Question on allocations for push pop of arrays

Need do confirm that if we do push! to array of a single element it reallocates with just one extra space for new element (in opposite to, say, preallocate 2 times more elements for better average performance).


Here’s how you can check:

using Plots
list = Int[]
plot([@allocated(push!(list, i)) for i in 1:1000], xaxis=:log10)
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To answer your question, you are probably looking for sizehint!, which lets you control how much extra space is preallocated.

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No, the allocation grows geometrically, by factors of 2, so that pushing n times does O(n) work.

So, I ran the following:

lst = Int64[]
size = 100000
plot([@allocated(push!(lst, i)) for i in 1:size])
plot([@allocated(pop!(lst)) for i in 1:size])

This outputs the following 2 plots. First appear to be confirming that capacity of array doubles, which is nice.

This is plot for when we do pop. I expected to also see relocation, but three are none. Can anyone help to explain?

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There aren’t allocations, just frees. Since you are shrinking, you only have to free the part of memory you aren’t using any more. You never need to reallocate anything new.

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