# Allocations in a copy! construct
**URL:** https://discourse.julialang.org/t/allocations-in-a-copy-construct/7323
**Category:** General Usage
**Tags:** memory-allocation
**Created:** [November 26, 2017, 3:44pm UTC](https://discourse.julialang.org/t/allocations-in-a-copy-construct/7323 "2017-11-26T15:44:03Z")
**Posts on this page:** 5
**Page:** 1
### Author:  [@mforets](https://discourse.julialang.org/u/mforets)
#### Post date: [November 26, 2017, 3:44pm UTC](https://discourse.julialang.org/t/allocations-in-a-copy-construct/7323/1 "2017-11-26T15:44:03Z")
Consider the following variations of in-place copying:
```julia
lower = 1.:3
upper = 12.:16
function a()
points = zeros(100)
allocs = @allocated copy!(points, [lower[1:end-1]; upper[1:end-1]])
println("case a: $allocs bytes")
resize!(points, length(lower)+length(upper)-2)
end
function b()
points = zeros(100)
allocs = @allocated begin
copy!(points, lower[1:end-1])
copy!(points, length(lower), upper[1:end-1])
end
println("case b: $allocs bytes")
resize!(points, length(lower)+length(upper)-2)
end
function c()
points = zeros(100)
allocs = @allocated begin
copy!(points, @view(lower[1:end-1]))
copy!(points, length(lower), @view(upper[1:end-1]))
end
println("case c: $allocs bytes")
resize!(points, length(lower)+length(upper)-2)
end
```
The result that i get (in the second run), is:
```julia
julia> a(); b(); c();
case a: 320 bytes
case b: 192 bytes
case c: 1728 bytes
```
- In case a, does `[lower[1:end-1]; upper[1:end-1]]` as a second argument to `copy!` create a temporary vector, while in case b it doesn’t, explaining that they differ roughly in twice as many allocations?
- For case c, why does it allocate that much, shouldn’t it be the same as case b?
---
### Author:  [@antoine-levitt](https://discourse.julialang.org/u/antoine-levitt)
#### Post date: [November 26, 2017, 4:07pm UTC](https://discourse.julialang.org/t/allocations-in-a-copy-construct/7323/2 "2017-11-26T16:07:59Z")
This is the global scope problems. If you declare lower and upper as const, only a() allocates.
---
### Author:  [@mforets](https://discourse.julialang.org/u/mforets)
#### Post date: [November 26, 2017, 4:17pm UTC](https://discourse.julialang.org/t/allocations-in-a-copy-construct/7323/3 "2017-11-26T16:17:12Z")
Huh, i thought that enclosing the `@allocated` macros inside the function was enough. Thanks for the tip!!
Now cases b and c allocate exactly 0 bytes, now i see that’s the behavior to be expected there.
---
### Author:  [@antoine-levitt](https://discourse.julialang.org/u/antoine-levitt)
#### Post date: [November 26, 2017, 4:30pm UTC](https://discourse.julialang.org/t/allocations-in-a-copy-construct/7323/4 "2017-11-26T16:30:00Z")
Yes, global variables are evil (hopefully they’ll get optimized a bit at some point). There’s a trick though : it only does not allocate because lower and upper are ranges. If they are arrays (with collect() in front of the ranges) then b and c allocate. b has to allocate because indexing is a copy, the fact that c allocates is a bug (I think it’s on the devs radar, but low priority)
---
### Author:  [@mforets](https://discourse.julialang.org/u/mforets)
#### Post date: [November 26, 2017, 4:42pm UTC](https://discourse.julialang.org/t/allocations-in-a-copy-construct/7323/5 "2017-11-26T16:42:43Z")
Ok, interesting. Then `copy!(points, lower[1:end-1])` didn’t create temporaries just because `lower` is a lazy array, i overlooked that. Trying with arrays behaves as you say, case c allocating less than b (but not exactly zero).