How to replicate pushing and deleting from an array of structs from Solidity in Vyper?

Question

I'm trying to re-create a Solidity contract in Vyper for a demonstration and am trying to figure out the best way to replicate adding/removing from an array of structs... As far as I can tell I'd have to use a hashmap of [uint256, mystruct] and then keep track of the index with a separate variable... is that the correct way to do this? What about for removing or zeroing a struct from the hashmap?

Solidity Code:

struct MyStruct {
    string name;
    uint age;
}
MyStruct[] public myStructs;
uint256 structCount;
// add
myStructs.push("Alice", 22);
structCount += 1;
// remove
delete myStructs[0];
structCount -= 1;

Work-in-progress Vyper Code:

struct MyStruct:
    name: String[64]
    age: uint256
myStructs: public(HashMap[uint256, MyStruct])
structIndex: uint256
structCount: uint256
add
self.myStructs[self.structIndex] = MyStruct({name: "Alice", age: 22})
self.structIndex += 1
self.structCount += 1
remove
thisStruct: MyStruct = self.myStructs[0]
is this the only way to do this?
thisStruct.name = ""
thisStruct.age = 0
self.structCount -= 1

Last time I checked vyper didn't have dynamic arrays. It was the language philosophy to use fixed length arrays. So you are doing exactly as expected using a mapping and some counters. — Ismael, Apr 28 '21 at 04:32

score 1 · Answer 1 · answered Apr 27 '21 at 17:03

Possibly someone else will chime in with Vyper suggestions but I think you might be starting out wrong-footed.

This doesn't do what I imagine you think it does:

// remove
delete myStructs[0];
structCount -= 1;

delete does not splice/re-organize the array as you might expect. If you have for example:

0: alice
1: bob
2: carol

Then, delete 0, you will have:

0: null
1: bob
2: carol

Although you decrement the count, the count is rather useless and the array hasn't become any shorter than it was before.

Implementing the remove function uses a well-established pattern that combines a mapping of keys and pointers with an array and reorganization process that is appropriate for the EVM.

See "Mapped Struct with Delete-enabled Index" over here: Are there well-solved and simple storage patterns for Solidity?

I imagine if you have the method sorted out, it should take only a modest effort to translate the steps into Vyper.

Hope it helps.

Yeah I should have probably been more descriptive in the example there... I understand that its not actually removing the element from the array, which is why I had the structCount variable since the array.length property can't be relied on. That posted you linked to is pretty cool though, definitely going to be useful for some other work. — Pathead, Apr 27 '21 at 17:15

How to replicate pushing and deleting from an array of structs from Solidity in Vyper?

add

remove

is this the only way to do this?

1 Answers1