Crestron SIMPL+ Programming Manual page 29

Crestron SIMPL+
Programming to anticipate
user errors and handle them
appropriately is called error-
trapping. It is a
recommended programming
practice.
Programming Guide – DOC. 5789A
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Expressions
EXPRESSION
a = 3
b*4 - a/3
1
0
One limitation with the if construct, as shown above, is that the code inside the if is
run whenever expression1 evaluates as TRUE, but any code after the closing braces
runs regardless. It is often useful to execute one set of code when a condition is
TRUE and then another set of code if that same condition is FALSE. For this
application, use the if-else construct, which looks like the following.
if (expression1)
{
// do something if expression1 is true
}
else
{
// do something else if expression1 is false
}
It should be clear that the code following the if runs whenever expression1 evaluates
to TRUE and the code following the else executes whenever expression1 evaluates
to FALSE. Obviously, there can never be a case where both sections of code execute
together.
The following example is designed to control a CD changer. Before telling the CD
player to go to a particular disc number, it checks to see that the analog value, which
represents the disc number, does not exceed the maximum value.
#DEFINE_CONSTANT NUMDISCS 100
ANALOG_INPUT disc_number;
STRING_OUTPUT CD_command, message;
CHANGE disc_number
{
if (disc_number <= NUMDISCS)
{
CD_command = "DISC " + itoa(disc_number) + "\r";
message = "Changing to disc " + itoa(disc_number) +
}
else
{
message = "Illegal disc number\n";
}
}
There is one last variation on the if-else statement. In the example above, the
decision to be made is binary. That is, do one thing if this is true, otherwise do
something else. In some cases decisions are not that straight forward. For example,
to check the current day of the week, execute one set of code if it is Saturday,
another set of code if it is Sunday, and yet some other code if it is any other day of
the week. One way to accomplish this is by using a series of if-else statements. For
this example, the code might look like the following.
EVALUATES TO
true if a=3, false otherwise
true as long as the result is non-zero
always true
always false
"\n";
Software
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• 25
SIMPL+