Perl Example #5
Subroutines and Parameter Passing

About the Program

All of the other subroutines, sub2, sub3, sub4, and sub5, receive a "flat" array-type parameter list referenced as @_ , and accesses individual arguments using scalar elements $_[0], $_[1], though. $_[n]. These subroutines show various techniques including return values, mixed parameter types, and the use of reference pointers.

#!/usr/bin/perl
# Subroutines, Parameters, and Reference Variables

$var1 = 13;      # Global Scalar Variable
$var2 = 51;      # Global Scalar Variable

@arr1 = qw(AAA BBB CCC DDD EEE FFF);  	# Global array
					# Uses "qw" command to quote words


# SUB1:  Shows use of subroutine variable "$var1" that has same name as the
#        global one.  The use of "my" forces scoping to subroutine only.

sub sub1
{
   my($var1) = 99;  # Reserved word "my" makes $var1 unique to subroutine

   print "  In sub1: Var1 = $var1   Var2 = $var2   Array1 = @arr1 \n\n";
}

# SUB2:  Demonstrates how to pass a scalar parameter to subroutine.  This
#        parameter does reference the original $var1, and will change the value.
#        Without preferred scoping  using "my", $var3 will also be known in main.
#        The variable @_ represents the full array of parameters passed.

sub sub2
{
   print "  In sub2: Parms = @_ \n";  # The full parameter list

   $var1 = $_[0]; # Passed scalars are referenced as $_[0], $_[1], $_[2], ...
   $var3 = $_[1]; 

   print "  In sub2: Var1 = $var1   Var2 = $var2   Var3 = $var3 \n\n";
}

# SUB3:   Demonstrates how to return a scalar from a subroutine call.  The
#         module modifies a subrange of @arr1, and returns number of changes. 

sub sub3
{  my ($diff);
   print "  In sub3: Parms = @_ \n";     # Print out parameter string
   print "  In sub3:  ";
   for ($_[0]..$_[1])			 # Step through subrange, one by one
      { 
        print $_, ". ",$arr1[$_], "   "; # Default variable is current counter

        $arr1[$_] = $arr1[$_]."XX";      # Concatenate operator is "." (dot)
       }

   print "\n";
   print "  In sub3: @arr1 \n";

   $diff = $_[1] - $_[0] + 1;		 # Determine number of items used

   return $diff;                         # Assign return value to subroutine
}

# SUB4:  Pass mixed parameters to subroutine... Scalar and array types
#        The variable "@_" is a default array of all things passed.
#        Demonstrates the difference between "my" and "local"

sub sub4
{ 
  my ($var1, @temp);    # Variables known only to this subroutine (preferred)
  local (@arr1);   # Variable known here as well as all subroutine calls (rare)

  print "  In sub4:  Parms = ", @_ , "  First = ", $_[0], "\n";

  $var1 = 0;
  @arr1 = qw (1,2,3,4,5);
  sub1;  # The "local" @arr1 will be known to sub1 and not the global @arr1

  $var1 = $_[0];
  @temp = @_[1..$#_];

  for($i = 0; $i <= $#temp; $i++)

     {chop($temp[$i]);
     }

  print "\n";
  $var1 = 99;
  print "  In sub4: Changing first parameter - ", $_[0], "\n";
  print "  In sub4: Temp = ", @temp, "\n";
  return @temp;
}
     

# SUB5: Working with references variables.  The backslash, or "\",  will 
#       send a pointer to a variable. The "$" dereferences that pointer. 
#       Note the difference if the backslash is used inside the print string
#       since "\$" prints the dollar sign character instead.

sub sub5
{
  print "  In sub5: parms = @_ \n";
  my($val1) = $_[0];
  $arry_ptr = $_[1];
  $var2_ptr = $_[2];
  print "  In sub5:  \$val1 = ", $val1, "Address = ", \$val1, "\n";  
  $val1 = $val1 - 1;
  print "  In sub5:  Var1 = ", $val1,  "\n";
  print "  In sub5:  Var2 = ", $var2_ptr, "   Contents = ", $$var2_ptr, "\n";
  $$var2_ptr = $$var2_ptr - 1;
  print "  In sub5:  Variable1 = ", $var2_ptr, "   Contents = ", $$var2_ptr, "\n";
  chop(@$arry_ptr); 
  print "  In sub5:  Array1 = ", $arry_ptr, "   Contents = ", @$arry_ptr, "\n";
}
  

   
# Main starts here:

print "Initial Values:\n";
print "In main: Var1 = $var1   Var2 = $var2   Array1 = @arr1 \n\n";

print "Calling sub1:\n";
sub1;

print "In main: Var1 = $var1   Var2 = $var2   Array1 = @arr1 \n\n";


print "Calling sub2:\n";
sub2($var2, 88);

print "In main: Var1 = $var1   Var2 = $var2  Var3 = $var3   Array1 = @arr1 \n\n";



print "Calling sub3:\n";
$var2 = sub3(2,4);

print "In main: Var1 = $var1   Var2 = $var2   Array1 = @arr1 \n\n";



print "Calling sub4:\n";
@arr1 = sub4($var1,@arr1);

print "In main: Var1 = $var1   Var2 = $var2   Array1 = @arr1 \n\n";




print "Reference values:\n";
print "In main: Var1 =  ", $var1, "\t Pointer = ", \$var1, "\n";
print "In main: Var2 =  ", $var2, "\t Pointer = ", \$var2, "\n";
print "In main: Array = ", @arr1, "\t Pointer = ",\@arr1, "\n";

print "Calling sub5:\n";

sub5($var1, \@arr1, \$var2);
print "In main: Var1 = $var1   Var2 = $var2   Array1 = @arr1 \n\n";

The actual program: ex5.pl

The output: ex5.out