How to loop through mapping in solidity with least gas cost possible?

Question

I have a mapping defined in my contract like below.

mapping(address => uint256) public stakers

I need to loop through it to get all of them for my frontend and other contract.

function getStakers() external returns(address[] memory, uint256[] memory) {
  // I need something like below
  for(i=0; i < stakers.length; i++) {
    sAddress[i] = address_from_mapping;
    sAmount[i] = uint256_from_mapping;
  }
  return (sAddress, sAmount);
}

Not sure how to implement it and also making it gas efficient as much as possible.

Answer 1

You can't call .length on stakers, since it's a mapping an not an array.

What I suggest you do is to use a struct and return an array of tuples.

// SPDX-License-Identifier: GPL-3.0

pragma solidity >=0.7.0 <0.9.0;

contract Stakers {

    struct Staker {
        address addrs;
        uint amount;
    }

    Staker[] public stakers;

    function addStaker(address _addr, uint _amount) external {
        stakers.push(Staker(_addr, _amount));
    }

    function getStakers() external view returns(Staker[] memory) {
        return stakers;
    }

}

You'll still be able to use the tuple return values in your front end.

Let me know if this helps.