       end
4-element Array{Int64,1}:
  11
  40
  33
 160

foreach is very similar to map, but is intended for use when a function is applied for its side effect, like printing or plotting, rather than its return value. An example with foreach:

julia> x, y = ["a", "b", "c"], 1:3;

julia> foreach(println ∘ ^, x, y)
a
bb
ccc

Note the use of the function composition operator in the foreach call.

How to iterate over two arrays in parallel

3 Answers