Resetting storage slot increases gas usage although should decrease it

Question

I have a simple contract that deletes the last element of the array:

pragma solidity^0.4.11;

contract GasRefundTest {

    uint[] myArray = [1, 2];

    function deleteLastElem() public returns(bytes32) {
        myArray.length--;
    }
}

Transaction cost for calling deleteLastElem() is 17182 gas.

When I change it to:

pragma solidity^0.4.11;

contract GasRefundTest {

    uint[] myArray = [1, 2];

    function deleteLastElem() public returns(bytes32) {
        delete myArray[1];
        myArray.length--;
    }
}

the transaction cost becomes 22480 gas.

I thought deleting storage slots should result in gas refund, instead I see gas increase.

Can anyone explain what's going on here.

Answer 1

Reducing the size of a dynamic-size array already zeroes out the elements that were "removed."

So the version of your code that first does a delete myArray[1] is just doing an extra write to storage that's about to be done anyway.

Fun things to try:

// This doesn't take more gas depending on how big you make the array.
myArray.length = 3; // or 300 or 3000

myArray = [1, 2, 3, 4, 5, 6, 7, 8, 9, 10];
...
// This takes more gas the more elements you're removing, because it has to zero
// out more positions in storage.
myArray.length = 9; // vs. 1

EDIT

I should point out that this last example is a bit confusing. The gas refund does kick in there, but it's limited to half the consumed gas.

Answer 2

To explain at the high level, you are doing 2 operations instead of 1, hence it requires more gas. All the code you write is compiled to low-level Ethereum Virtual Machine (EVM) commands, which then are interpreted by it. For each such command there is a particular gas price defined, look at this.

Now, in the second case, you use delete. From the docs,

delete a assigns the initial value for the type to a

It is important to note that delete a really behaves like an assignment to a, i.e. it stores a new object in a.

So you are putting 0 there first, before decreasing the length of an array. Purely for the sake of interest, you may try to use myArray[1] = 0; instead and see how this affects the gas used.