topic 4:subroutines cse2395/cse3395 perl programming learning perl 3rd edition chapter 4, pages...

Topic 4:SubroutinesTopic 4:Subroutines

CSE2395/CSE3395Perl Programming

CSE2395/CSE3395Perl Programming

Learning Perl 3rd edition chapter 4, pages 56-72, 88-91

Programming Perl 3rd edition pages 80-83, 217-233, 659, 742-745

perlsub manpage

2Original Slides by Debbie Pickett, Modified by David Abramson, 2006, Copyright Monash University

In this topicIn this topic

Command-line arguments The diamond operator <> Subroutines

► declaring► calling► returning from

Local variables Perl debugger

Command-line arguments The diamond operator <> Subroutines

► declaring► calling► returning from

Local variables Perl debugger


Command-line argumentsCommand-line arguments

Words on command line after script name are considered command-line arguments► perl -w mystery Agatha Arthur "Erle Stanley"

– three command-line arguments: “Agatha”, “Arthur”, “Erle Stanley”– broken up into space-separated words by shell– except where quoted or escaped

Command-line arguments are available to Perl in @ARGV array

► first argument is in $ARGV[0]– not like C, where argv[0] is program name– Perl script name can be found in special variable $0 (zero)

► unlike C, no need for argc parameter because @ARGV’s size is known

► if no command-line arguments, @ARGV is empty

Words on command line after script name are considered command-line arguments► perl -w mystery Agatha Arthur "Erle Stanley"

– three command-line arguments: “Agatha”, “Arthur”, “Erle Stanley”– broken up into space-separated words by shell– except where quoted or escaped

Command-line arguments are available to Perl in @ARGV array

► first argument is in $ARGV[0]– not like C, where argv[0] is program name– Perl script name can be found in special variable $0 (zero)

► unlike C, no need for argc parameter because @ARGV’s size is known

► if no command-line arguments, @ARGV is empty

Llama3 pages 90-91; Camel3 page 659; perlvar manpage


TimeoutTimeout

#!/usr/bin/perl -w print "Program name is: $0\n";print "Parameters are:\n";for ($i = 0; $i <= $#ARGV; $i++){ print " $i: $ARGV[$i]\n";}

#!/usr/bin/perl -w print "Program name is: $0\n";print "Parameters are:\n";for ($i = 0; $i <= $#ARGV; $i++){ print " $i: $ARGV[$i]\n";}


FiltersFilters

Many Unix programs can act as filters► if files named on command line, program processes

files in order– sort file1 file3 file4 # Sort all together

► if no files named on command line, program processes standard input– cat file1 file3 file4 | sort # Same

► filename “-” also means standard input– sort filling | cat slice1 - slice2 # Sandwich

Perl makes this easy with <> (“diamond”) operator► <> reads lines of input, like <STDIN>

Many Unix programs can act as filters► if files named on command line, program processes

files in order– sort file1 file3 file4 # Sort all together

► if no files named on command line, program processes standard input– cat file1 file3 file4 | sort # Same

► filename “-” also means standard input– sort filling | cat slice1 - slice2 # Sandwich

Perl makes this easy with <> (“diamond”) operator► <> reads lines of input, like <STDIN>

Llama3 pages 88-90; Camel3 pages 80-83


<> (diamond) operator<> (diamond) operator

If files named as command-line arguments► <> returns lines from file named in $ARGV[0]► then returns lines from file named in $ARGV[1]► until all lines in all files read, then returns undef► no need to open or close files or detect EOF between

files

If no command-line arguments► <> behaves the same as <STDIN>

If files named as command-line arguments► <> returns lines from file named in $ARGV[0]► then returns lines from file named in $ARGV[1]► until all lines in all files read, then returns undef► no need to open or close files or detect EOF between

files

If no command-line arguments► <> behaves the same as <STDIN>


TimeoutTimeout

#!/usr/bin/perl -w

# Unix cat program

while (<>){ print;}

#!/usr/bin/perl -w

# Unix cat program

while (<>){ print;}


TimeoutTimeout

#!/usr/bin/perl -w

# Even shorter Unix cat program

print while (<>); # That’s it.

#!/usr/bin/perl -w

# Even shorter Unix cat program

print while (<>); # That’s it.


SubroutinesSubroutines

In Perl, functions are called subroutines► declared with sub keyword

In Perl, functions are called subroutines► declared with sub keyword

sub name{ # Body goes here}

sub keyword declares the subroutine

put name of subroutine here

Llama3 page 57; Camel3 pages 217-218 ; perlsyn manpage


Calling subroutinesCalling subroutines

Subroutines officially begin with & character► $thing is scalar, @thing is array, &thing is

subroutine

In practice, & is almost never needed► not used when defining with sub keyword► not needed when calling because parentheses

identify it as a subroutine► &weekday(2004,6,1) or weekday(2004,6,1)

Subroutines officially begin with & character► $thing is scalar, @thing is array, &thing is

subroutine

In practice, & is almost never needed► not used when defining with sub keyword► not needed when calling because parentheses

identify it as a subroutine► &weekday(2004,6,1) or weekday(2004,6,1)

Llama3 pages57-58, 70-71; Camel3 page 218


Calling subroutinesCalling subroutines

Caller names arguments as list after subroutine name► $newyearsday = weekday($year, 1, 1);► as with built-in functions, can omit parentheses if

subroutine is predeclared Argument list is evaluated and placed in special

local array variable @_► argument list @_ is unrelated to default argument $_

Subroutine can access members of @_ to get parameter values► sub weekday { ($y, $m, $d) = @_; ... }► sub weekday { $y = $_[0]; ... }

Caller names arguments as list after subroutine name► $newyearsday = weekday($year, 1, 1);► as with built-in functions, can omit parentheses if

subroutine is predeclared Argument list is evaluated and placed in special

local array variable @_► argument list @_ is unrelated to default argument $_

Subroutine can access members of @_ to get parameter values► sub weekday { ($y, $m, $d) = @_; ... }► sub weekday { $y = $_[0]; ... }

Llama3 pages 60-62; Camel3 pages 219-221


Subroutine returnSubroutine return

Return value of a subroutine is the last expression evaluated► sub pi { 3.1415926535898; }

Can use return keyword to return sooner► sub absx { if ($x >= 0) { return $x; } else { return -$x; } }

Return value can be scalar or list► return (1, 2, 3);► return value is evaluated in scalar or list context

depending on context subroutine was called in► can use wantarray function to determine context

Return value of a subroutine is the last expression evaluated► sub pi { 3.1415926535898; }

Can use return keyword to return sooner► sub absx { if ($x >= 0) { return $x; } else { return -$x; } }

Return value can be scalar or list► return (1, 2, 3);► return value is evaluated in scalar or list context

depending on context subroutine was called in► can use wantarray function to determine context

Llama3 pages 58-60, 69-70; Camel3 pages 219,228


TimeoutTimeout

# Compute the hypotenuse of a triangle

# Declare the subroutinesub hypotenuse{ # Return root of sum of squares. sqrt($_[0] * $_[0] + $_[1] * $_[1]);}

# Read two numbers.print "Enter first number: ";chomp ($a = <STDIN>);print "Enter second number: ";chomp ($b = <STDIN>);print "Hypotenuse is ", hypotenuse($a, $b), "\n";


# Declare the subroutinesub hypotenuse{ # Return root of sum of squares. sqrt($_[0] * $_[0] + $_[1] * $_[1]);}



TimeoutTimeout


# Declare the subroutinesub hypotenuse{ # Give the parameters meaningful names. ($x, $y) = @_; # This has a slight bug. sqrt($x * $x + $y * $y);}



# Declare the subroutinesub hypotenuse{ # Give the parameters meaningful names. ($x, $y) = @_; # This has a slight bug. sqrt($x * $x + $y * $y);}



Local variables with myLocal variables with my

By default, all variables are global Variables can be declared local with my function

► my $x;

Variables declared with my are accessible only within enclosing block (usually until end of loop or subroutine)

► “lexical scoping”, like local variables in C► Perl also has “dynamic scoping” like shell, uses local function

Can localize more than one variable at once► my ($x, $y, @result); # Need brackets

Can localize and initialize in one step► my ($x, $y) = @_;

By default, all variables are global Variables can be declared local with my function

► my $x;

Variables declared with my are accessible only within enclosing block (usually until end of loop or subroutine)

► “lexical scoping”, like local variables in C► Perl also has “dynamic scoping” like shell, uses local function

Can localize more than one variable at once► my ($x, $y, @result); # Need brackets

Can localize and initialize in one step► my ($x, $y) = @_;

Llama3 pages 62-64; Camel3 pages 132-136; perlfunc manpage


TimeoutTimeout

# Sum a variable-length list of numbers.

# Read some numbers into @nums.while (<>){ chomp; push @nums, $_;}print "Sum is ", sum (@nums), "\n";

sub sum { my $sum = 0; # Iterate over parameter list @_. foreach my $num (@_) # With local iterator $num. { $sum += $num; # Add this element to running total. } return $sum;}

# Sum a variable-length list of numbers.

# Read some numbers into @nums.while (<>){ chomp; push @nums, $_;}print "Sum is ", sum (@nums), "\n";

sub sum { my $sum = 0; # Iterate over parameter list @_. foreach my $num (@_) # With local iterator $num. { $sum += $num; # Add this element to running total. } return $sum;}


Passing array parametersPassing array parameters

Arrays are flattened into lists before being put into parameter array @_► func($x, $y, @array)

– $_[0] = x; $_[1] = y; $_[2..@array+1] = @array;► sizes of arrays are lost in flattening► array must be reconstructed from @_ by subroutine

– e.g., my ($x, $y, @array) = @_;

If passing one array to a subroutine, make it the last argument► func($x, $y, @array) # Not func($x, @array, $y)

– elements $_[2] to end must be from @array

Don’t pass more than one array to a function► diff(@a, @b) won’t work because of list flattening

– diff won’t know where @a ends and where @b begins► can be solved using references (Topic 11)

Arrays are flattened into lists before being put into parameter array @_► func($x, $y, @array)

– $_[0] = x; $_[1] = y; $_[2..@array+1] = @array;► sizes of arrays are lost in flattening► array must be reconstructed from @_ by subroutine

– e.g., my ($x, $y, @array) = @_;

If passing one array to a subroutine, make it the last argument► func($x, $y, @array) # Not func($x, @array, $y)

– elements $_[2] to end must be from @array

Don’t pass more than one array to a function► diff(@a, @b) won’t work because of list flattening

– diff won’t know where @a ends and where @b begins► can be solved using references (Topic 11)

Camel3 pages 221,224-225


Subroutines in built-in functionsSubroutines in built-in functions

Anonymous subroutine bodies (“closures”) are used by some built-in Perl functions

map { code } @list► “modify” operation: create a new list with code applied to each

element grep { code } @list

► “filter” operation: create a new list from only the elements where code is true

sort { code } @list► custom sorting: use code to compare two elements of list to

determine their sorted order Can also use closures in user-defined functions

► requires use of references (Topic 11)

Anonymous subroutine bodies (“closures”) are used by some built-in Perl functions

map { code } @list► “modify” operation: create a new list with code applied to each

element grep { code } @list

► “filter” operation: create a new list from only the elements where code is true

sort { code } @list► custom sorting: use code to compare two elements of list to

determine their sorted order Can also use closures in user-defined functions

► requires use of references (Topic 11)

Llama3 pages 213-219, 236-238; Camel3 pages 730, 740-741; perlfunc manpage


TimeoutTimeout

# Censor: remove four-letter words from input.

while (<>){ # Break line into words. # split function explained in topic 7. @words = split /\s+/, $_;

# Put only words which match the expression # (length not equal to 4) into @safewords. # $_ is each element of @words in turn # (a bit like foreach loop iterator). @safewords = grep { length $_ != 4 } @words;

print "@safewords\n";}

# Censor: remove four-letter words from input.

while (<>){ # Break line into words. # split function explained in topic 7. @words = split /\s+/, $_;

# Put only words which match the expression # (length not equal to 4) into @safewords. # $_ is each element of @words in turn # (a bit like foreach loop iterator). @safewords = grep { length $_ != 4 } @words;

print "@safewords\n";}


TimeoutTimeout

# Sorting a list numerically

@list = (1, 64, 8, 2, 16, 128, 4, 32);

# sort function normally sorts items# alphabetically as strings.@alphabetically = sort @list;print "As strings: @alphabetically\n";

# Sorting comparison routine (automatically given two# parameters $a and $b) can specify how items collate.# -1: $a precedes $b; +1: $a follows $b; 0: equal@numerically = sort { if ($a < $b) { return -1; } elsif ($a > $b) { return 1; } else { return 0; }} @list;print "As numbers: @numerically\n";


@list = (1, 64, 8, 2, 16, 128, 4, 32);


# Sorting comparison routine (automatically given two# parameters $a and $b) can specify how items collate.# -1: $a precedes $b; +1: $a follows $b; 0: equal@numerically = sort { if ($a < $b) { return -1; } elsif ($a > $b) { return 1; } else { return 0; }} @list;print "As numbers: @numerically\n";


TimeoutTimeout


@list = (1, 64, 8, 2, 16, 128, 4, 32);


# Numerical sorting is so common there is# shorthand for it using the "spaceship"# operator <=> which returns -1, 0 or +1@numerically = sort { $a <=> $b } @list;print "As numbers: @numerically\n";


@list = (1, 64, 8, 2, 16, 128, 4, 32);


# Numerical sorting is so common there is# shorthand for it using the "spaceship"# operator <=> which returns -1, 0 or +1@numerically = sort { $a <=> $b } @list;print "As numbers: @numerically\n";


Perl debuggerPerl debugger

Perl has a built-in source-level debugger► perl -d script ...arguments...

Most useful commands► r (run)► n (next line, skip through nested subroutines)► s (step, stop at subroutine calls)► b line (break when line reached)► c (continue running)► p expr (print expression)► x list (examine list expression)► l (list code)► q (quit debugger)

Perl has a built-in source-level debugger► perl -d script ...arguments...

Most useful commands► r (run)► n (next line, skip through nested subroutines)► s (step, stop at subroutine calls)► b line (break when line reached)► c (continue running)► p expr (print expression)► x list (examine list expression)► l (list code)► q (quit debugger)

Llama3 page 294; Camel3 pages 506-517; perldebug manpage


Covered in this topicCovered in this topic

Command-line arguments► @ARGV array

The diamond operator <>► related to @ARGV

Subroutines► declaring with sub► calling► returning from

Local variables► my function

Built-in functions using closures► map, grep, sort

Perl debugger

Command-line arguments► @ARGV array

The diamond operator <>► related to @ARGV

Subroutines► declaring with sub► calling► returning from

Local variables► my function

Built-in functions using closures► map, grep, sort

Perl debugger


Going furtherGoing further

Prototypes► making user-defined subroutines behave more like builtins► Camel3 pages 225-228

Code generation► building Perl code on the fly with eval► Camel3 pages 705-707

Built-in functions► a myriad of standard subroutines provided by Perl► Camel3 pages 677-830

BEGIN and END► special functions that run before or after other code (see also awk)

► Camel3 pages 480-485

Prototypes► making user-defined subroutines behave more like builtins► Camel3 pages 225-228

Code generation► building Perl code on the fly with eval► Camel3 pages 705-707

Built-in functions► a myriad of standard subroutines provided by Perl► Camel3 pages 677-830

BEGIN and END► special functions that run before or after other code (see also awk)

► Camel3 pages 480-485


Next topicNext topic

Hashes► aka associative arrays► arrays indexed by strings, not numbers

Hash variables and functions

Hashes► aka associative arrays► arrays indexed by strings, not numbers

Hash variables and functions

Llama3 chapter 5, pages 73-85Camel3 pages 76-78, 697-700, 703-704, 733-734perldata manpage

topic 4:subroutines cse2395/cse3395 perl programming learning perl 3rd edition chapter 4, pages...

Documents