How can I make a variable always equal to the result of some calculations?

Question

In math, if z = x + y / 2, then z will always change whenever we replace the value of x and y. Can we do that in programming

Answer 1

There are two chief techniques:

1. Deferred calculation - instead of z being a simple variable, make it a function which calculates the value on demand (see other answers for examples). This can be source-code transparent if z is some proxy object with implicit conversion to the required type (as in Aconcagua's answer).

2. Explicit notification of changes. This requires x and y to be observable types; when either changes value, then z updates itself (and notifies its observers if applicable).

The first version is usually preferred, but the second may be more appropriate if you need z to be an observable type.