What does idx mean or do in this contract?

Question

Here is a contract from https://www.etherchain.org/account/0xc18a451d4ef2a85e7bf8404131c5f994a98bdb15#code

What exactly does idx mean or do?

    function enter() {
    if (msg.value < 500 finney) {
        msg.sender.send(msg.value);
        return;
    }

    // add a new participant to array
    uint idx = participants.length;
    participants.length += 1;
    participants[idx].etherAddress = msg.sender;
    participants[idx].amount = msg.value;

    // collect fees and update contract balance
    if (idx != 0) {
        collectedFees += msg.value / 10;
        balance += msg.value;
    } 
    else {
        //code 
    }

Answer 1

The line uint idx = participants.length; gets the length of the array and participants.length += 1; this one adds 1 to it. Finally participants[idx] adds a new key to the array and assign new participants to it.

An alternative will be a simple push and this #83 might be the reason why the coder avoid push