# Pluto and macros

**URL:** <https://discourse.julialang.org/t/pluto-and-macros/60600>\
**Category:** New to Julia\
**Tags:** pluto\
**Created:** [May 5, 2021, 5:38pm UTC](https://discourse.julialang.org/t/pluto-and-macros/60600 "2021-05-05T17:38:31Z")\
**Posts on this page:** 3\
**Page:** 1

<div class="post-metadata">

**Author:** ![gmouts](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/gmouts/32/9896_2.png) [@gmouts](https://discourse.julialang.org/u/gmouts)\
**Post date:** [May 5, 2021, 5:38pm UTC](https://discourse.julialang.org/t/pluto-and-macros/60600/1 "2021-05-05T17:38:31Z")

</div>

I need to benchmark a function and I decided to use Pluto. I get a variable not defined error that I don’t know how to avoid. A minimal (not) working example is:

```julia
for i in 1:length(ar)
	b = @benchmark i^2
end

```

Is there any way to use the benchmark macro in a loop in Pluto or should I just write a script and run it?

---

<div class="post-metadata">

**Author:** ![Mason](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/mason/32/2423_2.png) [@Mason](https://discourse.julialang.org/u/Mason)\
**Post date:** [May 5, 2021, 5:45pm UTC](https://discourse.julialang.org/t/pluto-and-macros/60600/2 "2021-05-05T17:45:40Z")

</div>

Did you try this outside of Pluto? I don’t think this has anything to do with Pluto.

`@benchmark` will actually involve using `eval`, so it doesn’t know about locally scoped variables like `i` here. However, you can interpolate those variables into the benchmarked expression like this:

```julia
for i in 1:length(ar)
	b = @benchmark ($i)^2
end

```

Now however, julia’s compiler is being given too much information and is totally optimizing this away:

```julia
julia> let i = 10
           @benchmark ($i)^2
       end
BenchmarkTools.Trial: 
  memory estimate: 0 bytes
  allocs estimate: 0
  --------------
  minimum time: 0.019 ns (0.00% GC)
  median time: 0.020 ns (0.00% GC)
  mean time: 0.024 ns (0.00% GC)
  maximum time: 0.120 ns (0.00% GC)
  --------------
  samples: 10000
  evals/sample: 1000

```

Whenever you see a benchmark that takes less than a nanosecond, that’s a strong indication that nothing actually happened in the benchmark loop.

In order to actually benchmark what you want to do, you have to block the compiler from seeing `i` as a constant. One way to do that would be to wrap it in a mutable `Ref` like this:

```julia
julia> let i = Ref(10)
           @benchmark ($i[])^2
       end
BenchmarkTools.Trial: 
  memory estimate: 0 bytes
  allocs estimate: 0
  --------------
  minimum time: 1.329 ns (0.00% GC)
  median time: 1.340 ns (0.00% GC)
  mean time: 1.360 ns (0.00% GC)
  maximum time: 157.327 ns (0.00% GC)
  --------------
  samples: 10000
  evals/sample: 1000

```

Now that is properly benchmarking the thing I think you want to measure.

---

<div class="post-metadata">

**Author:** ![gmouts](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/gmouts/32/9896_2.png) [@gmouts](https://discourse.julialang.org/u/gmouts)\
**Post date:** [May 5, 2021, 6:00pm UTC](https://discourse.julialang.org/t/pluto-and-macros/60600/3 "2021-05-05T18:00:39Z")

</div>

I tried it in the mean time in a script and I got the same error. ☹

How do I use `Ref`? Do I just write

```julia
xr = Ref(x)
@benchmark f($xr[])

```

instead of

```julia
@benchmark f(x)

```

?