# How to speedup setindex! and getindex?
**URL:** https://discourse.julialang.org/t/how-to-speedup-setindex-and-getindex/124413
**Category:** General Usage
**Tags:** speed-optimization
**Created:** [January 3, 2025, 4:29pm UTC](https://discourse.julialang.org/t/how-to-speedup-setindex-and-getindex/124413 "2025-01-03T16:29:23Z")
**Posts on this page:** 4
**Page:** 1
### Author:  [@Thomas\_Van\_Giel](https://discourse.julialang.org/u/Thomas_Van_Giel)
#### Post date: [January 3, 2025, 4:29pm UTC](https://discourse.julialang.org/t/how-to-speedup-setindex-and-getindex/124413/1 "2025-01-03T16:29:23Z")
I have a piece of code where the setindex! and getindex function take a lot of time to execute. Quite simplified, the code looks a bit like this:
```julia
using Random
function main()
A = rand(0:0.1:100000, 200,200)
for step in 1:100000
for i in 1:200
for j in 1:200
incr = (1 - 0.5) * 0.1
A[i,j] += incr
end
end
end
end
@profview main()
```
When I run the above code and profile the time it takes to execute, I get the following result:
Most of the time is spent using the setindex! function. Is there a way to speed this up, or do I have to live with the fact that it will be this slow.
---
### Author:  [@adienes](https://discourse.julialang.org/u/adienes)
#### Post date: [January 3, 2025, 4:41pm UTC](https://discourse.julialang.org/t/how-to-speedup-setindex-and-getindex/124413/2 "2025-01-03T16:41:52Z")
I will try two answers
- this whole loop looks equivalent to `A .+= steps * incr`, but I assume this block is just a placeholder for more complex logic?
- of course all the work is in `setindex!` — the loop is doing nothing else! so I would answer your question with a question “what else would you expect to see in the profile”
---
### Author:  [@mbauman](https://discourse.julialang.org/u/mbauman)
#### Post date: [January 3, 2025, 4:41pm UTC](https://discourse.julialang.org/t/how-to-speedup-setindex-and-getindex/124413/3 "2025-01-03T16:41:59Z")
> [@Thomas\_Van\_Giel](#):
>
> ```julia
> for i in 1:200
> for j in 1:200
> incr = (1 - 0.5) * 0.1
> A[i,j] += incr
>
> ```
Swap the order of your for loops; you’re not accessing sequential elements but rather skipping from column to column.
A decent rule of thumb is that you want the `i`s and `j`s to appear symmetrically in the order that’s written “closest” to the array, in both the indexing _and_ the for loops. In other words, you currently have the sequence `i` → `j` → `A` → `i` → `j`. This should be `j` → `i` → `A` → `i` → `j`
You can do even better by putting the `step` loop as the innermost one, but I’m guessing this is reflective of your real problem.
---
### Author:  [@DNF](https://discourse.julialang.org/u/DNF)
#### Post date: [January 3, 2025, 8:50pm UTC](https://discourse.julialang.org/t/how-to-speedup-setindex-and-getindex/124413/4 "2025-01-03T20:50:58Z")
> [@Thomas\_Van\_Giel](#):
>
> ```julia
> for i in 1:200
> for j in 1:200
>
> ```
It’s not obvious which parts of this MWE reflects your real problem, but both for safety and for efficiency, you should use `for i in axes(A, 1)` etc, or simply `eachindex`. Then you can avoid bounds checks, which can significantly improve performance.