1. Subroutines
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2. Subroutine: Intro What is a subroutine? Why use subroutine? Syntax
a separate body of code that perform a specific task
Why use subroutine?
Divide & Conquer:  Example
easier to read/write program
Repeated Task  Example
write once, use anywhere
Syntax
subroutine definition sub subroutine-name { statements }
subroutine invocation &subroutine-name;
cycle -> iterative
similar to system development cycle
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3. Subroutine: Forward Reference
Subroutines can be placed anywhere in a program, but it is best to put them before or after the main routine
If subroutine invocation is below subroutine definition, ‘&’ is not needed.
sub hello { print “hello.\n”; } hello;
When subroutine invocation is above subroutine definition, ‘&’ can be omitted if a forward reference is used.
sub hello; hello; sub hello { print “hello.\n”; }
cycle -> iterative
similar to system development cycle
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4. Subroutine’s Return Value
Use Return Value to pass data from subroutine to main routine
e.g. $price = &get_prices;
Default Return Value  Example
The value of the last expression evaluated in subroutine
Better to be explicit  Example
return_value; #last line of subroutine
return (return_value);  Example
returns return_value and exits subroutine
return_value can be a scalar or a list
cycle -> iterative
similar to system development cycle
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5. Global & Local Variables
Global variables
defined in the main routine
exists everywhere in a program
Local variables
defined with ‘local’ or ‘my’ statement
e.g. local $var1; my $var1;
‘my’ variable exists only in the subroutine where it is declared
‘local’ variable exists in the subroutine where it is declared and subroutines that are called by that subroutine
 Example
cycle -> iterative
similar to system development cycle
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6. Subroutine Arguments &subname (argument list)
use subroutine arguments to pass values from main routine to subroutines
passed argument list is stored in
&sub1($arg1, sub sub1 { my ($v1, … }
Good programming practice  Example
use global and local variables
use subroutine arguments
when in doubt, use return statements
cycle -> iterative
similar to system development cycle
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7. Passing by Value vs. Reference
is copied # - i.e. operations will not @array = (1,2,3,4); &sub1 sub sub1 { my }
Passing by Reference  Example
# reference is passed # is an alias in the subroutine # - i.e. operations will @array = (1,2,3,4); &sub1 sub sub1 { my ($list) }
cycle -> iterative
similar to system development cycle
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8. Passing arrays and hashes
All arguments passed to subroutine as a single list
&sub1
will be empty sub sub1 { my }
Good programming practice
list scalar arguments first
pass arrays and hashes by reference when appropriate
large array/hash
multiple array/hash
need to modify array/hash values
Examples #1 #2
cycle -> iterative
similar to system development cycle
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9. Special Subroutines BEGIN subroutine END subroutine
executes at program start
e.g. BEGIN { print “\n”; }
END subroutine
executes at program termination
e.g. END { print “\n”; }
END subroutine executes even after “die”
AUTOLOAD subroutine
executes when non-existing subroutine is called e.g. AUTOLOAD { print “subroutine $AUTOLOAD not found.\n”; }
 Example
 Review: Hash check example
cycle -> iterative
similar to system development cycle
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