# Field names and values of a struct defined as NTuple

**URL:** <https://discourse.julialang.org/t/field-names-and-values-of-a-struct-defined-as-ntuple/91284>\
**Category:** New to Julia\
**Tags:** question, struct, ntuple\
**Created:** [December 5, 2022, 11:38pm UTC](https://discourse.julialang.org/t/field-names-and-values-of-a-struct-defined-as-ntuple/91284 "2022-12-05T23:38:56Z")\
**Posts on this page:** 5\
**Page:** 1

<div class="post-metadata">

**Author:** ![mmv](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/mmv/32/26323_2.png) [@mmv](https://discourse.julialang.org/u/mmv)\
**Post date:** [December 5, 2022, 11:38pm UTC](https://discourse.julialang.org/t/field-names-and-values-of-a-struct-defined-as-ntuple/91284/1 "2022-12-05T23:38:57Z")

</div>

Hi! Let

```nohighlight
struct ex1
    name::String
    age::Int
end

```

I understand that

(1) the statements above define a new type of record containing a string and a number  
(2) an instance of that type can be created as

```nohighlight
a = ex1("John", 34)

```

(3) the field names of `ex1` can be listed with the function `fieldnames`  
(4) the values of those fields can be retrieved as `ex1.name` or `ex1.age`

However, if another structure is defined as

```nohighlight
struct ex2
    data::NTuple{4, UInt8}
end

```

then

```julia
julia> b = ex2(0, 0, 0, 0)
ERROR: MethodError: no method matching ex2(::Int64, ::Int64, ::Int64, ::Int64)
Closest candidates are:
  ex2(::Any) at REPL[1]:2
Stacktrace:
 [1] top-level scope
   @ REPL[3]:1

julia> b = ex2(0::UInt8, 0::UInt8, 0::UInt8, 0::UInt8)
ERROR: TypeError: in typeassert, expected UInt8, got a value of type Int64
Stacktrace:
 [1] top-level scope
   @ REPL[4]:1

```

Question 1: How do I define an instance of that `ex2` data type with all values set to 0 as a `UInt8` (or any other value, for that matter)? and why are those instantiation syntaxes problematic?

and

```julia
julia> fieldnames(ex2)
(:data,)

```

Question 2: How can I access individual values of `ex2` since they have no apparent individual names?

Thanks for any insight into these two questions.

---

<div class="post-metadata">

**Author:** ![fredrikekre](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/fredrikekre/32/1688_2.png) [@fredrikekre](https://discourse.julialang.org/u/fredrikekre)\
**Post date:** [December 6, 2022, 12:20am UTC](https://discourse.julialang.org/t/field-names-and-values-of-a-struct-defined-as-ntuple/91284/2 "2022-12-06T00:20:39Z")

</div>

> [@mmv](#):
>
> How do I define an instance of that `ex2` data type with all values set to 0 as a `UInt8`

The struct have one field, and if you check the constructor (with e.g. `methods(ex2)`) you see that it expects one argument – the tuple. You are passing 4 arguments. If you pass a tuple it works (and the `Int`s are converted to `UInt8`s, since the default constructor converts):

```julia
julia> struct ex2
           data::NTuple{4, UInt8}
       end

julia> a = ex2((1, 2, 3, 4)) # one argument, the tuple (1, 2, 3, 4)
ex2((0x01, 0x02, 0x03, 0x04))

```

> [@mmv](#):
>
> How can I access individual values of `ex2` since they have no apparent individual names?

You extract the tuple with `.data`, then use `getindex`:

```julia
julia> a.data[1]
0x01

julia> a.data[3]
0x03

```

---

<div class="post-metadata">

**Author:** ![mmv](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/mmv/32/26323_2.png) [@mmv](https://discourse.julialang.org/u/mmv)\
**Post date:** [December 6, 2022, 12:24am UTC](https://discourse.julialang.org/t/field-names-and-values-of-a-struct-defined-as-ntuple/91284/3 "2022-12-06T00:24:03Z")

</div>

Thanks a lot, @ [fredrikekre](https://discourse.julialang.org/u/fredrikekre): that’s very clear indeed!

---

<div class="post-metadata">

**Author:** ![uniment](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/uniment/32/24532_2.png) [@uniment](https://discourse.julialang.org/u/uniment)\
**Post date:** [December 6, 2022, 2:59am UTC](https://discourse.julialang.org/t/field-names-and-values-of-a-struct-defined-as-ntuple/91284/4 "2022-12-06T02:59:46Z")

</div>

Note 1:

Idiomatically, types in Julia should follow upper-CamelCase. So in your examples, the struct names would be `Ex1` and `Ex2`.

Note 2:

If you want, you can overload `Ex2`’s constructor and `Base.getindex` to obtain the behavior you expected:

```julia
julia> struct Ex2 data::NTuple{4, UInt8} end

julia> Ex2(a, b, c, d) = Ex2((a, b, c, d));

julia> Base.getindex(e::Ex2, i) = e.data[i]

julia> a = Ex2(0, 0, 0, 0)
Ex2((0x00, 0x00, 0x00, 0x00))

julia> a[1]
0x00

```

Keep in mind though, that other behaviors you’d likely expect (such as iteration, length measurement, etc.) would also have to be implemented. There are some macros out there to help with defining these I believe, but I’m not familiar with them.

---

<div class="post-metadata">

**Author:** ![mmv](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/mmv/32/26323_2.png) [@mmv](https://discourse.julialang.org/u/mmv)\
**Post date:** [December 6, 2022, 11:58pm UTC](https://discourse.julialang.org/t/field-names-and-values-of-a-struct-defined-as-ntuple/91284/5 "2022-12-06T23:58:42Z")

</div>

@uniment: Thanks for your input. My question arose in the context of writing a Julia wrapper for a C function, so I’ll try to remain as close as possible to the original code, but I’m always happy to learn something new. So, thanks a lot for that!