if x = 3 and z is unassigned, why does z = x-- - --x evaluates to 2?

Question

if x = 3 and z is unassigned,
why does z = x-- - --x evaluates to 2?

my professor is lecturing about this at the moment, and I'm currently stuck with this dilemma. Unfortunately, no one can explain why it happens.

Answer 1

on x--, x = 3, and after that it's 2. on --x, x = 1, because substraction (from 2) is done beforehand.

Therefore, 3 - 1 = 2.

Answer 2

Here is the order of operations, illustrated for better understanding:

• x-- - --x Hold value of x (lets call it tmpA). tmpA is 3.
• x-- - --x Decreases x. It is now 2.
• x-- - --x Decreases x. It is now 1.
• x-- - --x Hold value of x (lets call it tmpB). tmpB is 1.
• x-- - --x Performs substruction from calculated values. 3 - 1 = 2.

The -- prefix means the decrement will be done before evaluating the expression and the postfix -- means the decrement will be done after evaluation the expression.