Solidity – Understanding ‘=>’ in Mapping Function

contract-developmentsolidity

I'm very new to actual programming not as much blockchain. I know a little Java and am trying to learn solidity I am reading through the Intro to smart contracts turotial and on the 7th line (see below) they declare a variable using mapping but I don't understand how the => comes into play.

mapping (address => uint) public balances;

Best Answer

mapping is similar to Dictionary in Java where you specify key and value pair like

var mydictionary = new Dictionary(key,value);

while in solidity you use the same with => ; where Key = address and uint as value.

Related Solutions

Solidity – How to Loop Through Mapping in Solidity?

You can't loop through the keys or count the keys in a mapping (but patterns exist for accomplishing such functionality using a few more variables).

You're on the right path, tracking it yourself. For example:

pragma solidity ^0.4.8;

contract Vote {

  uint public totalVotes;

  mapping (address => uint) public voterCount;

  function vote(uint votes) returns(bool success) {
    totalVotes += votes;
    voterCount[msg.sender] += votes; // cumulative
    return true;
  }
}

You wouldn't want to iterate over the voters in the contract in any case because that would put an upper bound on the number of voters before the system stops working - due to rising transaction cost hitting the block gas limit.

I can show you how to make the voter list iterable but it's not obvious that it's really needed to count the votes as we go. Keeping it simple for first answer.

Hope it helps.

Token Airdrop Guide – Most Cost Effective Way to Perform an Airdrop

So this option will require a decent bit of setup but it would indeed be the cheapest to do. You could create a payment channel of sorts by committing a large amount of tokens to a smart contract. You could setup the smart contract such that only permissioned addresses can request an airdrop. In order to request the airdrop they must submit a message signed by you.

This would be incredibly cheap as each user would pay the transaction cost of sending the coins themselves.

example (update):

Each user will be given a particular message h which is a combination of \x19Ethereum Signed Message\n32 + keccak256('user-address', 'airdrop-value') that you have signed, along with the signature parts vrs. When they post to your contract to withdraw their funds, they provide the necessary signature data (h,v,r,s), along with the values that make up message h to so that they can verify the unique message to them, but also verify that it was signed by you.

I recently had to figure this out myself and made a quick python program used to sign data and extract the necessary parts:

https://github.com/postables/Postables-Payment-Channel/blob/develop/python/signer.py

Best Answer

Related Solutions

Solidity – How to Loop Through Mapping in Solidity?

Token Airdrop Guide – Most Cost Effective Way to Perform an Airdrop

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