Is it possible to calculate the gas costs of a transaction exactly upfront?

Question

In the Ethereum wallet there are the estimated gas costs. Why is it only an estimate? Is is possible to calculate the gas costs precisely upfront?

Answer 1

The short answer is: it depends, for some transactions state changes might change the pre calculated gas costs.

It is possible to calculate the gas costs of every transaction given the current state. However - since the state can change until the transaction is finally executed the state can change.

Consider the following code:

if a = true:
    cheap computation
else:
    expensive computation

Now it is possible that the state of a will change. However - there are a lot of transactions that have always the same costs independent of the state. E.g. a regular value transfer to an account without associated code will always have a fixed cost.