Perl

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Perl

• Lecture 25
• Major parts of this lecture adapted from
  http//www.scs.leeds.ac.uk/Perl/start.html

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Perl Example 1
!/usr/local/bin/perl Program to do the
obvious print 'Hello world.' Print a
message
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Comments on Hello, World

• Comments are to end of line
• Perl statements end with semicolons
• Perl is case-sensitive
• Perl is compiled and run in a single operation

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Perl Example 2
!/ex2/usr/bin/perl Remove blank lines from a
file Usage singlespace lt oldfile gt
newfile while (line ltSTDINgt) if (line
eq "\n") next print "line"
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More Perl notes

• Scalar variables start with
• Scalar variables hold strings or numbers, and
  they are interchangeable
• Examples
• priority 9
• priority '9'
• Array variables start with _at_

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Perl Example 3
!/usr/local/bin/perl Usage fixm ltfilenamesgt
Replace \r with \n -- replaces input
files foreach file (_at_ARGV) print
"Processing file\n" if (-e "fixm_temp")
die " File fixm_temp already exists!\n"
if (! -e file) die " No such file
file!\n" open DOIT, " tr \'\\015'
\'\\012' lt file gt fixm_temp" or die
" Can't tr '\015' '\012' lt infile gt
outfile\n" close DOIT open DOIT, " mv
-f fixm_temp file" or die " Can't mv -f
fixm_temp file\n" close DOIT
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Arithmetic in Perl
a 1 2 Add 1 and 2 and store in a a
3 - 4 Subtract 4 from 3 and store in
a a 5 6 Multiply 5 and 6 a 7 / 8
Divide 7 by 8 to give 0.875 a 9 10
Nine to the power of 10 a 5 2
Remainder of 5 divided by 2 a
Increment a and then return it a
Return a and then increment it --a
Decrement a and then return it a--
Return a and then decrement it
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String and assignment operators
a b . c Concatenate b and c a b x
c b repeated c times a b
Assign b to a a b Add b to a a
- b Subtract b from a a . b
Append b onto a
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Single and double quotes

• a 'apples'
• b 'bananas'
• print a . ' and ' . b
• prints apples and bananas
• print 'a and b'
• prints a and b
• print "a and b"
• prints apples and bananas

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Arrays

• _at_food ("apples", "bananas", "cherries")
• But
• print food1
• prints "bananas"
• _at_morefood ("meat", _at_food)
• _at_morefood ("meat", "apples", "bananas",
  "cherries")
• (a, b, c) (5, 10, 20)

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push and pop

• push adds one or more things to the end of a list
• push (_at_food, "eggs", "bread")
• push returns the new length of the list
• pop removes and returns the last element
• sandwich pop(_at_food)
• len _at_food returns length of _at_food
• food returns index of last element

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foreach
Visit each item in turn and call it
morsel foreach morsel (_at_food) print
"morsel\n" print "Yum yum\n"
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Tests

• "Zero" is false. This includes 0, '0', "0", '',
  ""
• Anything not false is true
• Use and ! for numbers, eq and ne for strings
• , , and ! are and, or, and not, respectively.

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for loops

• for loops are just as in C
• for (i 0 i lt 10 i) print
  "i\n"

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while loops
!/usr/local/bin/perl print "Password? " a
ltSTDINgt chop a Remove the
newline at end while (a ne "fred") print
"sorry. Again? " a ltSTDINgt chop
a
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until loops
!/usr/local/bin/perl do print
"Password? " a ltSTDINgt chop
a while (a ne "fred")
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if statements
if (a) print "The string is not
empty\n" else print "The string is
empty\n"
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if - elsif statements
if (!a) print "The string is empty\n"
elsif (length(a) 1) print "The string
has one character\n" elsif (length(a) 2)
print "The string has two characters\n" else
print "The string has many characters\n"
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Why Perl?

• Two factors make Perl important
• Pattern matching/string manipulation
• Based on regular expressions (REs)
• REs are similar in power to those in Formal
  Languages
• but have many convenience features
• Ability to execute UNIX commands
• Less useful outside a UNIX environment

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Basic pattern matching

• sentence /the/
• True if sentence contains "the"
• sentence "The dog bites."if (sentence
  /the/) is false
• because Perl is case-sensitive
• ! is "does not contain"

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RE special characters
. Any single character except a
newline The beginning of the line or
string The end of the line or
string Zero or more of the last
character One or more of the last
character ? Zero or one of the last
character
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Square brackets
qjk Either q or j or k qjk
Neither q nor j nor k a-z Anything
from a to z inclusive a-z No lower
case letters a-zA-Z Any letter a-z
Any non-zero sequence of
lower case letters
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More special characters
\n A newline \t A tab \w Any
alphanumeric same as a-zA-Z0-9_ \W Any
non-word char same as a-zA-Z0-9_ \d Any
digit. The same as 0-9 \D Any non-digit.
The same as 0-9 \s Any whitespace
character\S Any non-whitespace character \b
A word boundary, outside only \B No
word boundary
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Quoting special characters
\ Vertical bar \ An open square
bracket \) A closing parenthesis \
An asterisk \ A carat symbol \/ A
slash \\ A backslash
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Alternatives and parentheses
jellycream Either jelly or cream (egle)gs
Either eggs or legs (da)
Either da or dada or
dadada or...
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Substitution

• is a test, as in sentence /the/
• ! is the negated test, as in sentence !
  /the/
• is also used for replacement, as in
  sentence /london/London/
• This is an expression, whose value is the number
  of substitutions made (0 or 1)

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The _ variable

• Often we want to process one string repeatedly
• The _ variable holds the current string
• If a subject is omitted, _ is assumed
• Hence, the following are equivalent
• if (sentence /under/)
• _ sentence if (/under/) ...

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Global substitutions

• s/london/London/
• substitutes London for the first occurrence of
  london in _
• s/london/London/g
• substitutes London for each occurrence of london
  in _
• s/london/London/i
• case-insensitive substitution

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Remembering patterns

• The value of a substitution expression is the
  number of substitutions actually made
• Any part of the pattern enclosed in parentheses
  is assigned to the special variables 1, 2, 3,
  , 9
• In addition, during the match, the parts are also
  in \1, \2, \3, , \9
• Example /(\b.\b) \1/ matches repeated words

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tr

• tr does character-by-character translation
• tr returns the number of substitutions made
• sentence tr/abc/edf/
• replaces a with e, b with d, c with f
• count (sentence tr///)
• counts asterisks
• tr/a-z/A-Z/
• converts to all uppercase

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split

• split breaks a string into parts
• info "CaineMichaelActor14, Leafy
  Drive"_at_personal split(//, info)
• _at_personal ("Caine", "Michael", "Actor", "14,
  Leafy Drive")

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Associative arrays

• Associative arrays allow lookup by name rather
  than by index
• Associative array names begin with
• Example
• fruit ("apples", "red", "bananas", "yellow",
  "cherries", "red")
• Now, fruit"bananas" returns "yellow"
• Note braces, not parentheses

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Associative Arrays II

• Can be converted to normal arrays_at_food
  fruit
• Cannot index an associative array, but can use
  the keys and values functionsforeach f (keys
  fruit) print ("The color of f is " .
  fruitf . "\n")

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Associative Arrays III

• The function each gets key-value pairs
• while ((f, c) each(fruit)) print
  "f is p\n"

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Defining subroutines

• Parameters are put in the array _at__ which has
  nothing to do with _
• Subroutine definition
• sub printargs print "_at__\n"

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Calling subroutines

• Subroutine calls
• printargs("perly", "king")
• Prints "perly king"
• printargs("frog", "and", "toad")
• Prints "frog and toad"

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Returning a result

• The value of a subroutine is the value of the
  last expression that was evaluated

sub maximum if (_0 gt _1)
_0 else _1
biggest maximum(37, 24)
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Local variables

• _at__ is local to the subroutine, and
• so are _0, _1, _2,
• local creates local variables

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Example subroutine
sub inside local(a, b)
Make local variables (a, b)
(_0, _1) Assign values a s/
//g Strip spaces from
b s/ //g local
variables (a /b/ b /a/)
Is b inside a
or a inside
b? inside("lemon", "dole money")
true
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Perl V

• Perl 5 has modules
• Perl 5 is object oriented

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The End