# Why does Julia allocate memory when I already pre-allocated?
**URL:** https://discourse.julialang.org/t/why-does-julia-allocate-memory-when-i-already-pre-allocated/65223
**Category:** Performance
**Created:** [July 24, 2021, 8:43am UTC](https://discourse.julialang.org/t/why-does-julia-allocate-memory-when-i-already-pre-allocated/65223 "2021-07-24T08:43:00Z")
**Posts on this page:** 6
**Page:** 1
### Author:  [@greatpet](https://discourse.julialang.org/u/greatpet)
#### Post date: [July 24, 2021, 8:43am UTC](https://discourse.julialang.org/t/why-does-julia-allocate-memory-when-i-already-pre-allocated/65223/1 "2021-07-24T08:43:00Z")
Look at the code here.
```julia
julia> results = Vector{Int}(undef, 10^7);
julia> @time for i in 1:10^7
results[i] = i
end
0.315568 seconds (20.00 M allocations: 305.160 MiB, 5.86% gc time)
```
Why is there memory allocation and GC run, when the vector was allocated beforehand?
---
### Author:  [@Skoffer](https://discourse.julialang.org/u/Skoffer)
#### Post date: [July 24, 2021, 8:49am UTC](https://discourse.julialang.org/t/why-does-julia-allocate-memory-when-i-already-pre-allocated/65223/2 "2021-07-24T08:49:23Z")
`results` is global variable.
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### Author:  [@greatpet](https://discourse.julialang.org/u/greatpet)
#### Post date: [July 24, 2021, 9:00am UTC](https://discourse.julialang.org/t/why-does-julia-allocate-memory-when-i-already-pre-allocated/65223/3 "2021-07-24T09:00:54Z")
Do you mean that when an array is declared in global scope, I actually get an array of pointers to heap-allocated objects?
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### Author:  [@GunnarFarneback](https://discourse.julialang.org/u/GunnarFarneback)
#### Post date: [July 24, 2021, 9:07am UTC](https://discourse.julialang.org/t/why-does-julia-allocate-memory-when-i-already-pre-allocated/65223/4 "2021-07-24T09:07:08Z")
More likely there’s a bit of boxing and unboxing going on in each iteration of the loop while executing an unspecialized `setindex!`.
If you place the loop in a function with `results` as an argument or make `results` a const global you get more efficient code.
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### Author:  [@greatpet](https://discourse.julialang.org/u/greatpet)
#### Post date: [July 24, 2021, 9:20am UTC](https://discourse.julialang.org/t/why-does-julia-allocate-memory-when-i-already-pre-allocated/65223/5 "2021-07-24T09:20:19Z")
Thanks for the tips. I fixed the problem. I just had to add `const` before declaring the array `results`, and the memory allocation disappeared. I was aware that non-`const` globals are bad, but didn’t realize it would affect code as simple as this.
---
### Author:  [@Skoffer](https://discourse.julialang.org/u/Skoffer)
#### Post date: [July 24, 2021, 9:25am UTC](https://discourse.julialang.org/t/why-does-julia-allocate-memory-when-i-already-pre-allocated/65223/6 "2021-07-24T09:25:16Z")
The simplicity of code matters not. If variable is global it allocates, so you should either use `const` or wrap in `let` block or wrap in a function. You can read more here: [Performance Tips · The Julia Language](https://docs.julialang.org/en/v1/manual/performance-tips/#Avoid-global-variables)