# Struct copy with change in the value of fields
**URL:** https://discourse.julialang.org/t/struct-copy-with-change-in-the-value-of-fields/19592
**Category:** Performance
**Created:** [January 13, 2019, 8:58pm UTC](https://discourse.julialang.org/t/struct-copy-with-change-in-the-value-of-fields/19592 "2019-01-13T20:58:43Z")
**Posts on this page:** 9
**Page:** 1
### Author:  [@rveltz](https://discourse.julialang.org/u/rveltz)
#### Post date: [January 13, 2019, 8:58pm UTC](https://discourse.julialang.org/t/struct-copy-with-change-in-the-value-of-fields/19592/1 "2019-01-13T20:58:43Z")
Hi,
I have an anoying situation where I want to copy a variable of composed type by changing a value of the field which may in turn change the type. Here is an example
```julia
# in my application, the following struct contains ~20 fields...
struct A{T,vectype}
p::T
v::vectype
end
a = A(Int32(1), rand(10))
```
is of type `A{Int32,Array{Float64,1}}`. What I would like to do is something like this
```julia
b = copy(a, p = 0.21)
```
I face two issues. The first is I want to change an element, so writing this is anoying
```julia
b = A(p = 0.21, v = a.v) # I would have to write every field :frowning:
```
I cant do
```julia
b = deepcopy(a)
b.p = 0.21
```
as the types dont match.
Does anybody have a solution please?
Thank you,
---
### Author:  [@mohamed82008](https://discourse.julialang.org/u/mohamed82008)
#### Post date: [January 13, 2019, 9:10pm UTC](https://discourse.julialang.org/t/struct-copy-with-change-in-the-value-of-fields/19592/2 "2019-01-13T21:10:04Z")
Use Setfield.jl:
```julia
julia> using Setfield
julia> b = @set a.p = 0.21;
julia> typeof(b)
A{Float64,Array{Float64,1}}
```
---
### Author:  [@rveltz](https://discourse.julialang.org/u/rveltz)
#### Post date: [January 13, 2019, 9:38pm UTC](https://discourse.julialang.org/t/struct-copy-with-change-in-the-value-of-fields/19592/3 "2019-01-13T21:38:08Z")
Thank you!
---
### Author:  [@raythurnevoid](https://discourse.julialang.org/u/raythurnevoid)
#### Post date: [September 27, 2023, 1:14am UTC](https://discourse.julialang.org/t/struct-copy-with-change-in-the-value-of-fields/19592/4 "2023-09-27T01:14:36Z")
[JuliaObjects/Accessors.jl: Update immutable data (github.com)](https://github.com/JuliaObjects/Accessors.jl) as per 2023
---
### Author:  [@sadish-d](https://discourse.julialang.org/u/sadish-d)
#### Post date: [September 27, 2023, 1:15pm UTC](https://discourse.julialang.org/t/struct-copy-with-change-in-the-value-of-fields/19592/5 "2023-09-27T13:15:09Z")
You can define a constructor function to create a new struct instance using an old one’s default values. If you have a struct `S`:
```julia
struct S
a
b
S(a, b) = new(a, b)
end
```
You can define:
```julia
S(x::S; a=x.a, b=x.b) = S(a, b)
```
Then:
```julia
old = S("a", "b") # S("a", "b")
new = S(old; b=2) # S("a", 2)
```
---
### Author:  [@rveltz](https://discourse.julialang.org/u/rveltz)
#### Post date: [September 29, 2023, 1:39pm UTC](https://discourse.julialang.org/t/struct-copy-with-change-in-the-value-of-fields/19592/6 "2023-09-29T13:39:08Z")
yeah but imagine that there are 50 fields
---
### Author:  [@sadish-d](https://discourse.julialang.org/u/sadish-d)
#### Post date: [September 29, 2023, 11:04pm UTC](https://discourse.julialang.org/t/struct-copy-with-change-in-the-value-of-fields/19592/7 "2023-09-29T23:04:58Z")
It’s a pain if you have 50 fields. But if you just have one struct for which you need this, and don’t wish to download a package just for this one struct, this is an option.
This is an old post and looks like you found a solution. I just wanted to write my response in case someone with a similar problem lands on this thread. They can make a more informed choice knowing there are simpler options for simpler cases.
---
### Author:  [@Nikos\_Gianniotis](https://discourse.julialang.org/u/Nikos_Gianniotis)
#### Post date: [October 15, 2024, 8:32am UTC](https://discourse.julialang.org/t/struct-copy-with-change-in-the-value-of-fields/19592/8 "2024-10-15T08:32:33Z")
Thanks, this works well for me.
---
### Author:  [@tuckermcclure](https://discourse.julialang.org/u/tuckermcclure)
#### Post date: [August 14, 2025, 12:33am UTC](https://discourse.julialang.org/t/struct-copy-with-change-in-the-value-of-fields/19592/9 "2025-08-14T00:33:29Z")
In case you want something pretty general but don’t want to bring in a package, here’s a simple function that does this for you, assuming you can use keyword arguments for your constructor:
```julia-auto
julia> @kwdef struct AType; a::Float64; b::Int64; end;
julia> x = AType(1., 2);
julia> function copy_with_exceptions(x::T; kwargs...) where {T}
return T(; (f => getfield(x, f) for f in fieldnames(T))..., kwargs...)
end;
julia> copy_with_exceptions(x; b = 3)
AType(1.0, 3)
```
I’m not sure about performance here, but the pattern is simple enough if you don’t want to bring in Accessors.jl.