Say I have a mapping:

mapping (address => bytes32) public someMapping;

I then set someMapping[msg.sender] = 0

(let's say that address has never been set before). Will that cost gas, or will it figure out that no action is required?

1 Answer 1


Like all the actions that require a computation to be done by miners and that could change the state of the contract => It will cost a certain amount of gas, I tried quickly in remix with the following contract

pragma solidity ^0.4.17;

contract Test {

    mapping (uint256 => address) aMapping;

    function set() public {
        aMapping[1] = this;

    function testA() public  {
        aMapping[0] = 0;

        function testB() public  {
        aMapping[1] = 0;

            function testC() public  {
        aMapping[2] = 0;

And it costed me a little bit of gas each time I called testA/testB/testC

  • Thanks, I'll check this out and see if it's worth checking for a 0 value before simply writing 0.
    – MaxBov
    Commented Dec 10, 2017 at 3:08

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