safest and most convenient way to iterate through items

Question

So I am writing a contract on which I have to iterate through array items in order to check customer existence or remove a customer (and so on).

Remix tells me that my actual functions might consume a lot of gas, if that's the case, which is the best way to approach this?

Code:

    function IsUserDone(address user) private view returns(bool){
    for(uint256 i = 0; i < DoneCustomers.length; i++)
    {
        if(DoneCustomers[i] == user)
            return true;
    }
    return false;
}

function IsUserPending(address user) private view returns(bool){
    for(uint256 i = 0; i < PendingCustomers.length; i++)
    {
        if(PendingCustomers[i] == user)
            return true;
    }
    return false;
}

function TransferData(address dude) public {
    require(msg.sender == owner);
    require(IsUserPending(dude));

    for(uint256 i = 0; i < PendingCustomers.length; i++)
    {
        if(PendingCustomers[i] == dude){
            delete PendingCustomers[i];
            owner.transfer(total_price);
            AddDoneCustomers(dude);
        }
    }
}

Answer 1

You could try solidity inline assembly. It's not always more efficient, but in certain conditions it might save some gas, though it requires more knowledge of EVM.

Answer 2

In addition to storing that data in an array (which requires iterating over to find an item buried at the end of it), storing a mapping of items allows for a faster lookup, though requires more gas on initial creation since you're storing one more item to the blockchain:

mapping (address => bool) private pending

function isUserPending(address user) private view returns(bool) {
  return pending[user];
}