# Meta.parse + eval does not work as expected in the loop
**URL:** https://discourse.julialang.org/t/meta-parse-eval-does-not-work-as-expected-in-the-loop/71356
**Category:** New to Julia
**Created:** [November 11, 2021, 11:33pm UTC](https://discourse.julialang.org/t/meta-parse-eval-does-not-work-as-expected-in-the-loop/71356 "2021-11-11T23:33:37Z")
**Posts on this page:** 7
**Page:** 1
### Author:  [@ryszard314159](https://discourse.julialang.org/u/ryszard314159)
#### Post date: [November 11, 2021, 11:33pm UTC](https://discourse.julialang.org/t/meta-parse-eval-does-not-work-as-expected-in-the-loop/71356/1 "2021-11-11T23:33:37Z")
I would expect the first for loop below to work, but apparently for some reason loop index (`n`) is not recognized in eval(xpr). I am guessing it has something to do with scoping etc.
Is there a way to make it work “as expected” in this case i.e. to nudge `eval(Meta.parse())` to recognize loop index?
```julia
xpr = Meta.parse("3^n")
for n in 1:3 # ERROR: UndefVarError: n not defined
println("n= ", n, ", xpr= ", xpr, ", eval(xpr)= ", eval(xpr))
end
for nn in 1:3 # this works as expected
global n = nn
println("n= ", n, ", 3^n= ", eval(Meta.parse("3^n")))
end
```
---
### Author:  [@jling](https://discourse.julialang.org/u/jling)
#### Post date: [November 11, 2021, 11:51pm UTC](https://discourse.julialang.org/t/meta-parse-eval-does-not-work-as-expected-in-the-loop/71356/2 "2021-11-11T23:51:41Z")
`eval` always always happens in the global scope, don’t do `eval` in loop
---
### Author:  [@ryszard314159](https://discourse.julialang.org/u/ryszard314159)
#### Post date: [November 12, 2021, 1:12am UTC](https://discourse.julialang.org/t/meta-parse-eval-does-not-work-as-expected-in-the-loop/71356/3 "2021-11-12T01:12:51Z")
I do not really understand potential ramifications of allowing `eval()` work in local scope as well, I guess, but it would be very handy. For now, with the silly `global n = nn` above I can do what I need to do.
---
### Author:  [@Oscar\_Smith](https://discourse.julialang.org/u/Oscar_Smith)
#### Post date: [November 12, 2021, 1:16am UTC](https://discourse.julialang.org/t/meta-parse-eval-does-not-work-as-expected-in-the-loop/71356/4 "2021-11-12T01:16:22Z")
`eval` working in the local scope is exactly why languages like python and R are impossible to optimize. In general, the fact that you are trying to do this implies you are doing something wrong.
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### Author:  [@ryszard314159](https://discourse.julialang.org/u/ryszard314159)
#### Post date: [November 12, 2021, 2:09am UTC](https://discourse.julialang.org/t/meta-parse-eval-does-not-work-as-expected-in-the-loop/71356/5 "2021-11-12T02:09:27Z")
I would appreciate some suggestion what can I do to avoid using `eval()` in this case.
Basically, I want the user of the function (invoked from the command line script) to be able to pass simple (but arbitrary) function (`ufun`) to evaluate some value dependent on the loop index. For example:
```julia
function foo(maxit, ufun)
for n in 1:maxit
v = ufun(n)
[...]
end
end
```
where `ufun(n)` can be e.g. `2^n`, `n^2`, `6*n`, etc.
---
### Author:  [@dlakelan](https://discourse.julialang.org/u/dlakelan)
#### Post date: [November 12, 2021, 2:16am UTC](https://discourse.julialang.org/t/meta-parse-eval-does-not-work-as-expected-in-the-loop/71356/6 "2021-11-12T02:16:03Z")
parse the code, then construct a function at the global scope level, then in your code call that function.
```julia
expr = Meta.parse("x^2")
fdefexpr = :(x -> $(expr))
f = eval(fdefexpr)
f(2)
```
Be very careful about the fact that you are evaluating potentially arbitrary code provided by the user…
---
### Author:  [@ryszard314159](https://discourse.julialang.org/u/ryszard314159)
#### Post date: [November 12, 2021, 1:22pm UTC](https://discourse.julialang.org/t/meta-parse-eval-does-not-work-as-expected-in-the-loop/71356/7 "2021-11-12T13:22:02Z")
Thanks a ton! It is very helpful. […]
> Be very careful about the fact that you are evaluating potentially arbitrary code provided by the user…
I am the only user so far… but definitely very good point!