setting the length property of array

Question

What does setting the length property of an array do? In most other languages length is a function that return the length of an array.

I have the following code:

struct ledger_t {
  uint8 WinningNum1;
  uint8 WinningNum2;
  uint8 WinningNum3;
  uint8 WinningNum4;
  bytes32 ClosingHash;
  bytes32 OpeningHash;
  mapping(address=>bet_t) bets;
  uint Guess4OutOf4;
  uint Guess3OutOf4;
  uint Guess2OutOf4;
  uint Guess1OutOf4;
  uint PriceOfTicket;
  uint ExpirationTime; //for eventual refunds only, ~2 weeks after draw announced
}
ledger_t[] public ledger;

//constructor
function TheEthereumLottery() {
  owner = msg.sender;
  ledger.length = 0;
  IndexOfCurrentDraw = 0;
  referral_fee = 90;
}

Answer 1

What does setting the length property of an array do.

It sets the length of the array.

The proviso is that the array must be dynamic, and must be a storage rather than memory variable.

From the documentation:

You can resize a dynamic array in storage (i.e. an array declared at the contract level) with arrayname.length = <some new length>;. If you get the “lvalue” error, you are probably doing one of two things wrong.

• You might be trying to resize an array in “memory”, or
• You might be trying to resize a non-dynamic array.