Is the gas usage constant for this function regardless of mapping/array size (assuming non-zero array entries)?

items is an array, and presence is a mapping

function clear() public {
    for (uint i = 0; i < items.length; i++) {
        presence[items[i]] = 0;

    delete items;

In other words, is the gas refund from deleting the data greater than the gas consumed when iterating and querying the data?

This function is a snippet from this Set implementation.

1 Answer 1


A quick analysis shows that the function call gas usage isn't O(1) and also isn't close to O(1). After more than 2 items, the gas usage starts rising for every additional item.

Here is a table of gas usage for clearing a varying number of items (tested using https://remix.ethereum.org):

Number of items Gas used to clear items
0 23823
1 18644
2 23976
3 29308
4 34640

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