# BenchmarkTools keep failing in a loop
**URL:** https://discourse.julialang.org/t/benchmarktools-keep-failing-in-a-loop/41261
**Category:** New to Julia
**Created:** [June 12, 2020, 11:43am UTC](https://discourse.julialang.org/t/benchmarktools-keep-failing-in-a-loop/41261 "2020-06-12T11:43:41Z")
**Posts on this page:** 5
**Page:** 1
### Author:  [@tirthasheshpatel](https://discourse.julialang.org/u/tirthasheshpatel)
#### Post date: [June 12, 2020, 11:43am UTC](https://discourse.julialang.org/t/benchmarktools-keep-failing-in-a-loop/41261/1 "2020-06-12T11:43:41Z")
I am trying to use `BenchmarkTools` to benchmark some of my algorithms but I keep getting an error I don’t really understand. Why does it keep failing in a loop? I think the `@benchmark` macro isn’t a blocking line due to which `len` is going out of scope. I am not sure but is this expected?
```julia
julia> using BenchmarkTools
julia> for len ∈ 1:10:1000
@benchmark sort!(a) setup=(a=randn(len));
end
ERROR: UndefVarError: len not defined
Stacktrace:
[1] ##sample#256(::BenchmarkTools.Parameters) at C:\Users\tirth\.julia\packages\BenchmarkTools\eCEpo\src\execution.jl:373
[2] _run(::BenchmarkTools.Benchmark{Symbol("##benchmark#254")}, ::BenchmarkTools.Parameters; verbose::Bool, pad::String, kwargs::Base.Iterators.Pairs{Symbol,Integer,NTuple{4,Symbol},NamedTuple{(:samples, :evals, :gctrial, :gcsample),Tuple{Int64,Int64,Bool,Bool}}}) at C:\Users\tirth\.julia\packages\BenchmarkTools\eCEpo\src\execution.jl:405
[3] (::Base.var"#inner#2"{Base.Iterators.Pairs{Symbol,Integer,NTuple{5,Symbol},NamedTuple{(:verbose, :samples, :evals, :gctrial, :gcsample),Tuple{Bool,Int64,Int64,Bool,Bool}}},typeof(BenchmarkTools._run),Tuple{BenchmarkTools.Benchmark{Symbol("##benchmark#254")},BenchmarkTools.Parameters}})() at .\essentials.jl:715
[4] #invokelatest#1 at .\essentials.jl:716 [inlined]
[5] #run_result#37 at C:\Users\tirth\.julia\packages\BenchmarkTools\eCEpo\src\execution.jl:32 [inlined]
[6] run(::BenchmarkTools.Benchmark{Symbol("##benchmark#254")}, ::BenchmarkTools.Parameters; progressid::Nothing, nleaves::Float64, ndone::Float64, kwargs::Base.Iterators.Pairs{Symbol,Integer,NTuple{5,Symbol},NamedTuple{(:verbose, :samples, :evals, :gctrial, :gcsample),Tuple{Bool,Int64,Int64,Bool,Bool}}}) at C:\Users\tirth\.julia\packages\BenchmarkTools\eCEpo\src\execution.jl:94
[7] #warmup#45 at C:\Users\tirth\.julia\packages\BenchmarkTools\eCEpo\src\execution.jl:141 [inlined]
[8] warmup(::BenchmarkTools.Benchmark{Symbol("##benchmark#254")}) at C:\Users\tirth\.julia\packages\BenchmarkTools\eCEpo\src\execution.jl:141
[9] top-level scope at C:\Users\tirth\.julia\packages\BenchmarkTools\eCEpo\src\execution.jl:287
[10] top-level scope at REPL[2]:2
```
I didn’t see many docs but a quick solution will be appreciated!
---
### Author:  [@Skoffer](https://discourse.julialang.org/u/Skoffer)
#### Post date: [June 12, 2020, 11:50am UTC](https://discourse.julialang.org/t/benchmarktools-keep-failing-in-a-loop/41261/2 "2020-06-12T11:50:54Z")
```julia
for len ∈ 1:10:1000
@benchmark sort!(a) setup=(a=randn($len));
end
```
---
### Author:  [@tirthasheshpatel](https://discourse.julialang.org/u/tirthasheshpatel)
#### Post date: [June 12, 2020, 11:52am UTC](https://discourse.julialang.org/t/benchmarktools-keep-failing-in-a-loop/41261/3 "2020-06-12T11:52:41Z")
Oh, I was not aware of `$` syntax. Thank you so much @Skoffer
---
### Author:  [@Skoffer](https://discourse.julialang.org/u/Skoffer)
#### Post date: [June 12, 2020, 11:54am UTC](https://discourse.julialang.org/t/benchmarktools-keep-failing-in-a-loop/41261/4 "2020-06-12T11:54:43Z")
Just in case, relevant part of the documentation: [https://github.com/JuliaCI/BenchmarkTools.jl/blob/master/doc/manual.md#interpolating-values-into-benchmark-expressions](https://github.com/JuliaCI/BenchmarkTools.jl/blob/master/doc/manual.md#interpolating-values-into-benchmark-expressions)
---
### Author:  [@tirthasheshpatel](https://discourse.julialang.org/u/tirthasheshpatel)
#### Post date: [June 12, 2020, 11:59am UTC](https://discourse.julialang.org/t/benchmarktools-keep-failing-in-a-loop/41261/5 "2020-06-12T11:59:05Z")
Thanks for that too! 🙂