# In-place assignment, without allocation
**URL:** https://discourse.julialang.org/t/in-place-assignment-without-allocation/103811
**Category:** Performance
**Tags:** indexing, inplace
**Created:** [September 13, 2023, 12:37pm UTC](https://discourse.julialang.org/t/in-place-assignment-without-allocation/103811 "2023-09-13T12:37:06Z")
**Posts on this page:** 9
**Page:** 1
### Author:  [@user664303](https://discourse.julialang.org/u/user664303)
#### Post date: [September 13, 2023, 12:37pm UTC](https://discourse.julialang.org/t/in-place-assignment-without-allocation/103811/1 "2023-09-13T12:37:06Z")
I’m trying to do `y = x[ind]` in-place, where `y, x, ind` are all vectors. By in-place, I mean no memory is allocated. I’m able to achieve this using a hand-written for loop (`assign2!` below). I suspect there is a way to achieve this using broadcasting, which could potentially generate better code. However, I haven’t been able to find it. `assign!` below is my attempt at it. Is there a simple line of code using broadcasting that works (doesn’t allocate, & is fast)? Why doesn’t the `@.` find it?
```julia
using BenchmarkTools, Random
x = randn(100)
y = zeros(200)
ind = vcat(randperm(100), randperm(100))
assign!(y, x, ind) = @inbounds @. y = x[ind]
@btime assign!($y, $x, $ind);
function assign2!(y, x, ind)
@simd for i in eachindex(ind)
@inbounds y[i] = x[ind[i]]
end
end
@btime assign2!($y, $x, $ind);
```
---
### Author:  [@skleinbo](https://discourse.julialang.org/u/skleinbo)
#### Post date: [September 13, 2023, 1:01pm UTC](https://discourse.julialang.org/t/in-place-assignment-without-allocation/103811/2 "2023-09-13T13:01:30Z")
```julia
assign!(y,x,ind) = y.=getindex.(Ref(x), ind)
```
is allocation free. The problem in your example is that `x[ind]` is not a `broadcasted`, hence it doesn’t fuse with the assignment.
By the way, the `@simd` is useless here.
---
### Author:  [@frylock](https://discourse.julialang.org/u/frylock)
#### Post date: [September 13, 2023, 1:02pm UTC](https://discourse.julialang.org/t/in-place-assignment-without-allocation/103811/3 "2023-09-13T13:02:09Z")
EDIT: I think I misunderstood the question - sorry.
Granted, this has one allocation, but this seems pretty fast to me:
```julia
x = randn(10)
y = zeros(10)
@btime y .= x
22.051 ns (1 allocation: 16 bytes)
```
---
### Author:  [@frylock](https://discourse.julialang.org/u/frylock)
#### Post date: [September 13, 2023, 1:11pm UTC](https://discourse.julialang.org/t/in-place-assignment-without-allocation/103811/4 "2023-09-13T13:11:19Z")
Why is it that this does not allocate:
```julia
@btime assign!(y, x, 3)
10.063 ns (0 allocations: 0 bytes)
```
… but this does?
```julia
@btime y .= getindex.(x, 3)
19.719 ns (1 allocation: 32 bytes)
```
---
### Author:  [@user664303](https://discourse.julialang.org/u/user664303)
#### Post date: [September 13, 2023, 1:12pm UTC](https://discourse.julialang.org/t/in-place-assignment-without-allocation/103811/5 "2023-09-13T13:12:41Z")
> [@frylock](#):
>
> Granted, this has one allocation, but this seems pretty fast to me
Regardless of the speed, it is strange to me that this allocates.
---
### Author:  [@DanielVandH](https://discourse.julialang.org/u/DanielVandH)
#### Post date: [September 13, 2023, 1:15pm UTC](https://discourse.julialang.org/t/in-place-assignment-without-allocation/103811/6 "2023-09-13T13:15:07Z")
You need to do `@btime $y .= $x`, which shows no allocations.
---
### Author:  [@frylock](https://discourse.julialang.org/u/frylock)
#### Post date: [September 13, 2023, 1:15pm UTC](https://discourse.julialang.org/t/in-place-assignment-without-allocation/103811/7 "2023-09-13T13:15:45Z")
I’m not yet good enough to unravel everything, but the best I can figure is if I wrote it in C, it would look something like:
```plaintext
int x[10];
int y[10]; // or whatever
for (int i = 0; i < 10; ++i)
y[i] = x[i];
```
The allocation would be the loop variable?
---
### Author:  [@skleinbo](https://discourse.julialang.org/u/skleinbo)
#### Post date: [September 13, 2023, 1:15pm UTC](https://discourse.julialang.org/t/in-place-assignment-without-allocation/103811/8 "2023-09-13T13:15:54Z")
> [@frylock](#):
>
> Granted, this has one allocation
> [@user664303](#):
>
> Regardless of the speed, it is strange to me that this allocates.
> [@frylock](#):
>
> … but this does?
It doesn’t, but you must remember to interpolate external variables in benchmark expressions
```julia
julia> @btime $y .= $x
15.415 ns (0 allocations: 0 bytes)
```
---
### Author:  [@skleinbo](https://discourse.julialang.org/u/skleinbo)
#### Post date: [September 13, 2023, 1:17pm UTC](https://discourse.julialang.org/t/in-place-assignment-without-allocation/103811/9 "2023-09-13T13:17:45Z")
> [@frylock](#):
>
> The allocation would be the loop variable?
No, “allocation” means heap allocation which need to be tracked and eventually garbage collected. That counter would live happily on the stack.