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:
Often Used Functions (alphabetic listing)
# 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.';
For specific information on a function, use perldoc -f function_name to get a concise summary.
Creating Your Own Functions
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 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
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:
Output:
$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"; Scalar Variable String Interpolation
I have 240 Potato
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:
Operations on Scalar Variables
print "I have \$total_potatoes \$p\n"; # prints: I have $total_potatoes $p
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:
Output:
$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"; Preincrement vs Postincrement
From one potato come 240.
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:
The decrement (--) operator works the same way.
$potatoes = 80; # $potatoes holds 80 $onions = ++$potatoes; # $onions holds 81, $potatoes holds 81 $parsnips = $potatoes++; # parsnips holds 81, $potatoes holds 82
Weird Perl Assignment Idioms
Modify a Value and Save the Original in One Operation
Swap the Values of Two Variables
$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' # $reversec holds 'CCCAAAAGACTC'
Here's a simple way to swap the values of two variables in one fast step:
NOTE: The obvious alternative DOES NOT work:
($onions,$potatoes) = ($potatoes,$onions); # $onions now holds the original value of $potatoes, and vice-versa
Here a correct non-idiomatic alternative:
$onions = $potatoes; $potatoes = $onions; # oops!
Rotate the Values of Three Variables
$tmp = $onions; $onions = $potatoes; $potatoes = $tmp;
($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:Computing the Hypotenuse of a Right Triangle
(~) 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 .
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.
