# Struct with property based on other properties
**URL:** https://discourse.julialang.org/t/struct-with-property-based-on-other-properties/93181
**Category:** New to Julia
**Created:** [January 18, 2023, 9:04pm UTC](https://discourse.julialang.org/t/struct-with-property-based-on-other-properties/93181 "2023-01-18T21:04:15Z")
**Posts on this page:** 8
**Page:** 1
### Author:  [@Eveee](https://discourse.julialang.org/u/Eveee)
#### Post date: [January 18, 2023, 9:04pm UTC](https://discourse.julialang.org/t/struct-with-property-based-on-other-properties/93181/1 "2023-01-18T21:04:15Z")
Hi all,
I am learning to use structs in Julia, specifically I have a set of integers that I would like to access throughout some calculations:
```julia
struct MyNumbers
a::Int64
b::Int64
c=a+b
end
```
However, the assignment of c gives me a syntax error, what would be the correct way to do this?
---
### Author:  [@pdeffebach](https://discourse.julialang.org/u/pdeffebach)
#### Post date: [January 18, 2023, 9:09pm UTC](https://discourse.julialang.org/t/struct-with-property-based-on-other-properties/93181/2 "2023-01-18T21:09:18Z")
You would use an “inner constructor” to ensure `c` always has the right value. And an “outer constructor” to create a new instance where `c` is given the correct value. See the docs [here](https://docs.julialang.org/en/v1/manual/constructors/#man-inner-constructor-methods).
```julia
julia> struct MyNumbers
a::Int64
b::Int64
c::Int64
MyNumbers(a, b, c) = begin
@assert c == a + b
new(a, b, c)
end
end;
julia> MyNumbers(a, b) = MyNumbers(a, b, a + b);
julia> MyNumbers(1, 2, 5)
ERROR: AssertionError: c == a + b
Stacktrace:
[1] MyNumbers(a::Int64, b::Int64, c::Int64)
@ Main ./REPL[1]:6
[2] top-level scope
@ REPL[3]:1
julia> MyNumbers(1, 2, 3)
MyNumbers(1, 2, 3)
julia> MyNumbers(1, 2)
MyNumbers(1, 2, 3)
```
---
### Author:  [@cjdoris](https://discourse.julialang.org/u/cjdoris)
#### Post date: [January 18, 2023, 9:14pm UTC](https://discourse.julialang.org/t/struct-with-property-based-on-other-properties/93181/3 "2023-01-18T21:14:38Z")
Or simply
```julia
struct MyNumbers
a::Int64
b::Int64
c::Int64
MyNumbers(a, b) = new(a, b, a+b)
end
```
---
### Author:  [@Eveee](https://discourse.julialang.org/u/Eveee)
#### Post date: [January 18, 2023, 9:20pm UTC](https://discourse.julialang.org/t/struct-with-property-based-on-other-properties/93181/4 "2023-01-18T21:20:51Z")
Thank you both!
---
### Author:  [@stephenll](https://discourse.julialang.org/u/stephenll)
#### Post date: [January 18, 2023, 10:58pm UTC](https://discourse.julialang.org/t/struct-with-property-based-on-other-properties/93181/5 "2023-01-18T22:58:17Z")
I’ve always used `getproperty` to accomplish this if using a `mutable struct`.
```julia
function Base.getproperty(x::MyNumbers, sym::Symbol)
if sym == :c
returns x.a + x.b
else
return getfield(x, sym)
end
end
```
---
### Author:  [@mikmoore](https://discourse.julialang.org/u/mikmoore)
#### Post date: [January 19, 2023, 4:37pm UTC](https://discourse.julialang.org/t/struct-with-property-based-on-other-properties/93181/6 "2023-01-19T16:37:04Z")
If your extra field requires some expensive calculation, then you can precompute it using a constructor, as people have suggested.
However, for simple calculations, the `getproperty` approach suggested above is nicer because it doesn’t actually use the extra memory and won’t become wrong if you change `a` or `b` (although in your example, `MyNumbers` is immutable so this won’t happen).
`getproperty` can sometimes be a little tedious to define, though. So my preferred approach (and the most flexible) is to access objects through accessor functions. Personally, I almost never use direct field access outside of defining an accessor. For example:
```julia
geta(x::MyNumbers) = x.a
getb(x::MyNumbers) = x.b
getc(x::MyNumbers) = geta(x) + getb(x) # or x.a + x.b
```
---
### Author:  [@astro-kevin](https://discourse.julialang.org/u/astro-kevin)
#### Post date: [May 21, 2024, 1:45pm UTC](https://discourse.julialang.org/t/struct-with-property-based-on-other-properties/93181/7 "2024-05-21T13:45:18Z")
Is there a way to avoid having to type out the entire constructor again? For example something like
```julia
struct MyNumbers
a::Int64
b::Int64
c::Int64
d::Int64
e::Int64
MyNumbers(a, b, ...) = new(a, b, ..., a+b)
end
```
which obviously doesn’t work, but would be useful if there are a lot of properties that you don’t want to type out all over again.
---
### Author:  [@Dan](https://discourse.julialang.org/u/Dan)
#### Post date: [May 21, 2024, 2:03pm UTC](https://discourse.julialang.org/t/struct-with-property-based-on-other-properties/93181/8 "2024-05-21T14:03:39Z")
Something like:
```julia
julia> struct MyNumbers
a::Int64
b::Int64
c::Int64
d::Int64
e::Int64
MyNumbers(a, b, args...) = new(a, b, args..., a+b)
end
julia> MyNumbers(1,2,3,4)
MyNumbers(1, 2, 3, 4, 3)
```