# Metaprogramming using eval and struct: BUG getting UndefVarError: not defined
**URL:** https://discourse.julialang.org/t/metaprogramming-using-eval-and-struct-bug-getting-undefvarerror-not-defined/37937
**Category:** General Usage
**Tags:** question, bug, eval, struct
**Created:** [April 21, 2020, 12:23am UTC](https://discourse.julialang.org/t/metaprogramming-using-eval-and-struct-bug-getting-undefvarerror-not-defined/37937 "2020-04-21T00:23:17Z")
**Posts on this page:** 20
**Page:** 1
### Author:  [@JosephPollacco](https://discourse.julialang.org/u/JosephPollacco)
#### Post date: [April 21, 2020, 12:23am UTC](https://discourse.julialang.org/t/metaprogramming-using-eval-and-struct-bug-getting-undefvarerror-not-defined/37937/1 "2020-04-21T00:23:17Z")
> I am trying to read parameters from a .csv file for which the name of parameter and its values are given in the .csv file.
>
> For that I am trying to simply evaluate a simple expression involving putting the value of the parameter in a **struct** , but I get error.
> I will be delighted if you could provide me a solution to this simple problem?
```julia
# Defining structure
mutable struct PARAM
θs :: Vector{Float64}
end
θs = zeros(Float64, 1)
hydro = PARAM(θs)
Evaluate =" hydro.θs[1]=0.5"
Evaluate_Parse = Meta.parse(Evaluate)
eval(Evaluate_Parse)
```
. Nevertheless, I get the following error message
_ERROR: LoadError: UndefVarError: hydro not defined_
---
### Author:  [@KDr2](https://discourse.julialang.org/u/KDr2)
#### Post date: [April 21, 2020, 1:16am UTC](https://discourse.julialang.org/t/metaprogramming-using-eval-and-struct-bug-getting-undefvarerror-not-defined/37937/2 "2020-04-21T01:16:27Z")
It runs well if just paste your code in REPL, do you have the definition of `hydro` and the execution of `eval` in different modules?
---
### Author:  [@dpsanders](https://discourse.julialang.org/u/dpsanders)
#### Post date: [April 21, 2020, 1:26am UTC](https://discourse.julialang.org/t/metaprogramming-using-eval-and-struct-bug-getting-undefvarerror-not-defined/37937/3 "2020-04-21T01:26:51Z")
Why don’t you just use a `Dict` and avoid metaprogramming?
---
### Author:  [@JosephPollacco](https://discourse.julialang.org/u/JosephPollacco)
#### Post date: [April 21, 2020, 3:25am UTC](https://discourse.julialang.org/t/metaprogramming-using-eval-and-struct-bug-getting-undefvarerror-not-defined/37937/4 "2020-04-21T03:25:31Z")
There seem to be a bug since you are correct the code works well in REPL but fails in a function.
Yes I have the definition of hydro and the execution of eval in a different modules
---
### Author:  [@WschW](https://discourse.julialang.org/u/WschW)
#### Post date: [April 21, 2020, 3:29am UTC](https://discourse.julialang.org/t/metaprogramming-using-eval-and-struct-bug-getting-undefvarerror-not-defined/37937/5 "2020-04-21T03:29:49Z")
`eval` can only see variables in the global scope, it cannot see local variables.
---
### Author:  [@JosephPollacco](https://discourse.julialang.org/u/JosephPollacco)
#### Post date: [April 21, 2020, 3:36am UTC](https://discourse.julialang.org/t/metaprogramming-using-eval-and-struct-bug-getting-undefvarerror-not-defined/37937/6 "2020-04-21T03:36:06Z")
Thanks for shedding light to the problem, how can I make **mutable struct** into **global scope**?
---
### Author:  [@KDr2](https://discourse.julialang.org/u/KDr2)
#### Post date: [April 21, 2020, 4:17am UTC](https://discourse.julialang.org/t/metaprogramming-using-eval-and-struct-bug-getting-undefvarerror-not-defined/37937/7 "2020-04-21T04:17:34Z")
It’s not about `mutable sutrct PARAM`, it’s about the variable `hydro`.
- `hydro` must be global in some Module
- `eval` must be ensured to evaluate an expression in that Module.
In the code below, `hydro` is a global var in module `T`, and `T.eval` is used to ensure that the parsed expression are evaluated in module `T`.
```julia
julia> module T
mutable struct PARAM
θs :: Vector{Float64}
end
θs = zeros(Float64, 1)
hydro = PARAM(θs)
end
Main.T
julia> Evaluate =" hydro.θs[1]=0.5"
" hydro.θs[1]=0.5"
julia> Evaluate_Parse = Meta.parse(Evaluate)
:(hydro.θs[1] = 0.5)
julia> eval(Evaluate_Parse)
ERROR: UndefVarError: hydro not defined
Stacktrace:
[1] top-level scope at none:1
[2] eval at ./boot.jl:331 [inlined]
[3] eval(::Expr) at ./client.jl:451
[4] top-level scope at REPL[4]:1
julia> T.eval(Evaluate_Parse)
0.5
julia> T.hydro
Main.T.PARAM([0.5])
julia>
```
---
### Author:  [@WschW](https://discourse.julialang.org/u/WschW)
#### Post date: [April 21, 2020, 4:48am UTC](https://discourse.julialang.org/t/metaprogramming-using-eval-and-struct-bug-getting-undefvarerror-not-defined/37937/8 "2020-04-21T04:48:24Z")
Generally it is advisable to avoid using `eval`. From your example I do not see why you need `eval` at all as directly using `hydro.θs[1] = 0.5` without using `eval` would work.
If your use case requires names that you will only know after you load the CSV I would suggest storing the data in either a `Dict` or a `DataFrame` depending on your use case. Both of these allow you to index by name.
---
### Author:  [@JosephPollacco](https://discourse.julialang.org/u/JosephPollacco)
#### Post date: [April 21, 2020, 8:42pm UTC](https://discourse.julialang.org/t/metaprogramming-using-eval-and-struct-bug-getting-undefvarerror-not-defined/37937/9 "2020-04-21T20:42:04Z")
Thanks for your great help and your time. Unfortunately when I introduced your recommendation into the complex program, I was not able to make it work, I am still getting the same error. I think that I would need more clarity on the meaning of **Main.T**?
---
### Author:  [@mbauman](https://discourse.julialang.org/u/mbauman)
#### Post date: [April 21, 2020, 8:52pm UTC](https://discourse.julialang.org/t/metaprogramming-using-eval-and-struct-bug-getting-undefvarerror-not-defined/37937/10 "2020-04-21T20:52:14Z")
`T` is just the name of the module that KDr2 is using to hold onto your global variable. All variables you want to change/assign/update/touch with `eval` would need to be globals there — and you need to ensure that you’re explicitly `eval`ing into that module.
But I really think you’ll want to change your angle of attack here — using `eval` like this isn’t very sustainable or maintainable. The other alternatives proposed here will likely make your life much easier. Another option would be a file format like [TOML](https://github.com/JuliaLang/TOML.jl) that can work nicely for what I think you’re trying to do.
---
### Author:  [@JosephPollacco](https://discourse.julialang.org/u/JosephPollacco)
#### Post date: [April 24, 2020, 9:24am UTC](https://discourse.julialang.org/t/metaprogramming-using-eval-and-struct-bug-getting-undefvarerror-not-defined/37937/11 "2020-04-24T09:24:28Z")
**PROBLEM RESOLVED 😄**
I want to thank you for inviting me to take a different approach which I solved as follow:
```julia
mutable struct PARAM
θs :: Vector{Float64}
end
setfield!(hydro, Symbol(θs), Parameter)
```
I get the value of the parameter θs from from a .csv file. This effort is to perform multistep optimisation.
---
### Author:  [@dpsanders](https://discourse.julialang.org/u/dpsanders)
#### Post date: [April 24, 2020, 2:18pm UTC](https://discourse.julialang.org/t/metaprogramming-using-eval-and-struct-bug-getting-undefvarerror-not-defined/37937/12 "2020-04-24T14:18:35Z")
@JosephPollacco Note that this is the same as
`hydro.θs = parameter`
if I understand correctly
---
### Author:  [@JosephPollacco](https://discourse.julialang.org/u/JosephPollacco)
#### Post date: [April 30, 2020, 2:21am UTC](https://discourse.julialang.org/t/metaprogramming-using-eval-and-struct-bug-getting-undefvarerror-not-defined/37937/13 "2020-04-30T02:21:03Z")
You are correct that it is the same as:
```julia
hydro.θs = parameter
```
The advantage of
```julia
setfield!(hydro, Symbol("θs"), Parameter)
```
Is that you can perform _Metaprogramming_ by avoiding _eval_. I can now read the symbol directly from a csv file and put it automatically into the structure _hydro_
---
### Author:  [@dpsanders](https://discourse.julialang.org/u/dpsanders)
#### Post date: [April 30, 2020, 4:01am UTC](https://discourse.julialang.org/t/metaprogramming-using-eval-and-struct-bug-getting-undefvarerror-not-defined/37937/14 "2020-04-30T04:01:37Z")
But that only works if you have already defined a `struct` with the correct field name beforehand.
---
### Author:  [@JosephPollacco](https://discourse.julialang.org/u/JosephPollacco)
#### Post date: [April 30, 2020, 4:07am UTC](https://discourse.julialang.org/t/metaprogramming-using-eval-and-struct-bug-getting-undefvarerror-not-defined/37937/15 "2020-04-30T04:07:26Z")
You are correct, it is the downside of the method. 😞
If you have a method to create `struct` on the fly that will be great. My challenge is that some parameters are vectors.
---
### Author:  [@dpsanders](https://discourse.julialang.org/u/dpsanders)
#### Post date: [April 30, 2020, 4:13am UTC](https://discourse.julialang.org/t/metaprogramming-using-eval-and-struct-bug-getting-undefvarerror-not-defined/37937/16 "2020-04-30T04:13:10Z")
Just use a `Dict`, as was already mentioned several times:
```julia
d = Dict()
d["θs"] = 3
d["hello"] = [4, 5]
```
Of course this will be more efficient if all the values are known to be of a certain type:
```julia
d = Dict{String, Int64}()
d["a"] = 3
d["b"] = -17
```
---
### Author:  [@dpsanders](https://discourse.julialang.org/u/dpsanders)
#### Post date: [April 30, 2020, 4:14am UTC](https://discourse.julialang.org/t/metaprogramming-using-eval-and-struct-bug-getting-undefvarerror-not-defined/37937/17 "2020-04-30T04:14:18Z")
Or you can use a `NamedTuple`, which is similar to a struct.
NamedTupleTools.jl helps with that.
---
### Author:  [@JosephPollacco](https://discourse.julialang.org/u/JosephPollacco)
#### Post date: [April 30, 2020, 4:21am UTC](https://discourse.julialang.org/t/metaprogramming-using-eval-and-struct-bug-getting-undefvarerror-not-defined/37937/18 "2020-04-30T04:21:40Z")
**Thanks for the suggestion of using Dict**
My understanding is that one of the limitation of _Dict_ is that one cannot use Vectors. “θs” is a parameter describing the maximum saturated soil moisture of a layer in a soil. I have many soil layers, so my understanding is that _Dict_ might not be the best solution for using Vectors, but I might be wrong. For e.g. this may not be possible:
```julia
d["θs[1]"] = 1
d["θs[2]"] = 2
```
---
### Author:  [@Oscar\_Smith](https://discourse.julialang.org/u/Oscar_Smith)
#### Post date: [April 30, 2020, 4:28am UTC](https://discourse.julialang.org/t/metaprogramming-using-eval-and-struct-bug-getting-undefvarerror-not-defined/37937/19 "2020-04-30T04:28:10Z")
Just do `d["θs"]=[1,2]`, ie store the vector as the value.
---
### Author:  [@dpsanders](https://discourse.julialang.org/u/dpsanders)
#### Post date: [April 30, 2020, 4:32am UTC](https://discourse.julialang.org/t/metaprogramming-using-eval-and-struct-bug-getting-undefvarerror-not-defined/37937/20 "2020-04-30T04:32:17Z")
Did you actually try to run that code? It works fine.
```julia
> d["θs[1]"] = 1
1
> d["θs[2]"] = 2
2
> d
Dict{Any,Any} with 2 entries:
"θs[2]" => 2
"θs[1]" => 1
```
But that’s not what you really want (probably).
I think you want
```julia
> d = Dict()
Dict{Any,Any} with 0 entries
> d["θs"] = [3, 4, 5]
3-element Array{Int64,1}:
3
4
5
> d["θs"][1]
3
> d["θs"][2]
4
```
`d["θs"]` is now a `Vector` whose elements you can extract in the usual way.
[Next page](https://discourse.julialang.org/t/metaprogramming-using-eval-and-struct-bug-getting-undefvarerror-not-defined/37937.md?page=2)