Efficiently loop through structs in Julia

Question

I have a simple question. I have defined a struct, and I need to inititate a lot (in the order of millions) of them and loop over them.

I am initiating one at a time and going through the loop as follows:

using Distributions

mutable struct help_me{Z<:Bool}
    can_you_help_me::Z
    millions_of_thanks::Z
end

for i in 1:max_iter
    tmp_help = help_me(rand(Bernoulli(0.5),1)[1],rand(Bernoulli(0.99),1)[1])
    # many follow-up processes 
end

The memory allocation scales up in max_iter. For my purpose, I do not need to save each struct. Is there a way to "re-use" the memory allocation used by the struct?

Answer 1

Your main problem lies here:

rand(Bernoulli(0.5),1)[1], rand(Bernoulli(0.99),1)[1]

You are creating a length-1 array and then reading the first element from that array. This allocates unnecessary memory and takes time. Don't create an array here. Instead, write

rand(Bernoulli(0.5)), rand(Bernoulli(0.99))

This will just create random scalar numbers, no array.

Compare timings here:

julia> using BenchmarkTools

julia> @btime rand(Bernoulli(0.5),1)[1]
  36.290 ns (1 allocation: 96 bytes)
false

julia> @btime rand(Bernoulli(0.5))
  6.708 ns (0 allocations: 0 bytes)
false

6 times as fast, and no memory allocation.

This seems to be a general issue. Very often I see people writing rand(1)[1], when they should be using just rand().

Also, consider whether you actually need to make the struct mutable, as others have mentioned.