# Picking a random position in an Array

I need to pick a random position (not a random element) in an `Array`. I’ll assume a two dimensional array for this question. The first that came to mind was

``````function ridx(A)
return (rand(1:size(A,1)),rand(1:size(A,2)))
end
``````

But then, learning about `CartesianIndex`, I found one can also do

``````function ridx2(A)
return rand(CartesianIndices(A))
end
``````

which is simpler to understand and works for arrays of any dimension. I also thought it would be faster than `ridx`, with only one call to `rand` instead of 2. However, the opposite seems to be the case (by a tiny but consistent across several test difference).

So my question is, is `ridx2` a correct, reasonable and efficient use of `rand` and `CartesianIndices`, or am I missing something here?

3 Likes

both of them are basically constant and really fast, but for clarity you should consider using `ridx2`

``````julia> a = @btime (rand(axes(A,1)), rand(axes(A,2))) setup=(A=rand(50, 50))
6.577 ns (0 allocations: 0 bytes)
(7, 38)

julia> a = @btime (rand(axes(A,1)), rand(axes(A,2))) setup=(A=rand(500, 500))
6.633 ns (0 allocations: 0 bytes)
(132, 186)

julia> @btime rand(CartesianIndices(A)) setup=(A=rand(10,10));
8.075 ns (0 allocations: 0 bytes)

julia> @btime rand(CartesianIndices(A)) setup=(A=rand(100, 100))
8.080 ns (0 allocations: 0 bytes)
CartesianIndex(92, 66)

julia> @btime rand(CartesianIndices(A)) setup=(A=rand(500, 500))
8.098 ns (0 allocations: 0 bytes)
CartesianIndex(364, 243)
``````

I don’t believe 2ns will be your bottleneck

1 Like

On my laptop, the results are reversed. `rand(CartesianIndices(A))` is faster. As I would expect, since it only calls `rand` once.

2 Likes

I agree 2ns is not much (even if I will be using this in a Monte Carlo program where it will be called 10 to 100 million times in a typical run). Still, it doesn’t make sense to me that calling `rand` twice be faster than calling it once…

in case it is , and if `A` is a dense array, using linear indexing is faster (and IMO better readable)

``````julia> a = @btime (A[rand(eachindex(A))]) setup=(A=rand(500, 500))
8.500 ns (0 allocations: 0 bytes)

julia> a = @btime (A[rand(axes(A,1)), rand(axes(A,2))]) setup=(A=rand(500, 500))
12.137 ns (0 allocations: 0 bytes)

julia> @btime (A[rand(CartesianIndices(A))]) setup=(A=rand(500, 500))
11.178 ns (0 allocations: 0 bytes)
``````
1 Like

Interesting… Yes, I would have expected that to be the case.

Thanks @Eben60 but I need the indexes, not the element. This is because I am using the array to represent a lattice, and I need to select a site and later identify its neighbors. So I will need to convert to a tuple or `CartesianIndex` at some point.

I mean, counting surface number of “count” is not so useful… you don’t know what’s happening under the hood

Since linear indices work for arrays of any dimension in Julia, one can simply write `rand(eachindex(A))`. This will give you a single number as index. It is 2X faster than both `CartesianIndices` and calling `rand` twice on my PC and works for arrays of any dimensions.

it does not give you 2D location

``````julia> rand(CartesianIndices(rand(2,2)))
CartesianIndex(1, 1)

julia> rand(eachindex(rand(2,2)))
4
``````

I understand the 2D location is required here for accessing the neighbors later, then `CartesianIndices` would be the easiest. But linear indices will be faster in general and accessing the neighbors can still be easy by adding/subtracting `n` for right and left, or adding/subtracting `1` for below and above.

1 Like

Usual optimization when heavier random generation is used, is to batch random generation, even at the cost of memory:

``````julia> @btime maximum(i->(rand(1:size(A,1)),rand(1:size(A,2))),1:1024) setup=(A=rand(500, 500))
15.383 μs (0 allocations: 0 bytes)
(499, 495)

julia> @btime maximum(divrem.(rand(eachindex(A),1024),500)) setup=(A=rand(500, 500))
14.497 μs (2 allocations: 24.25 KiB)
(499, 488)
``````

Even the addition of `divrem` to obtain coordinate per dimension, isn’t enough to make it slower than individual calls to `rand`.

1 Like

This advice isn’t always applicable, especially since `@btime` only reports the minimum time. If you are performing this in an inner loop, excessive allocations lead to more GC calls, slowing the program down. `@benchmark` is much more enlightening (looking at mean or median for example) when comparing allocating vs non-allocating methods, especially if these two methods are being called many times and not just once.

Sure, you are right. GC adds complications. Using `rand!` on a preallocated vector solves this.

Thank you @jling @Seif_Shebl @Dan @jmair for the input. Bottom line seems to be: `CartesianIndices` are easier and clearer, but `LinearIndices` are faster. I’ll consider whether to reimplement neighbour finding using linear indices (the trickier part are of course the boundaries).

If using cartesian indices are more appropriate to your problem, I would suggest to use them… and it’s more ergonomic, e.g. you can easily get a vector (`rand(CartesianIndices(a), 100)`, which as noted in a previous post should be faster than individual calls to `rand`), or fill a preallocated vector (`rand!(output, CartesianIndices(a))`).

How much slower is the resolution of your problem with `CartesianIndices` compared to using linear indices?
If really necessary, you can cook a `rand` method for `CartesianIndices` which should close the performance gap:

``````import Random: Sampler
using Random: SamplerSimple

@generated function Sampler(::Type{RNG}, xx::CartesianIndices{N}, rep::Random.Repetition) where {N, RNG<:AbstractRNG}
samplers = [:(Sampler(RNG, axes(xx, \$ii))) for ii = 1:N]
:(SamplerSimple(xx, tuple(\$(samplers...))))
end

@generated function Base.rand(rng::AbstractRNG, sp::SamplerSimple{<:CartesianIndices{N}}) where {N}
rands = [:(rand(rng, sp.data[\$ii])) for ii = 1:N]
:(tuple(\$(rands...)))
end
``````

But it’s type piracy, so depending on the context, applying this to another type mimicking `CartesianIndices` could be more appropriate.

Thank you @rfourquet for you suggestion, I’l look into it. As for how slower my application is, in my first try it seemed to run about 30% slower. But it since to use CartesianIndices I had to make several changes, it is hard to know where to put the blame for the performance loss; I may have made a poor coding choice at some point. I tried several bits of code, and picking the random position seemed to be one of the culprits, so that’s why I asked the question.

I am now trying to introduce CartesianIndices making a minimum of changes at a time and checking for performance at each step, but this will take a while since I have other projects to deal with, and at least I have a functioning version now.

My current code (w/o CartesianIndices) is in GitHub if anyone wants to have a look.