Can I modify a struct property and send it back to the contract?

Question

we are experiencing an issue dealing with structs. We expect to modify a struct property after getting it from an event in web3js, but, it seems that the modify does not apply.

Any ideas?

Contract code:

pragma solidity ^0.5.3;
pragma experimental ABIEncoderV2;

contract Example {
  struct Structure {
    uint value;
  }

  event Created(
    Structure structure
  );

  event Modified(
    Structure structure
  );

  constructor() public {}

  function create() public {
    emit Created(Structure(42));
  }

  function modify(Structure memory structure) public {
    emit Modified(structure);
  }
}

web3js code:

  let Example = artifacts.require('Example')
  let accounts = await web3.eth.getAccounts()

  Example.deployed()
    .then(instance => {
      instance.Created(async (err, evt) => {
        let structure = evt.returnValues.structure
        console.log('Created: ' + structure.value)
        structure.value = 21
        console.log('Modify: ' + structure.value)
        await instance.modify(structure, { from: accounts[0] })
      })

      instance.Modified((err, evt) => {
        let structure = evt.returnValues.structure
        console.log('Modified: ' + structure.value)
        callback()
      })

      instance.create({ from: accounts[0] })
    })

log:

Created: 42
Modify: 21
Modified: 42

We expected the modified value equals to 21

Answer 1

As smarx said, using structure[0] = 21 it works, because structure is an array. With more properties, the order is the same on the struct definition, so, with the struct:

struct Example {
  uint first;
  uint n;
  uint last;
}

those equals are verified:

struct[0] == struct.first
struct[1] == struct.n
struct[2] == struct.last

and to change the value we must do:

struct[0] = 1 // sets struct.first
struct[1] = 2 // sets struct.n
struct[2] = 3 // sets struct.last