Parameter -> equal : (X, X) in Buffer Documentation

Developers
1

Hello, sorry for this probably easy question but i can’t find the solution.

I add some Principals in a Buffer and I would like to remove duplicates Principals… There is many way to do, but Buffer methods always need the equal : (X, X) parameter… And I don’t know what is that and I didn’t easily find examples.

let buff : Buffer.Buffer<Principal> = Buffer.Buffer(10);
[...]
let principal1 = "xxxx-xxxx-..." 
let principal2 = "xxxx-xxxx-..." 

if( Buffer.contains(buff, principal1, ???) == false) // check if the principal is in the buffer
{
buff.add(principal1); //add if it's not
}

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How can I properly use these methods please ? Could you give an easy example like this one ?

2

The type signature for equal is (X, X) -> Bool which means it is a function that takes 2 arguments of type X and returns a Bool.

In this case X is Principal so you can use Principal.equal:

If you never want duplicates you could consider using a Set data structure instead. I could only find this implementation:

1 Like
3

Thank you for this answer. This is was I tried to use : Principal.equal, as I used it to instanciate my Hashmaps, so, the problem is not from there…

Ok, I found immediately so, I had just forget to put var x = before my method :
var x = Buffer.contains(buff, q2.answers[i].principal, Principal.equal);

Thank you also for the explanation about Set, it seems to be what I was looking for !

4

I’m also unable to use this… I understand a little better these methods than before… But not perfectly yet.

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I tried :

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I get the error :
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I don’t really understand why it expect Nat32 type instead of Hash type like it’s written in the prototype.

5

The argument xh has the type Hash. If we follow the imports we can see that that’s a type alias for Nat32:

You’re providing Hash.hash, which is a function that takes a Nat and returns a Hash:

Instead, you probably want to provide the equivalent of Principal.hash(q2.answers[i].principal) for that argument.

6

Evginey’s hashmap has a set implementation: GitHub - ZhenyaUsenko/motoko-hash-map: Stable hash maps for Motoko

7

Thanks :pray: :sparkles: very much