Thread: >>Perl Basics 3<<

Functions

In addition to its operators, Perl has many functions. Functions have a human-readable name, such as print and take one or more arguments passed as a list. A function may return no value, a single value (AKA "scalar"), or a list (AKA "array"). You can enclose the argument list in parentheses, or leave the parentheses off.A few examples:

# The function is print. Its argument is a string. # The effect is to print the string to the terminal.print "The rain in Spain falls mainly on the plain.\n"; # Same thing, with parentheses.print("The rain in Spain falls mainly on the plain.\n"); # You can pass a list to print. It will print each argument. # This prints out "The rain in Spain falls 6 times in the plain."print "The rain in Spain falls ",2*4-2," times in the plain.\n"; # Same thing, but with parentheses.print ("The rain in Spain falls ",2*4-2," times in the plain.\n"); # The length function calculates the length of a string, # yielding 45.length "The rain in Spain falls mainly on the plain.\n"; # The split function splits a string based on a delimiter pattern # yielding the list ('The','rain in Spain','falls mainly','on the plain.')split '/','The/rain in Spain/falls mainly/on the plain.';

Often Used Functions (alphabetic listing)

For specific information on a function, use perldoc -f function_name to get a concise summary.

abs	absolute value
chdir	change current directory
chmod	change permissions of file/directory
chomp	remove terminal newline from string variable
chop	remove last character from string variable
chown	change ownership of file/directory
close	close a file handle
closedir	close a directory handle
cos	cosine
defined	test whether variable is defined
delete	delete a key from a hash
die	exit with an error message
each	iterate through keys & values of a hash
eof	test a filehandle for end of file
eval	evaluate a string as a perl expression
exec	quit Perl and execute a system command
exists	test that a hash key exists
exit	exit from the Perl script
glob	expand a directory listing using shell wildcards
gmtime	current time in GMT
grep	filter an array for entries that meet a criterion
index	find location of a substring inside a larger string
int	throw away the fractional part of a floating point number
join	join an array together into a string
keys	return the keys of a hash
kill	send a signal to one or more processes
last	exit enclosing loop
lc	convert string to lowercase
lcfirst	lowercase first character of string
length	find length of string
local	temporarily replace the value of a global variable
localtime	return time in local timezone
log	natural logarithm
m//	pattern match operation
map	perform on operation on each member of array or list
mkdir	make a new directory
my	create a local variable
next	jump to the top of enclosing loop
open	open a file for reading or writing
opendir	open a directory for listing
pack	pack a list into a compact binary representation
package	create a new namespace for a module
pop	pop the last item off the end of an array
print	print to terminal or a file
printf	formatted print to a terminal or file
push	push a value onto the end of an array
q/STRING/	generalized single-quote operation
qq/STRING/	generalized double-quote operation
qx/STRING/	generalized backtick operation
qw/STRING/	turn a space-delimited string of words into a list
rand	random number generator
read	read binary data from a file
readdir	read the contents of a directory
readline	read a line from a text file
readlink	determine the target of a symbolic link
redo	restart a loop from the top
ref	return the type of a variable reference
rename	rename or move a file
require	load functions defined in a library file
return	return a value from a user-defined subroutine
reverse	reverse a string or list
rewinddir	rewind a directory handle to the beginning
rindex	find a substring in a larger string, from right to left
rmdir	remove a directory
s///	pattern substitution operation
scalar	force an expression to be treated as a scalar
seek	reposition a filehandle to an arbitrary point in a file
select	make a filehandle the default for output
shift	shift a value off the beginning of an array
sin	sine
sleep	put the script to sleep for a while
sort	sort an array or list by user-specified criteria
splice	insert/delete array items
split	split a string into pieces according to a pattern
sprintf	formatted string creation
sqrt	square root
stat	get information about a file
sub	define a subroutine
substr	extract a substring from a string
symlink	create a symbolic link
system	execute an operating system command, then return to Perl
tell	return the position of a filehandle within a file
tie	associate a variable with a database
time	return number of seconds since January 1, 1970
tr///	replace characters in a string
truncate	truncate a file (make it smaller)
uc	uppercase a string
ucfirst	uppercase first character of a string
umask	change file creation mask
undef	undefine (remove) a variable
unlink	delete a file
unpack	the reverse of pack
untie	the reverse of tie
unshift	move a value onto the beginning of an array
use	import variables and functions from a library module
values	return the values of a hash variable
wantarray	return true in an array context
warn	print a warning to standard error
write	formatted report generation

Creating Your Own Functions


You can define your own functions or redefine the built-in ones using the sub function. This is described in more detail in a later lecture.

Variables

A variable is a symbolic placeholder for a value, a lot like the variables in algebra. Perl has several built-in variable types:
Scalars: $variable_nameA single-valued variable, always preceded by a $ sign.Arrays: @array_nameA multi-valued variable indexed by integer, preceded by an @ sign.Hashes: %hash_nameA multi-valued variable indexed by string, preceded by a % sign.Filehandle: FILEHANDLE_NAMEA file to read and/or write from. Filehandles have no special prefix, but are usually written in all uppercase.We discuss arrays, hashes and filehandles later.Scalar Variables

Scalar variables have names beginning with $. The name must begin with a letter or underscore, and can contain as many letters, numbers or underscores as you like. These are all valid scalars:

• $foo
• $The_Big_Bad_Wolf
• $R2D2
• $_____A23

$Once_Upon_a_Midnight_Dreary_While_I_Pondered_Weak _and_Weary

You assign values to a scalar variable using the = operator (not to be confused with ==, which is numeric comparison). You read from scalar variables by using them wherever a value would go.
A scalar variable can contain strings, floating point numbers, integers, and more esoteric things. You don't have to predeclare scalars. A scalar that once held a string can be reused to hold a number, and vice-versa:


Code:

$p = 'Potato'; # $p now holds the string "potato" $bushels = 3; # $bushels holds the value 3 $potatoes_per_bushel = 80; # $potatoes_per_bushel contains 80; $total_potatoes = $bushels * $potatoes_per_bushel; # 240 print "I have $total_potatoes $p\n";

Output:

I have 240 Potato

Scalar Variable String Interpolation

The example above shows one of the interesting features of double-quoted strings. If you place a scalar variable inside a double quoted string, it will be interpolated into the string. With a single-quoted string, no interpolation occurs.
To prevent interpolation, place a backslash in front of the variable:

print "I have \$total_potatoes \$p\n"; # prints: I have $total_potatoes $p

Operations on Scalar Variables

You can use a scalar in any string or numeric expression like $hypotenuse = sqrt($x**2 + $y**2) or $name = $first_name . ' ' . $last_name. There are also numerous shortcuts that combine an operation with an assignment:


$a++Increment $a by one$a--Decrement $a by one$a += $bModify $a by adding $b to it.$a -= $bModify $a by subtracting $b from it.$a *= $bModify $a by multiplying $b to it.$a /= $bModify $a by dividing it by $b.$a .= $bModify the string in $a by appending $b to it.

Example Code:

$potatoes_per_bushel = 80; # $potatoes_per_bushel contains 80; $p = 'one'; $p .= ' '; # append a space $p .= 'potato'; # append "potato" $bushels = 3; $bushels *= $potatoes_per_bushel; # multiply print "From $p come $bushels.\n";

Output:

From one potato come 240.

Preincrement vs Postincrement

The increment (++) operator can be placed before or after the variable name, and in either case, the effect on the variable is to bump it up by one. However, when you put the operator before the variable name, the value of the expression as a whole is the value of the variable after the operation (preincrement). If you put the operator after the variable name, the value of the expression is the value of the variable before it was incremented:

$potatoes = 80; # $potatoes holds 80 $onions = ++$potatoes; # $onions holds 81, $potatoes holds 81 $parsnips = $potatoes++; # parsnips holds 81, $potatoes holds 82

The decrement (--) operator works the same way.
Weird Perl Assignment Idioms



Modify a Value and Save the Original in One Operation

$potatoes = 80; # $potatoes holds 80 ($onions = $potatoes) += 10; # $onions now 90, and $potatoes still 80 $sequence = 'GAGTCTTTTGGG'; ($reversec = reverse $sequence) =~ tr/GATC/CTAG/; # reverse reverses a string # tr/// translates one set of characters into another # $sequence holds 'GAGTCTTTTGGG' # $reversec holds 'CCCAAAAGACTC'

Swap the Values of Two Variables

Here's a simple way to swap the values of two variables in one fast step:

($onions,$potatoes) = ($potatoes,$onions); # $onions now holds the original value of $potatoes, and vice-versa

NOTE: The obvious alternative DOES NOT work:

$onions = $potatoes; $potatoes = $onions; # oops!

Here a correct non-idiomatic alternative:

$tmp = $onions; $onions = $potatoes; $potatoes = $tmp;

Rotate the Values of Three Variables

($onions,$potatoes,$turnips) = ($potatoes,$turnips,$onions); # $onions <- $potatoes # $potatoes <- $turnips # $turnips <- $onions

Processing Command Line Arguments

When a Perl script is run, its command-line arguments (if any) are stored in an automatic array called @ARGV. You'll learn how to manipulate this array later. For now, just know that you can call the shift function repeatedly from the main part of the script to retrieve the command line arguments one by one.

Printing the Command Line Argument

Code:

#!/usr/bin/perl # file: echo.pl $argument = shift; print "The first argument was $argument.\n";

Output:

(~) 50% chmod +x echo.pl(~) 51% echo.pl tunaThe first argument was tuna.(~) 52% echo.pl tuna fishThe first argument was tuna.(~) 53% echo.pl 'tuna fish'The first argument was tuna fish.(~) 53% echo.plThe first argument was .

Computing the Hypotenuse of a Right Triangle

Code:

#!/usr/bin/perl # file: hypotenuse.pl $x = shift; $y = shift; $x>0 and $y>0 or die "Must provide two positive numbers"; print "Hypotenuse=",sqrt($x**2+$y**2),"\n";

Output:

(~) 82% hypotenuse.plMust provide two positive numbers at hypotenuse.pl line 6.(~) 83% hypotenuse.pl 1Must provide two positive numbers at hypotenuse.pl line 6.(~) 84% hypotenuse.pl 3 4Hypotenuse=5(~) 85% hypotenuse.pl 20 18Hypotenuse=26.9072480941474(~) 86% hypotenuse.pl -20 18Must provide two positive numbers at hypotenuse.pl line 6.

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