CS105 Computer Programming Perl

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CS105 Computer Programming Perl
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What is PERL?

• Practical Extraction Report Language
• Pathologically Eclectic Rubbish Lister
• Practically Everything Really Likable
• (about the shells, awk, sed, grep, and C)
• Written by Larry Wall
• Perl is a GNU product!.
• Perl borrows ideas from all over the place.
• Perl runs on many platforms.

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What is PERL?

• Perl is not an acronym it is what we call a
  backronym
• Objective Making Easy Things Easy and Difficult
  Things Possible
• Motto TMTOWTDI There's More Than One Way To Do
  It

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Why use PERL?

• Easy - for daily use
• Nearly unlimited - for high-level programming (no
  real time, no kernel, no device drivers please)
• Mostly fast - fast enough for the tasks you would
  do with Perl
• Not too nice - the symbol of Perl is the camel,
  ugly but can handle difficult environment

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Who uses PERL?

• System administrators
• Many other UNIX users.
• Data Miners
• Database Engineers
• Biologist
• You!

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Setting Up a CS Account.

• Get an account from the CS department
• CS department offers guest account
• Do all assignments on CS Unix machines
• Environment in which assignments will be graded
• CS account comes with webspace
• Needed for some assignments

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Running Perl in Unix

• Three ways to run a perl program
• Execute program stored in a file
• gt perl myfile.pl
• Execute as a single statement
• gt perl e print Hello World.\n
• Useful in determining if a statement will work.
• Execute as a script/shell program.

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Execute as a Shell Program

• First line in the file must tell the OS where
  Perl is located
• !/usr/bin/perl
• Each statement must end with a semicolon ().
• In some cases the statement must be on one line.
• Comments begin with and everything is case
  sensitive.
• Must tell OS that the file is executable.
• gt chmod 700 myfile.pl
• gt myfile.pl mydata.txt

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Some Useful Options

• By default, Perl wont give any warnings for
  syntax or programming errors
• This makes catching bugs hard
• Check but dont execute
• gt perl c myfile.pl
• Useful for finding syntax errors.
• Check for common mistakes
• gt perl w myfile.pl
• Check for common programming errors and
  suspicious code.

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Quick and dirty (EX - replace file content)

•   You have a bunch of files in your directory ment
  ioning the name        "President Bush"    Yo
  u are told to replace that by         "President
   Reagen"    perl -i -p -e "s/Bush/Reagen/g"
   

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Quick and dirty(EX - process a csv file)

• You have a number of csv files, you want to print 
  the 3rd field of each row of each file.    perl
  -F, -ane 'print "F2\n"' .csv
• You want to make sure all the rows are 4 elements 
  long. Print out file name and line number of all 
  the bad rows.    perl -F, -ane 'print
  "ARGV.\n" if _at_F ! 4' .csv