Propeller Manual

Operators – Spin Language Reference
Increment, pre- or post- ‘+ +’
The Increment operator is a special, immediate operator that increments a variable by one and
assigns the new value to that same variable. It can only be used in run-time variable
expressions. Increment has two forms, pre-increment and post-increment, depending on
which side of the variable it appears on. The pre-increment form appears to the left of a
variable and the post-increment form appears to the right of a variable. This is extremely
useful in programming since there are many situations that call for the incrementing of a
variable right before or right after the use of that variable’s value. For example:
Y := ++X - 4
The above shows the pre-increment form; it means “increment before providing the value for
the next operation”. It increments 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
6 in
X, then the expression, 6 - 4 is evaluated, finally writing the result, 2, into Y. After this
statement,
X equals 6 and Y equals 2.
Y := X++ - 4
The above shows the post-increment form; it means “increment after providing the value for
the next operation”. It provides the current value of
X for the next operation in the
expression, then increments the value of
X by one and writes that result to X. If X started out
as 5 in this example,
X++ would provide the current value for the expression (5 - 4) to be
evaluated later, then would store 6 in
X. The expression 5 - 4 is then evaluated and the result,
1, is stored into
Y. After this statement, X equals 6 and Y equals 1.
Since Increment 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 6, then 6 + 6 is evaluated and Y is set to 12.
Y := X++ + X
Here, X’s current value, 5, is saved for the next operation (the Add) and X itself is incremented
to 6, then 5 + 6 is evaluated and
Y is set to 11.
Page 152 · Propeller Manual v1.1