# UndefVarError when using @turbo on nested for loop whose inner loop depends on value in outer loop
**URL:** https://discourse.julialang.org/t/undefvarerror-when-using-turbo-on-nested-for-loop-whose-inner-loop-depends-on-value-in-outer-loop/125477
**Category:** General Usage
**Tags:** package, loopvectorization
**Created:** [February 2, 2025, 4:34pm UTC](https://discourse.julialang.org/t/undefvarerror-when-using-turbo-on-nested-for-loop-whose-inner-loop-depends-on-value-in-outer-loop/125477 "2025-02-02T16:34:04Z")
**Posts on this page:** 3
**Page:** 1
### Author:  [@leespen1](https://discourse.julialang.org/u/leespen1)
#### Post date: [February 2, 2025, 4:34pm UTC](https://discourse.julialang.org/t/undefvarerror-when-using-turbo-on-nested-for-loop-whose-inner-loop-depends-on-value-in-outer-loop/125477/1 "2025-02-02T16:34:04Z")
I am trying to optimize some for-loops in a package I am working on, so I decided to check out `LoopVectorization`. I ran into some errors, and constructed the following MWE:
```julia
using LoopVectorization
function f(jmax,kmax)
@assert jmax >= 1
@assert kmax >= 1
s = 0
@turbo for j in 1:jmax
for k in 1:kmax
s += 1
end
end
return s
end
function g(jmax)
@assert jmax >= 1
s = 0
@turbo for j in 1:jmax
for k in 1:j
s += 1
end
end
return s
end
sf = f(5,5)
println("sf=",sf)
sg = g(5)
println("sg=",sg)
```
When I run this script, I get
```julia
ERROR: LoadError: UndefVarError: `j` not defined in `Main`
Suggestion: check for spelling errors or missing imports.
Stacktrace:
[1] macro expansion
@ ~/.julia/packages/LoopVectorization/tIJUA/src/condense_loopset.jl:1179 [inlined]
[2] g(jmax::Int64)
@ Main ~/Research/LoopVectorization/loop_vectorization.jl:20
[3] top-level scope
@ ~/Research/LoopVectorization/loop_vectorization.jl:30
[4] include(fname::String)
@ Main ./sysimg.jl:38
[5] top-level scope
@ REPL[8]:1
in expression starting at /home/spencer/Research/LoopVectorization/loop_vectorization.jl:30
```
Could someone help me understand why this fails? Using `@turbo` on a nested format should be fine, see the following example from the documentation.
```julia
function A_mul_B!(C, A, B)
@turbo for n ∈ indices((C,B), 2), m ∈ indices((C,A), 1)
Cmn = zero(eltype(C))
for k ∈ indices((A,B), (2,1))
Cmn += A[m,k] * B[k,n]
end
C[m,n] = Cmn
end
end
```
The README on github warns against the following:
1. Are not indexing an array out of bounds. `@turbo` does not perform any bounds checking.
2. Are not iterating over an empty collection. Iterating over an empty loop such as `for i ∈ eachindex(Float64[])` is undefined behavior, and will likely result in the out of bounds memory accesses. Ensure that loops behave correctly.
3. Are not relying on a specific execution order. `@turbo` can and will re-order operations and loops inside its scope, so the correctness cannot depend on a particular order. You cannot implement `cumsum` with `@turbo`.
4. Are not using multiple loops at the same level in nested loops.
Certainly 1. and 2. do not apply.
I don’t think 3. applies. The values `k` can take depend on which iteration of the outer loop we are on, but which order those happen in doesn’t matter. I.e. it only matters that we do `s += 1` for `(j,k)` equal to `(1,1)`, `(2,1)`, `(2,2)`, `(3,1)`, `(3,2)`, `(3,2)`, regardless of which order that iteration is performed.
I don’t think 4. applies, if I understand correctly. I think it is warning against doing something like
```julia
for i in range1
for j in range2
[...]
end
for k in range3
[...]
end
end
```
In any case, it’s conceivable to me that a function `g` might be something `LoopVectorization` wasn’t designed to handle, but the `UndefVarError` doesn’t tell me anything, and makes it look like I made a syntax mistake more than anything else.
---
### Author:  [@danielwe](https://discourse.julialang.org/u/danielwe)
#### Post date: [February 2, 2025, 5:42pm UTC](https://discourse.julialang.org/t/undefvarerror-when-using-turbo-on-nested-for-loop-whose-inner-loop-depends-on-value-in-outer-loop/125477/2 "2025-02-02T17:42:00Z")
I suppose it’s not mentioned in the README, but see the docs [Getting Started · LoopVectorization.jl](https://juliasimd.github.io/LoopVectorization.jl/stable/getting_started/)
> Currently LoopVectorization only supports rectangular iteration spaces, although I plan on extending it to triangular and ragged iteration spaces in the future. This means that if you nest multiple loops, the number of iterations of the inner loops shouldn’t be a function of the outer loops.
---
### Author:  [@leespen1](https://discourse.julialang.org/u/leespen1)
#### Post date: [February 2, 2025, 8:57pm UTC](https://discourse.julialang.org/t/undefvarerror-when-using-turbo-on-nested-for-loop-whose-inner-loop-depends-on-value-in-outer-loop/125477/3 "2025-02-02T20:57:54Z")
Thanks! It would be nice if the macro could catch the non-rectangular nested loop and give a more descriptive error. Not that I’m complaining; this package does low-level optimizations on a level that is magic to me.