Propeller Manual

2: Spin Language Reference – Operators
This would negate the value of A and store the result back to A.
Decrement, pre- or post- ‘
- -’
The Decrement operator is a special, immediate operator that decrements a variable by one
and assigns the new value to that same variable. It can only be used in run-time variable
expressions. Decrement has two forms, pre-decrement and post-decrement, depending on
which side of the variable it appears on. The pre-decrement form appears to the left of a
variable and the post-decrement form appears to the right of a variable. This is extremely
useful in programming since there are many situations that call for the decrementing of a
variable right before or right after the use of that variable’s value. For example:
Y := --X + 2
The above shows the pre-decrement form; it means “decrement before providing the value for
the next operation”. It decrements the value of
X by one, writes that result to X and provides
that result to the rest of the expression. If
X started out as 5 in this example, --X would store 4
in
X, then the expression, 4 + 2 is evaluated, finally writing the result, 6, into Y. After this
statement,
X equals 4 and Y equals 6.
Y := X-- + 2
The above shows the post-decrement form; it means “decrement after providing the value for
the next operation”. It provides the current value of
X for the next operation in the
expression, then decrements the value of
X by one and writes that result to X. If X began as 5
in this example,
X-- would provide the current value for the expression (5 + 2) to be
evaluated later, then would store 4 in
X. The expression 5 + 2 is then evaluated and the result,
7, is stored into
Y. After this statement, X equals 4 and Y equals 7.
Since Decrement is always an assignment operator, the rules of Intermediate Assignments
(see page 147) apply here. Assume
X started out as 5 for the following examples.
Y := --X + X
Here, X would first be set to 4, then 4 + 4 is evaluated and Y is set to 8.
Y := X-- + X
Here, X’s current value, 5, is saved for the next operation (the Add) and X itself is
decremented to 4, then 5 + 4 is evaluated and
Y is set to 9.
Propeller Manual v1.1 · Page 151