# Write Perl Functions

You can create your own functions in order to perform complex operations in answer keys, to perform the same steps or calculations multiple times in a question, or to perform the same steps or calculations in multiple questions.

To create your own Perl functions, you use the sub keyword. Your function will have a name and might accept parameters or return one or more values. After creating your function, you can use it by referencing the function name and optionally passing one or more parameters, just like any of the predefined Perl functions. For example:

$myvalue = myfunction(parameter, parameter); Function names must follow the same rules used for variable names: • Function names must contain only letters (a-z, A-Z), underscores (_), and numeric digits (0-9). • The first character of a function name must be a letter (a-z, A-Z) or underscore (_). • Function names are case-sensitive, so myfunction is not the same as MyFunction. 1. In Question, write your function in an <eqn> tag using syntax like the following: sub function_name { my ($parameter_name, $parameter_name ...) = @_; function_statements return value; }  where • function_name is the name of your function. • parameter_name is the variable name of any parameter that is passed to the function. List the parameters in the order in which they will be passed to the function. For example, if your function receives x and y coordinates in that order, you would specify my ($xvalue, $yvalue) = @_; . Note The special array @_ always contains all the values that are passed to a function, but otherwise behaves like any other array. For example, $_[2] contains the third parameter passed to the function.
Best Practice Precede any variable assignments in the function with the keyword my, for example, my $a = 7;. This identifies the variable as private to the function, meaning that the variable exists only within the scope of the function and does not affect a variable with the same name outside of the function. • function_statements are the statements in the body of the function that determine what actions are performed by the function. • value is one or more values that are returned by the function. For example, the following function calculates the area of an ellipse, given radii of r1 and r2: sub ellipse_area { my ($r1, $r2) = @_; my$area = $r1 *$r2 * $pi; return$area;
}
2. In Question, Answer, or Solution, call the function in an <EQN> or <eqn> tag using syntax like the following:
$variable = function_name(parameter_value, parameter_value ...) where • variable is the name of the variable that should get the value returned by your function. To receive multiple values, you can specify an array or a list of variables. • function_name is the name of your function. • parameter_value is the value of any parameter that is passed to the function. For example, the following statement calls the ellipse_area function shown above, passes two radius values to the function, and assigns a value to$key:
$key = ellipse_area(4,5); Note If your function does not return a value, you do not need to assign it to a variable to call it. Instead, simply use the function name and any required parameters. ## Example Question With Function The following table summarizes an actual question. QID Numerical  sub orbital_period { my ($a) = @_; my $period = decform($a**(3/2),0); return \$period; } @a = (randnum(320,990,1)/10, randnum(320,990,1)/10, randnum(320,990,1)/10); '' Use Kepler's Third Law to estimate the periods of hypothetical minor planets with the following semimajor axes:

AU: <_> years
AU: <_> years
AU: <_> years