Lecture%20Seven%20(II)

1
Lecture Seven (II)

• Perl Programming

2
Introduction

• Perl is an interpreted language for scanning
  files, extracting information from those text
  files, and printing reports based on that
  information.
• Perl is mainly used to handle text files, but can
  be used for binary files (databases), process
  manipulation, network tasks.

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Introduction

• Perl is not a strong-typed language, like C, C,
  Java
• Perl does not arbitrarily impose limit on data
  structures. Strings have unlimited length.
  Recursion is of unlimited depth.
• Information source http//www.perl.com/perl/faq/,
  http//www.phlab.missouri.edu/cgi-bin/perlman

4
A simple example

• perl -e print hello dolly\n
• hello dolly
• "-e" is followed by Perl command (execute
  command line instead of reading from a script
  file).

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Compared with sed and awk

• Unlike sed and awk, where a sequence of commands
  is executed for each line of input, Perl executes
  each command just once.
• Use the -n option to implicitly loop over a file,
  or use an explicit loop to iterate the file.

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Another simple example

• cat foo
• this is the first line
• second line
• perl -ne print foo
• -n option is used to implicitly loop through
  the file a line at a time.
• this is the first line
• second line

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Example continued

• perl -ne print if /this/ foo
• this is the first line
• date perl -ne print Today is _
• _ is a variable referring to input
• Today is October 15, 1997

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A Simple Script - foo.perl

• !/usr/contrib/bin/perl -w
• This is a comment line
• print Hello\n
• perl -c foo.perl
• foo.perl syntax ok
• foo.perl

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Perl IO

• PERL works with stdin, stdout, stderr, but does
  not access the streams directly
• Filehandles STDIN, STDOUT, STDERR
• Printing output
• any unknown word (not a filehandle, label,
  reserved word) is treated as if surrounded by
  single quotes
• perl words ARE case sensitive
• single quotes - all characters treated as
  literals
• double quotes - all chars literal except variable
  substitution, escape sequences

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Special Literals

• \t tab
• \n newline
• \\ backslash
• special literals (should not be double quoted)
• _LINE_ current line number
• _FILE_ current filename

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IO

• print Hello, world, \n
• HelloWorld
• print Hello world\n
• Hello world
• print Hello, world,\n no comma allowed after
  filehandle (ERROR filehandle must be specified
  when literals not quoted)
• print STDOUT Hello, world,\n
• Helloworld

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IO

• print The price is 100.\n
• The price is . (looks for scalar variable 100)
• print The price is \100.\n
• The price is 100.
• print The price is ,100,.\n
• The price is 100
• print The number is ,0777,.\n
• The number is 511 (octal due to leading 0)

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IO

• print The number is ,0xAbcF,.\n
• The number is 43983 (hex due to leading 0x)
• print The unformatted number is ,14.56,.\n
• The unformatted number is 14.5600000. (use
  default format)
• print \tIn double quotes\t\n
• In double quotes
• print \tIn single quotes\t\n
• \tIn single quotes\t\n

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IO

• print Current line number is ,_LINE,\n
• Current line number is 7 (interpret, literal
  not in quotes)

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Printf print formatted string

• printf(Names s\t Number d\n,John,50)
• Name John Number 50
• c - character
• s - string
• d - decimal
• x - hex
• o - octal
• e - floating point, scientific notation
• f - floating point

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Three Types Variables

• Scalars can be numeric or character as determined
  by context
• 123 12.4 5E-10 0xff (hex) 0377 (octal)
• 'What you see is (almost) what \n you get
• 'Don\'t Walk
• "How are you?"
• "Substitute values of x and \n in \" quotes.
• date
• uptime -u
• du -sk filespec sort -n
• x
• list_of_things5
• lookup'key'

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Three Types Variables

• Double-quotes allow substitution of variables and
  control codes.
• Back-quotes also allow substitution, then try to
  execute the result as a system command, returning
  as the final value whatever the system command
  outputs.
• Arrays of scalars (also called lists) are
  sequentially-arranged scalars
• _at_dow('Sunday', 'Monday', 'Tuesday')
• (13,14,15,16,17,18,19) equivalent to (13..19)
• (13,14,15,16,17,18,19)2..4 equivalent to
  (15,16,17)
• _at_dow the entire array

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Three Types Variables

• Associative arrays (also called hashes)
  facilitate arbitrary recall
• states(CA,California,ME,Maine,.)
• statesME

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Example

• salary 5000
• _at_months (Mar, Apr, May)
• states (CA,California,ME,Maine,MT,Mon
  tana)
• print salary\n
• print _at_months\n
• print months0\n
• print statesCA\n
• print x 3,\n x initialized to 0 since
  context is numeric
• print name\n name initialized to empty
  string due to context
• 5000
• Mar Apr May
• Mar
• California
• 3

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Name Convention

• Scalar variables start with '', even when
  referring to an array element. The variable name
  reference for a whole list starts with '_at_', and
  the variable name reference for a whole
  associative array starts with ''.
• Lists are indexed with square brackets enclosing
  a number, normally starting with 0. In Perl 5,
  negative subscripts count from the end. Thus,
  things5 is the 6th element of array _at_things,
  and ('Sun','Mon','Tue','Wed','Thu','Fri','Sat')1
  equals 'Mon'.
• Associative arrays are indexed with curly
  brackets enclosing a string. whatever,
  _at_whatever, and whatever are three different
  variables.

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Name Convention

• Examples
• _at_days (31,28,31,30,31,30,31,31,30,31,30,31)
• A list with 12 elements.
• days Last index of _at_days 11 for
  above list
• days 7 shortens or lengthens list _at_days
  to 8 elements
• _at_days (days0, days1,... )
• _at_days3,4,5 (30,31,30)
• _at_days'a','c' same as (days'a',days'c')
• days (key1, value1, key2, value2,
  ...)

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Name Convention

• Case is significant--
• FOO, Foo and foo are all different variables.
• If a letter or underscore is the first character
  after the , _at_, or , the rest of the name may
  also contain digits and underscores. If this
  character is a digit, the rest must be digits.
• Perl has several dozen special variables whose
  second character is non-alphanumeric. For
  example, / is the input record separator,
  newline "\n" by default. An uninitialized
  variable has a special "undefined" value which
  can be detected by the function defined().
  Undefined values convert depending on context to
  0, null, or false.

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Name Convention

• The variable "_" Perl presumes when needed
  variables are not specified. Thus
• ltSTDINgt assigns a record from
  filehandle STDIN to _
• print prints the curent value of
  _
• chop removes the last character
  from _
• _at_things split parses _ into white-space
  delimited words, which become successive
• elements of list _at_things.

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Name Convention

• Subroutines and functions are referenced with an
  initial '', which is optional if reference is
  obviously a subroutine or function such as
  following the sub, do, and sort directives sub
  square return _0 2 print "5 squared is
  ", square(5)
• Filehandles don't start with a special character,
  and so as to not conflict with reserved words are
  most reliably specified as uppercase names
  INPUT, OUTPUT, STDIN, STDOUT, STDERR, etc.

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Number and Characters

• Example
• !/usr/local/bin/perl
• print '007',' has been portrayed by at least ',
  004, ' actors. '
• print 73, ' ', 73, ' ', 7/3, ' ', 73, ' ',
  73, ' '
• x 7
• print x
• print ' Doesn\'t resolve variables like x and
  backslashes \n. '
• print "Does resolve x and backslash\n"
• y "A line containing x and ending with line
  feed.\n"
• print y
• y "Con" . "cat" . "enation!\n"
• print y

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Another Example

• !/usr/contrib/bin/perl -w
• if (ARGV gt 0) no args -1
• who join(' ', _at_ARGV)
• else
• who 'World'
• print "Hello, who!\n"

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Substitution Example

• Substitutes Text In Multiple File
• perl -e 's/gopher/World Wide Web/gi' -p -i.bak
  .html
• "-e execute command line instead of reading
  from a file
• "s substitutes "gopher" with "World Wide Web
• "g" is global across each line,
• "i" turn off case sensitiveity.
• "-i make a copy of the file with the name
  ltoriginal file namegt.ltextension givengt
• "-p loop through the files line by line

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Another Example

• !/usr/local/bin/perl -w
• original'gopher'
• replacement"World Wide Web"
• nchanges 0
• undefine /
• for (i0 i lt ARGV i)
• if (! open(INPUT,"lt ARGVi ") )
• print STDERR "Can't open input file
  bakfile\n"
• next
• dataltINPUTgt Read input file as one
  long record.
• close INPUT

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continued

• if (data s/original/replacement/gi)
• bakfile "file.bak"
• if (! rename(file,bakfile)) die
  "Can't rename file !"
• if (! open(OUTPUT,"gtfile") ) die
  "Can't open output file file\n"
• print OUTPUT data
• close OUTPUT
• print STDERR "file changed\n"
• nchanges
• else print STDERR "file unchanged\n"
• print STDERR "nchanges files changed.\n"
• exit(0)

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Expressiveness Sometimes Means Confusing

• The same expression can be written in many
  different ways in Perl
• if (x 0) y 10 else y 20
• y x0 ? 10 20
• y 20 y 10 if x0
• unless (x 0) y20 else y10
• if (x) y20 else y10
• y (10, 20)x ! 0

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Comparison

• Output
• Hello
• x is 7
• x is now 5
• Use eq, ne, lt, gt, etc for strings.

• x 'operator'
• print ltltTHATSALL
• Hello
• THATSALL
• x 7
• if (x 7) print "x is x\n"
• if (x 5) print "x is now x\n
• x 'stuff'
• if (x eq 'stuff')
• print "Use eq, ne, lt, gt, etc for
  strings.\n"

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Using Arrays

• _at_stuff ('This', 'is', 'a', 'list.')
• print "So \stuff1 stuff1, ", "and
  \stuff stuff.\n"
• print _at_stuff,"\n"
• print join('...',_at_stuff),"\n"
• splice(_at_stuff, 3, 0, ('fine', 'little'))
• replace 0 elements from index 3 by 'fine'...
  
• print join('...',_at_stuff),"\n"
• Output
• So stuff1 is, and stuff 3.
• Thisisalist.
• This...is...a...list.
• This...is...a...fine...little...list.