# Access a struct field from a string
**URL:** https://discourse.julialang.org/t/access-a-struct-field-from-a-string/44337
**Category:** New to Julia
**Tags:** struct
**Created:** [August 5, 2020, 2:49pm UTC](https://discourse.julialang.org/t/access-a-struct-field-from-a-string/44337 "2020-08-05T14:49:09Z")
**Posts on this page:** 7
**Page:** 1
### Author:  [@Luigi\_Bottecchia](https://discourse.julialang.org/u/Luigi_Bottecchia)
#### Post date: [August 5, 2020, 2:49pm UTC](https://discourse.julialang.org/t/access-a-struct-field-from-a-string/44337/1 "2020-08-05T14:49:09Z")
Hey Everyone!
I have incurred in an issue that I believe is conceptual.
I have a struct that I want to fill from a dictionary and i’m encountering problems on accessing the struct fields. The main issue is that the dictionary is variable so I need the function to be quite flexible.
Here is what I was trying to do:
```julia
mydict = Dict{Any,Any}("name"=>"joe","color"=>"blue")
struct MyStruct
name::String
color::String
age::Float64
end
function myfunction(dictionary::Dict)
result = Array{MyStruct,1}(undef,1)
for i in keys(dictionary)
if Symbol(i) in fieldnames(MyStruct)
field_name = Symbol(i)
result[1].field_name = dictionary[i]
else
break
end
end
return result
end
my_result = myfunction(mydict)
```
basically the function first create a 1 element array of MyStruct, then check whether the keys of the dictionary are present in the struct fields. (Here a simplified version that breaks in case this is not true)
In case they are present the function should write in the struct the value of the dictionary for that key.
I’m getting an UndefRefError: access to undefined reference
I got what is the issue (Julia does not recognize field\_name has field of MyStruct) But I haven’t find a solution for this.
Thanks for your help!
Luigi
---
### Author:  [@rdeits](https://discourse.julialang.org/u/rdeits)
#### Post date: [August 5, 2020, 3:00pm UTC](https://discourse.julialang.org/t/access-a-struct-field-from-a-string/44337/2 "2020-08-05T15:00:50Z")
The `foo.bar` syntax only works if “bar” is the actual name of the field, not a variable holding the name of the field. Instead, you want the `setproperty!` function, which takes the field name as an argument:
```julia
julia> mutable struct MyStruct
name::String
end
julia> m = MyStruct("hello")
MyStruct("hello")
julia> field = :name
:name
julia> setproperty!(m, field, "world")
"world"
julia> m
MyStruct("world")
```
A few things to note:
- Your struct must be a `mutable struct` since you’re mutating it
- The argument to `setproperty!` should be a `Symbol`, not a string. It’s easy to convert a string to a `Symbol`, e.g. `setproperty!(m, Symbol("name"), "world")` but you might be better off making the keys of `mydict` `Symbols` instead of strings to begin with.
---
### Author:  [@rdeits](https://discourse.julialang.org/u/rdeits)
#### Post date: [August 5, 2020, 3:02pm UTC](https://discourse.julialang.org/t/access-a-struct-field-from-a-string/44337/3 "2020-08-05T15:02:08Z")
In other words, `foo.bar = "hello"` is just a shorthand for:
```julia
setproperty!(foo, :bar, "hello")
```
which itself will just call the built-in function:
```julia
setfield!(foo, :bar, "hello")
```
---
### Author:  [@AndiMD](https://discourse.julialang.org/u/AndiMD)
#### Post date: [August 5, 2020, 3:08pm UTC](https://discourse.julialang.org/t/access-a-struct-field-from-a-string/44337/4 "2020-08-05T15:08:04Z")
Also, you cannot index the Array at the #undef element with `result[1]`
You could use a default value instead of `undef` like this: `result = [MyStruct("Default","Default",0)]`
---
### Author:  [@Luigi\_Bottecchia](https://discourse.julialang.org/u/Luigi_Bottecchia)
#### Post date: [August 5, 2020, 3:26pm UTC](https://discourse.julialang.org/t/access-a-struct-field-from-a-string/44337/5 "2020-08-05T15:26:38Z")
@rdeits @AndiMD Thanks a lot! you both helped solving the question! Is there a Way I can put “two solutions”?
I solved it by calling setting the struct to mutable and bye initializing it as @AndiMD said. while in the function instead of result[1].field\_name = dictionary[i] I put setproperty!(result[1],Symbol(i),dictionary[i]) as suggested by @rdeits
Can I ask if you could clarify the pros/cons of using mutable structs? I read the documentations but is still not 100% clear to me!
Thanks a lot
---
### Author:  [@rdeits](https://discourse.julialang.org/u/rdeits)
#### Post date: [August 5, 2020, 4:14pm UTC](https://discourse.julialang.org/t/access-a-struct-field-from-a-string/44337/6 "2020-08-05T16:14:26Z")
The advantages of a `struct` over a `mutable struct` are:
- A `struct` can be cheaper to construct. This matters a lot if you are constantly creating new objects inside your loop.
- A `struct` is immutable, which can help prevent subtle bugs when you accidentally change a field without meaning to. Many people find code that uses immutable objects to be easier to reason about.
Neither of these advantages are overwhelming, so feel free to pick whichever type suits your needs.
---
### Author:  [@Luigi\_Bottecchia](https://discourse.julialang.org/u/Luigi_Bottecchia)
#### Post date: [August 6, 2020, 12:22pm UTC](https://discourse.julialang.org/t/access-a-struct-field-from-a-string/44337/7 "2020-08-06T12:22:49Z")
Thank you very much!