i++ vs. ++i

Last Updated on 24. March 2025 by Mario Oettler

At the first glance incrementing a variable i with i++ or ++i seems to be the same. But in the background things are different.

i++ is called post-increment operator. It creates a copy of i and returns it. Then it increments i.

++i is called pre-increment operator. It increments the value immediately and returns the new value of i. The following code example shows the difference.

pragma solidity 0.8.29;

contract Incrementor{
    uint256 i = 1;
    uint256 j = 1;
    uint256 a = 1;
    function increment_01() public returns(uint256, uint256){
        a = i++;
        return (a, i);
    }

    function increment_02() public returns(uint256, uint256){
        a = ++j;
        return (a, j);
    }
}

Function increment_01() uses the post-increment operator. It returns the following values:

a = 1,

i = 2

Function increment_02() uses the pre-increment operator. It returns the following values:

a = 2

j = 2

So, what does this have to do with gas costs? The post-increment operator (i++) makes a copy of the variable in memory. This costs gas. Hence, the pre-increment operator (++i) is cheaper as it doesn’t make a copy.

The following table shows that gas cost for a single increment.

Operator	Gas Costs
i++	5585
++i	5558