G4M3R
23-03-13, 05:29 PM
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 (http://stein.cshl.org/cgi-bin/perlref?func=abs)
absolute value
chdir (http://stein.cshl.org/cgi-bin/perlref?func=chdir)
change current directory
chmod (http://stein.cshl.org/cgi-bin/perlref?func=chmod)
change permissions of file/directory
chomp (http://stein.cshl.org/cgi-bin/perlref?func=chomp)
remove terminal newline from string variable
chop (http://stein.cshl.org/cgi-bin/perlref?func=chop)
remove last character from string variable
chown (http://stein.cshl.org/cgi-bin/perlref?func=chown)
change ownership of file/directory
close (http://stein.cshl.org/cgi-bin/perlref?func=close)
close a file handle
closedir (http://stein.cshl.org/cgi-bin/perlref?func=closedir)
close a directory handle
cos (http://stein.cshl.org/cgi-bin/perlref?func=cos)
cosine
defined (http://stein.cshl.org/cgi-bin/perlref?func=defined)
test whether variable is defined
delete (http://stein.cshl.org/cgi-bin/perlref?func=delete)
delete a key from a hash
die (http://stein.cshl.org/cgi-bin/perlref?func=die)
exit with an error message
each (http://stein.cshl.org/cgi-bin/perlref?func=each)
iterate through keys & values of a hash
eof (http://stein.cshl.org/cgi-bin/perlref?func=eof)
test a filehandle for end of file
eval (http://stein.cshl.org/cgi-bin/perlref?func=eval)
evaluate a string as a perl expression
exec (http://stein.cshl.org/cgi-bin/perlref?func=exec)
quit Perl and execute a system command
exists (http://stein.cshl.org/cgi-bin/perlref?func=exists)
test that a hash key exists
exit (http://stein.cshl.org/cgi-bin/perlref?func=exit)
exit from the Perl script
glob (http://stein.cshl.org/cgi-bin/perlref?func=glob)
expand a directory listing using shell wildcards
gmtime (http://stein.cshl.org/cgi-bin/perlref?func=gmtime)
current time in GMT
grep (http://stein.cshl.org/cgi-bin/perlref?func=grep)
filter an array for entries that meet a criterion
index (http://stein.cshl.org/cgi-bin/perlref?func=index)
find location of a substring inside a larger string
int (http://stein.cshl.org/cgi-bin/perlref?func=int)
throw away the fractional part of a floating point number
join (http://stein.cshl.org/cgi-bin/perlref?func=join)
join an array together into a string
keys (http://stein.cshl.org/cgi-bin/perlref?func=keys)
return the keys of a hash
kill (http://stein.cshl.org/cgi-bin/perlref?func=kill)
send a signal to one or more processes
last (http://stein.cshl.org/cgi-bin/perlref?func=last)
exit enclosing loop
lc (http://stein.cshl.org/cgi-bin/perlref?func=lc)
convert string to lowercase
lcfirst (http://stein.cshl.org/cgi-bin/perlref?func=lcfirst)
lowercase first character of string
length (http://stein.cshl.org/cgi-bin/perlref?func=length)
find length of string
local (http://stein.cshl.org/cgi-bin/perlref?func=local)
temporarily replace the value of a global variable
localtime (http://stein.cshl.org/cgi-bin/perlref?func=localtime)
return time in local timezone
log (http://stein.cshl.org/cgi-bin/perlref?func=log)
natural logarithm
m// (http://stein.cshl.org/cgi-bin/perlref?func=m//)
pattern match operation
map (http://stein.cshl.org/cgi-bin/perlref?func=map)
perform on operation on each member of array or list
mkdir (http://stein.cshl.org/cgi-bin/perlref?func=mkdir)
make a new directory
my (http://stein.cshl.org/cgi-bin/perlref?func=my)
create a local variable
next (http://stein.cshl.org/cgi-bin/perlref?func=next)
jump to the top of enclosing loop
open (http://stein.cshl.org/cgi-bin/perlref?func=open)
open a file for reading or writing
opendir (http://stein.cshl.org/cgi-bin/perlref?func=opendir)
open a directory for listing
pack (http://stein.cshl.org/cgi-bin/perlref?func=pack)
pack a list into a compact binary representation
package (http://stein.cshl.org/cgi-bin/perlref?func=package)
create a new namespace for a module
pop (http://stein.cshl.org/cgi-bin/perlref?func=pop)
pop the last item off the end of an array
print (http://stein.cshl.org/cgi-bin/perlref?func=print)
print to terminal or a file
printf (http://stein.cshl.org/cgi-bin/perlref?func=printf)
formatted print to a terminal or file
push (http://stein.cshl.org/cgi-bin/perlref?func=push)
push a value onto the end of an array
q/STRING/ (http://stein.cshl.org/cgi-bin/perlref?func=q/STRING/)
generalized single-quote operation
qq/STRING/ (http://stein.cshl.org/cgi-bin/perlref?func=qq/STRING/)
generalized double-quote operation
qx/STRING/ (http://stein.cshl.org/cgi-bin/perlref?func=qx/STRING/)
generalized backtick operation
qw/STRING/ (http://stein.cshl.org/cgi-bin/perlref?func=qw/STRING/)
turn a space-delimited string of words into a list
rand (http://stein.cshl.org/cgi-bin/perlref?func=rand)
random number generator
read (http://stein.cshl.org/cgi-bin/perlref?func=read)
read binary data from a file
readdir (http://stein.cshl.org/cgi-bin/perlref?func=readdir)
read the contents of a directory
readline (http://stein.cshl.org/cgi-bin/perlref?func=readline)
read a line from a text file
readlink (http://stein.cshl.org/cgi-bin/perlref?func=readlink)
determine the target of a symbolic link
redo (http://stein.cshl.org/cgi-bin/perlref?func=redo)
restart a loop from the top
ref (http://stein.cshl.org/cgi-bin/perlref?func=ref)
return the type of a variable reference
rename (http://stein.cshl.org/cgi-bin/perlref?func=rename)
rename or move a file
require (http://stein.cshl.org/cgi-bin/perlref?func=require)
load functions defined in a library file
return (http://stein.cshl.org/cgi-bin/perlref?func=return)
return a value from a user-defined subroutine
reverse (http://stein.cshl.org/cgi-bin/perlref?func=reverse)
reverse a string or list
rewinddir (http://stein.cshl.org/cgi-bin/perlref?func=rewinddir)
rewind a directory handle to the beginning
rindex (http://stein.cshl.org/cgi-bin/perlref?func=rindex)
find a substring in a larger string, from right to left
rmdir (http://stein.cshl.org/cgi-bin/perlref?func=rmdir)
remove a directory
s/// (http://stein.cshl.org/cgi-bin/perlref?func=s///)
pattern substitution operation
scalar (http://stein.cshl.org/cgi-bin/perlref?func=scalar)
force an expression to be treated as a scalar
seek (http://stein.cshl.org/cgi-bin/perlref?func=seek)
reposition a filehandle to an arbitrary point in a file
select (http://stein.cshl.org/cgi-bin/perlref?func=select)
make a filehandle the default for output
shift (http://stein.cshl.org/cgi-bin/perlref?func=shift)
shift a value off the beginning of an array
sin (http://stein.cshl.org/cgi-bin/perlref?func=sin)
sine
sleep (http://stein.cshl.org/cgi-bin/perlref?func=sleep)
put the script to sleep for a while
sort (http://stein.cshl.org/cgi-bin/perlref?func=sort)
sort an array or list by user-specified criteria
splice (http://stein.cshl.org/cgi-bin/perlref?func=splice)
insert/delete array items
split (http://stein.cshl.org/cgi-bin/perlref?func=split)
split a string into pieces according to a pattern
sprintf (http://stein.cshl.org/cgi-bin/perlref?func=sprintf)
formatted string creation
sqrt (http://stein.cshl.org/cgi-bin/perlref?func=sqrt)
square root
stat (http://stein.cshl.org/cgi-bin/perlref?func=stat)
get information about a file
sub (http://stein.cshl.org/cgi-bin/perlref?func=sub)
define a subroutine
substr (http://stein.cshl.org/cgi-bin/perlref?func=substr)
extract a substring from a string
symlink (http://stein.cshl.org/cgi-bin/perlref?func=symlink)
create a symbolic link
system (http://stein.cshl.org/cgi-bin/perlref?func=system)
execute an operating system command, then return to Perl
tell (http://stein.cshl.org/cgi-bin/perlref?func=tell)
return the position of a filehandle within a file
tie (http://stein.cshl.org/cgi-bin/perlref?func=tie)
associate a variable with a database
time (http://stein.cshl.org/cgi-bin/perlref?func=time)
return number of seconds since January 1, 1970
tr/// (http://stein.cshl.org/cgi-bin/perlref?func=tr///)
replace characters in a string
truncate (http://stein.cshl.org/cgi-bin/perlref?func=truncate)
truncate a file (make it smaller)
uc (http://stein.cshl.org/cgi-bin/perlref?func=uc)
uppercase a string
ucfirst (http://stein.cshl.org/cgi-bin/perlref?func=ucfirst)
uppercase first character of a string
umask (http://stein.cshl.org/cgi-bin/perlref?func=umask)
change file creation mask
undef (http://stein.cshl.org/cgi-bin/perlref?func=undef)
undefine (remove) a variable
unlink (http://stein.cshl.org/cgi-bin/perlref?func=unlink)
delete a file
unpack (http://stein.cshl.org/cgi-bin/perlref?func=unpack)
the reverse of pack
untie (http://stein.cshl.org/cgi-bin/perlref?func=untie)
the reverse of tie
unshift (http://stein.cshl.org/cgi-bin/perlref?func=unshift)
move a value onto the beginning of an array
use (http://stein.cshl.org/cgi-bin/perlref?func=use)
import variables and functions from a library module
values (http://stein.cshl.org/cgi-bin/perlref?func=values)
return the values of a hash variable
wantarray (http://stein.cshl.org/cgi-bin/perlref?func=wantarray)
return true in an array context
warn (http://stein.cshl.org/cgi-bin/perlref?func=warn)
print a warning to standard error
write (http://stein.cshl.org/cgi-bin/perlref?func=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.
<<----- Perl Basics (http://mmofuse.net/forums/f63/perl-basics-1-a-19913/)2 (http://mmofuse.net/forums/f63/perl-basics-2-a-19914/)
Perl Basics 4 ----->> (http://mmofuse.net/forums/f63/perl-basics-4-a-19916/)
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 (http://stein.cshl.org/cgi-bin/perlref?func=abs)
absolute value
chdir (http://stein.cshl.org/cgi-bin/perlref?func=chdir)
change current directory
chmod (http://stein.cshl.org/cgi-bin/perlref?func=chmod)
change permissions of file/directory
chomp (http://stein.cshl.org/cgi-bin/perlref?func=chomp)
remove terminal newline from string variable
chop (http://stein.cshl.org/cgi-bin/perlref?func=chop)
remove last character from string variable
chown (http://stein.cshl.org/cgi-bin/perlref?func=chown)
change ownership of file/directory
close (http://stein.cshl.org/cgi-bin/perlref?func=close)
close a file handle
closedir (http://stein.cshl.org/cgi-bin/perlref?func=closedir)
close a directory handle
cos (http://stein.cshl.org/cgi-bin/perlref?func=cos)
cosine
defined (http://stein.cshl.org/cgi-bin/perlref?func=defined)
test whether variable is defined
delete (http://stein.cshl.org/cgi-bin/perlref?func=delete)
delete a key from a hash
die (http://stein.cshl.org/cgi-bin/perlref?func=die)
exit with an error message
each (http://stein.cshl.org/cgi-bin/perlref?func=each)
iterate through keys & values of a hash
eof (http://stein.cshl.org/cgi-bin/perlref?func=eof)
test a filehandle for end of file
eval (http://stein.cshl.org/cgi-bin/perlref?func=eval)
evaluate a string as a perl expression
exec (http://stein.cshl.org/cgi-bin/perlref?func=exec)
quit Perl and execute a system command
exists (http://stein.cshl.org/cgi-bin/perlref?func=exists)
test that a hash key exists
exit (http://stein.cshl.org/cgi-bin/perlref?func=exit)
exit from the Perl script
glob (http://stein.cshl.org/cgi-bin/perlref?func=glob)
expand a directory listing using shell wildcards
gmtime (http://stein.cshl.org/cgi-bin/perlref?func=gmtime)
current time in GMT
grep (http://stein.cshl.org/cgi-bin/perlref?func=grep)
filter an array for entries that meet a criterion
index (http://stein.cshl.org/cgi-bin/perlref?func=index)
find location of a substring inside a larger string
int (http://stein.cshl.org/cgi-bin/perlref?func=int)
throw away the fractional part of a floating point number
join (http://stein.cshl.org/cgi-bin/perlref?func=join)
join an array together into a string
keys (http://stein.cshl.org/cgi-bin/perlref?func=keys)
return the keys of a hash
kill (http://stein.cshl.org/cgi-bin/perlref?func=kill)
send a signal to one or more processes
last (http://stein.cshl.org/cgi-bin/perlref?func=last)
exit enclosing loop
lc (http://stein.cshl.org/cgi-bin/perlref?func=lc)
convert string to lowercase
lcfirst (http://stein.cshl.org/cgi-bin/perlref?func=lcfirst)
lowercase first character of string
length (http://stein.cshl.org/cgi-bin/perlref?func=length)
find length of string
local (http://stein.cshl.org/cgi-bin/perlref?func=local)
temporarily replace the value of a global variable
localtime (http://stein.cshl.org/cgi-bin/perlref?func=localtime)
return time in local timezone
log (http://stein.cshl.org/cgi-bin/perlref?func=log)
natural logarithm
m// (http://stein.cshl.org/cgi-bin/perlref?func=m//)
pattern match operation
map (http://stein.cshl.org/cgi-bin/perlref?func=map)
perform on operation on each member of array or list
mkdir (http://stein.cshl.org/cgi-bin/perlref?func=mkdir)
make a new directory
my (http://stein.cshl.org/cgi-bin/perlref?func=my)
create a local variable
next (http://stein.cshl.org/cgi-bin/perlref?func=next)
jump to the top of enclosing loop
open (http://stein.cshl.org/cgi-bin/perlref?func=open)
open a file for reading or writing
opendir (http://stein.cshl.org/cgi-bin/perlref?func=opendir)
open a directory for listing
pack (http://stein.cshl.org/cgi-bin/perlref?func=pack)
pack a list into a compact binary representation
package (http://stein.cshl.org/cgi-bin/perlref?func=package)
create a new namespace for a module
pop (http://stein.cshl.org/cgi-bin/perlref?func=pop)
pop the last item off the end of an array
print (http://stein.cshl.org/cgi-bin/perlref?func=print)
print to terminal or a file
printf (http://stein.cshl.org/cgi-bin/perlref?func=printf)
formatted print to a terminal or file
push (http://stein.cshl.org/cgi-bin/perlref?func=push)
push a value onto the end of an array
q/STRING/ (http://stein.cshl.org/cgi-bin/perlref?func=q/STRING/)
generalized single-quote operation
qq/STRING/ (http://stein.cshl.org/cgi-bin/perlref?func=qq/STRING/)
generalized double-quote operation
qx/STRING/ (http://stein.cshl.org/cgi-bin/perlref?func=qx/STRING/)
generalized backtick operation
qw/STRING/ (http://stein.cshl.org/cgi-bin/perlref?func=qw/STRING/)
turn a space-delimited string of words into a list
rand (http://stein.cshl.org/cgi-bin/perlref?func=rand)
random number generator
read (http://stein.cshl.org/cgi-bin/perlref?func=read)
read binary data from a file
readdir (http://stein.cshl.org/cgi-bin/perlref?func=readdir)
read the contents of a directory
readline (http://stein.cshl.org/cgi-bin/perlref?func=readline)
read a line from a text file
readlink (http://stein.cshl.org/cgi-bin/perlref?func=readlink)
determine the target of a symbolic link
redo (http://stein.cshl.org/cgi-bin/perlref?func=redo)
restart a loop from the top
ref (http://stein.cshl.org/cgi-bin/perlref?func=ref)
return the type of a variable reference
rename (http://stein.cshl.org/cgi-bin/perlref?func=rename)
rename or move a file
require (http://stein.cshl.org/cgi-bin/perlref?func=require)
load functions defined in a library file
return (http://stein.cshl.org/cgi-bin/perlref?func=return)
return a value from a user-defined subroutine
reverse (http://stein.cshl.org/cgi-bin/perlref?func=reverse)
reverse a string or list
rewinddir (http://stein.cshl.org/cgi-bin/perlref?func=rewinddir)
rewind a directory handle to the beginning
rindex (http://stein.cshl.org/cgi-bin/perlref?func=rindex)
find a substring in a larger string, from right to left
rmdir (http://stein.cshl.org/cgi-bin/perlref?func=rmdir)
remove a directory
s/// (http://stein.cshl.org/cgi-bin/perlref?func=s///)
pattern substitution operation
scalar (http://stein.cshl.org/cgi-bin/perlref?func=scalar)
force an expression to be treated as a scalar
seek (http://stein.cshl.org/cgi-bin/perlref?func=seek)
reposition a filehandle to an arbitrary point in a file
select (http://stein.cshl.org/cgi-bin/perlref?func=select)
make a filehandle the default for output
shift (http://stein.cshl.org/cgi-bin/perlref?func=shift)
shift a value off the beginning of an array
sin (http://stein.cshl.org/cgi-bin/perlref?func=sin)
sine
sleep (http://stein.cshl.org/cgi-bin/perlref?func=sleep)
put the script to sleep for a while
sort (http://stein.cshl.org/cgi-bin/perlref?func=sort)
sort an array or list by user-specified criteria
splice (http://stein.cshl.org/cgi-bin/perlref?func=splice)
insert/delete array items
split (http://stein.cshl.org/cgi-bin/perlref?func=split)
split a string into pieces according to a pattern
sprintf (http://stein.cshl.org/cgi-bin/perlref?func=sprintf)
formatted string creation
sqrt (http://stein.cshl.org/cgi-bin/perlref?func=sqrt)
square root
stat (http://stein.cshl.org/cgi-bin/perlref?func=stat)
get information about a file
sub (http://stein.cshl.org/cgi-bin/perlref?func=sub)
define a subroutine
substr (http://stein.cshl.org/cgi-bin/perlref?func=substr)
extract a substring from a string
symlink (http://stein.cshl.org/cgi-bin/perlref?func=symlink)
create a symbolic link
system (http://stein.cshl.org/cgi-bin/perlref?func=system)
execute an operating system command, then return to Perl
tell (http://stein.cshl.org/cgi-bin/perlref?func=tell)
return the position of a filehandle within a file
tie (http://stein.cshl.org/cgi-bin/perlref?func=tie)
associate a variable with a database
time (http://stein.cshl.org/cgi-bin/perlref?func=time)
return number of seconds since January 1, 1970
tr/// (http://stein.cshl.org/cgi-bin/perlref?func=tr///)
replace characters in a string
truncate (http://stein.cshl.org/cgi-bin/perlref?func=truncate)
truncate a file (make it smaller)
uc (http://stein.cshl.org/cgi-bin/perlref?func=uc)
uppercase a string
ucfirst (http://stein.cshl.org/cgi-bin/perlref?func=ucfirst)
uppercase first character of a string
umask (http://stein.cshl.org/cgi-bin/perlref?func=umask)
change file creation mask
undef (http://stein.cshl.org/cgi-bin/perlref?func=undef)
undefine (remove) a variable
unlink (http://stein.cshl.org/cgi-bin/perlref?func=unlink)
delete a file
unpack (http://stein.cshl.org/cgi-bin/perlref?func=unpack)
the reverse of pack
untie (http://stein.cshl.org/cgi-bin/perlref?func=untie)
the reverse of tie
unshift (http://stein.cshl.org/cgi-bin/perlref?func=unshift)
move a value onto the beginning of an array
use (http://stein.cshl.org/cgi-bin/perlref?func=use)
import variables and functions from a library module
values (http://stein.cshl.org/cgi-bin/perlref?func=values)
return the values of a hash variable
wantarray (http://stein.cshl.org/cgi-bin/perlref?func=wantarray)
return true in an array context
warn (http://stein.cshl.org/cgi-bin/perlref?func=warn)
print a warning to standard error
write (http://stein.cshl.org/cgi-bin/perlref?func=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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