Why does ++i cost less gas than i++?

Question

Why in solidity, i++ & ++i works the same but one costs a little bit more gas?

Take a look at this contract:

contract foobar {
uint[] public arr = [1,2,3,4,5];

function find() public view returns(bool) {
        for(uint i = 0; i < arr.length; i++ /*++i*/) {
        if(arr[i] == 5) {
            return true;
        }
    }
    return false;
}

}

When I run this contract using i++ this is what the contract's cost is:

// i++
// gas  388824 gas
// transaction cost 338107 gas 
// execution cost   338107 gas 
// foobar.find() execution cost 36221 gas

And when I run the same function using ++i, this is what the cost is:

// ++i
// gas  388327 gas
// transaction cost 337675 gas 
// execution cost   337675 gas
// foobar.find() execution cost 36201 gas

Clearly, i++ costs more gas than ++i, But Why? Thanks

Answer 1

They do not work the same. i++ gets compiled to something like

j = i;
i = i + 1;
return j

while ++i gets compiled to something like

i = i + 1;
return i;

Long story short, i++ returns the non-incremented value, and ++i returns the incremented value, so for example, doing

i = 0;
array[i++];

will access the value at index 0 in the array, while array[++i] will access the value at index 1.