# Filling an array of structs from file contents

**URL:** <https://discourse.julialang.org/t/filling-an-array-of-structs-from-file-contents/78757>\
**Category:** New to Julia\
**Tags:** question\
**Created:** [March 30, 2022, 5:55pm UTC](https://discourse.julialang.org/t/filling-an-array-of-structs-from-file-contents/78757 "2022-03-30T17:55:07Z")\
**Posts on this page:** 4\
**Page:** 1

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**Author:** ![qt\_codes](https://avatars.discourse-cdn.com/v4/letter/q/e9a140/32.png) [@qt\_codes](https://discourse.julialang.org/u/qt_codes)\
**Post date:** [March 30, 2022, 5:55pm UTC](https://discourse.julialang.org/t/filling-an-array-of-structs-from-file-contents/78757/1 "2022-03-30T17:55:07Z")

</div>

Hi, I have code with a custom struct with different data types

```julia
mutable struct myStruct
    name::String = ""
    id::Int = -1
    ....
    # several more entries of varied types
    ....
end

```

I want to read a file and use several of these structs to capture the data in an accesible way. I do not know how many structs I will need at run-time. I want to read through the file, then add `myStruct` structs to a `stored_array` one at a time as I complete them. Something along the lines of

```julia
function add_element(stored_data::Array{myStruct}, new_data::myStruct)
    push!(stored_data, new_data)
    return stored_array
end

```

I have tried this, but I get an error that `push!` is not defined for these types. If I try to get define a new version of `push!`, my program does not use it. See these attempted definitions and error

```julia
# not all of these were used at the same time
stored_array = Array{myStruct}
stored_array = Array{myStruct, length} # where length is an Int
stored_array = Array{myStruct}(undef, length)

```

```julia
function push!(stored_array::Type{Array{myStruct}}, data::myStruct)
    ....
end

function push!(stored_array::Array{myStruct}, data::myStruct)
    ...
end

```

```julia
ERROR: LoadError: MethodError: no method matching push!(::Type{Array{myStruct}}, ::myStruct)
Closest candidates are:
  push!(::Type{Vector{myStruct}}, ::myStruct) at C:\Users\qt\Desktop\Julia\code\Debug\decoder.jl:122
  push!(::Vector{myStruct}, ::myStruct) at C:\Users\qt\Desktop\Julia\code\Debug\decoder.jl:116

```

I feel that I am missing something much simpler. I have seen posts suggesting that looping over the array to initialize it is the best way to go, but with that I am having trouble initializing the array correctly.

Could someone who is more familiar with the language help me here? I’m open to changing my flow if there is a more “Julia” way to handle this use case.

---

<div class="post-metadata">

**Author:** ![stepanzh](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/stepanzh/32/34851_2.png) [@stepanzh](https://discourse.julialang.org/u/stepanzh)\
**Post date:** [March 30, 2022, 6:24pm UTC](https://discourse.julialang.org/t/filling-an-array-of-structs-from-file-contents/78757/2 "2022-03-30T18:24:50Z")

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I tried to reproduce `add_element` function, and it worked for me

```julia
mutable struct Foo
    name::String
    id::Int
end

add_element!(stored::Array{Foo}, x::Foo) = push!(stored, x)
add_element2!(stored::Vector{Foo}, x::Foo) = push!(stored, x)

A = [Foo("a", 1)]
add_element!(A, Foo("b", 2))
add_element2!(A, Foo("c", 3))
@show A # A = Foo[Foo("a", 1), Foo("b", 2), Foo("c", 3)]

A = Foo[]
add_element!(A, Foo("a", 1))
@show A # A = Foo[Foo("a", 1)]

```

About Julia-ways. If you just want to append a structure just use `push!` as it is common. Also, does `myStruct` is defined once you read a new line?

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<div class="post-metadata">

**Author:** ![qt\_codes](https://avatars.discourse-cdn.com/v4/letter/q/e9a140/32.png) [@qt\_codes](https://discourse.julialang.org/u/qt_codes)\
**Post date:** [March 30, 2022, 6:34pm UTC](https://discourse.julialang.org/t/filling-an-array-of-structs-from-file-contents/78757/3 "2022-03-30T18:34:24Z")

</div>

This worked, and I can now just use `push!` instead of wrapping it in a function. The main difference was how you defined the array I’m pushing too.

I have a parsing function that fills the struct I push to the array that handles newlines and other variations. Thank you for the help on this!

Do you know why my versions of `push!` caused those errors? I tried to type them as arrays but the errors only mention vectors.

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<div class="post-metadata">

**Author:** ![stepanzh](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/stepanzh/32/34851_2.png) [@stepanzh](https://discourse.julialang.org/u/stepanzh)\
**Post date:** [March 30, 2022, 6:55pm UTC](https://discourse.julialang.org/t/filling-an-array-of-structs-from-file-contents/78757/4 "2022-03-30T18:55:35Z")

</div>

> [@qt\_codes](#):
>
> `ERROR: LoadError: MethodError: no method matching push!(::Type{Array{myStruct}}, ::myStruct)`

This error caused by pushing to **type** , not array (e.g. `push!(Vector{Int}, 1)`). Perhaps, this caused by invalid initialization of `stored_array`, see the difference

- `stored_array = Array{myStruct}` — this is `UnionAll` **type** ;
- `stored_array = Array{myStruct, length}` — this is type too, it defines **type** of `length`- **dimensional** array of `myStruct` (see help for `Array`);
- `stored_array = Array{myStruct}(undef, length)` — this is correct. It defines **uninitialised array** of dimension 1 with `length` elements.

In last case I recommend to use `stored_array = Vector{myStruct}(undef, length)`, because it is clear that array dimension is one. `Vector{T}` is just alias for `Array{T,1}`, same for `Matrix`.