# Redefining == operator doesn't behave expectedly
**URL:** https://discourse.julialang.org/t/redefining-operator-doesnt-behave-expectedly/108303
**Category:** New to Julia
**Tags:** question
**Created:** [January 3, 2024, 1:14pm UTC](https://discourse.julialang.org/t/redefining-operator-doesnt-behave-expectedly/108303 "2024-01-03T13:14:16Z")
**Posts on this page:** 5
**Page:** 1
### Author:  [@Joslacrosse](https://discourse.julialang.org/u/Joslacrosse)
#### Post date: [January 3, 2024, 1:14pm UTC](https://discourse.julialang.org/t/redefining-operator-doesnt-behave-expectedly/108303/1 "2024-01-03T13:14:16Z")
I have a problem with a coding execise from Exercism, i habe been trying to solve it for some hours now
I think the problem lies with the connectd == and != as if i tests the statements individually they work out.
Does anyone know how to redefine the == so that the test will go through
The code doesnt clear one of the tests:
````julia
function toIntArray(ISBN::String)
dig ="0123456789"
isbn=[]
for i in 1:length(ISBN)
if in(ISBN[i],dig)
append!(isbn,Int(ISBN[i]-48))
elseif ISBN[i]=='X' && (i-length(ISBN))==0
append!(isbn,10)
elseif ISBN[i] =='-'
else
throw(DomainError("Snake on the plane"))
end
end
return(isbn)
end
struct ISBN
isbn::Array{Int}
function ISBN(ISBN::String)
storage=0
isbn = toIntArray(ISBN)
if length(isbn)!= 10
throw(DomainError("tooloongortooshoort"))
end
for i in 1:length(isbn)
storage=storage+ isbn[i]*(11-i)
end
if storage %11 ==0
return(true)
else
throw(DomainError("Thats the wrong number"))
end
end
end
function Base.:(==)(i1::ISBN, i2::ISBN)
return toIntArray(i1)==toIntArray(i2)
end
function Base.:(!=)(i1::ISBN, i2::ISBN)
return toIntArray(i1)!=toIntArray(i2)
end
´´´
The test is
´´´ ISBN("3-598-21508-8") == ISBN("3598215088") != ISBN("3-598-21507-X") ```
(should result in true but results in false)
the ecercise can be found at https://exercism.org/tracks/julia/exercises/isbn-verifier
````
---
### Author:  [@abraemer](https://discourse.julialang.org/u/abraemer)
#### Post date: [January 3, 2024, 1:48pm UTC](https://discourse.julialang.org/t/redefining-operator-doesnt-behave-expectedly/108303/2 "2024-01-03T13:48:29Z")
Welcome to the forum! 🙂
I don’t want to solve the exercise for you so I will just give you some hints:
1. You should check what `ISBN("3-598-21508-8")` returns. It is not of type `ISBN` because your constructor (the `ISBN` function inside the struct’s definition) never calls `new` to actually construct an instance. Have a look at [the manual’s section on constructors](https://docs.julialang.org/en/v1/manual/constructors/#man-inner-constructor-methods)
2. Once you fix that, you will notice that your definitions of `==` and `!=` don’t make sense as you didn’t define a method for `toIntArray` that takes `ISBN` types. So you’ll probably want to rethink what these methods should do.
Come back if you need more hints!
Edit: When you solved it, you can also post your solution and I will give you some pointers how to make it more idiomatic and point out some beginner mistakes i.e. concerning the typing of variables 🙂
---
### Author:  [@Joslacrosse](https://discourse.julialang.org/u/Joslacrosse)
#### Post date: [January 3, 2024, 3:15pm UTC](https://discourse.julialang.org/t/redefining-operator-doesnt-behave-expectedly/108303/3 "2024-01-03T15:15:44Z")
Hey abraemer,
Great advice
i have succceeded to resolve the problem with your help i think its not the most elegant solution but it works
I changes the constructor from returning the ISBN to new(isbn) # I think that leads to a new instance being created ?
I overloaded the toIntArray function to facilitate it to turn instances of isbn into an IntArray
Thank you very much vor your help! 😀
Here is the current code that manages to pass all the tests:
```julia
struct ISBN
isbn::Array{Int}
function ISBN(ISBN::String)
storage=0
isbn = toIntArray(ISBN)
if length(isbn)!= 10
throw(DomainError("tooloongortooshoort"))
end
for i in 1:length(isbn)
storage=storage+ isbn[i]*(11-i)
end
if storage %11 ==0
new(isbn)
else
throw(DomainError("Thats the wrong number"))
end
end
end
function toIntArray(ISBN::String)
dig ="0123456789"
isbn=[]
for i in 1:length(ISBN)
if in(ISBN[i],dig)
append!(isbn,Int(ISBN[i]-48))
elseif ISBN[i]=='X' && (i-length(ISBN))==0
append!(isbn,10)
elseif ISBN[i] =='-'
else
throw(DomainError("Snake on the plane"))
end
end
return(isbn)
end
function toIntArray(isbn::ISBN)
dig ="0123456789"
result=[]
for i in 1:10
append!(result, Int(isbn.isbn[i]))
end
return(result)
end
function Base.:(==)(i1::ISBN, i2::ISBN)
return toIntArray(i1)==toIntArray(i2)
end
```
---
### Author:  [@abraemer](https://discourse.julialang.org/u/abraemer)
#### Post date: [January 3, 2024, 4:21pm UTC](https://discourse.julialang.org/t/redefining-operator-doesnt-behave-expectedly/108303/4 "2024-01-03T16:21:19Z")
Glad my hints where helpful 🙂
As promised some comments:
- General logic: your ISBN struct stores the digits of the ISBN, so when you want to compare to instances, you could just compare their arrays directly. Your function `toIntArray(isbn::ISBN)` really just performs a slow copy of the stored array. So you can just do:
```julia
Base.:(==)(i1::ISBN, i2::ISBN) = i1.isbn == i2.isbn
```
- Typing: You annotated the field `isbn` in your `ISBN` struct with `Array{Int}` which is not a good annotation because `Array` denotes a multi-dimensional array and its full type signature carrys the dimension as well. So it would be better (for performance) to write the type as `Array{Int, 1}` or just `Vector{Int}` which means the same thing. However you could also just leave the type out if you don’t care about performance. Julia does not require you to give fields a type.
- You could shorten the `toIntArray` function a bit using Julia’s `isdigit` function and a bit of [broadcasting magic (see manual)](https://docs.julialang.org/en/v1/manual/arrays/#Broadcasting):
```julia
function toIntArray(ISBN::String)
digits_string = filter(isdigit, ISBN)
digits = parse.(Int, collect(digits_string)) # note that strings are scalars for broadcasting, so I use collect to turn it into a Vector{Char}
if ISBN[end] == 'X'
push!(digits, 10)
end
return digits
end
```
- you could consider making the verification of an ISBN its own function. That way you could use that functionality elsewhere and it also would clear up the constructor a bit.
---
### Author:  [@Joslacrosse](https://discourse.julialang.org/u/Joslacrosse)
#### Post date: [January 4, 2024, 7:18am UTC](https://discourse.julialang.org/t/redefining-operator-doesnt-behave-expectedly/108303/5 "2024-01-04T07:18:55Z")
Thanks for the great feedback! I‘ll try to implement it! 😀