Lecture 11: Perl

1
Lecture 11 Perl

• Referring to and opening files
• The Perl match operator
• Perl arrays and list data
• Built-in Perl functions
• Control-flow statements
• Associative Arrays

2
The notion of a string

• continent a new nation conceived in liberty
• in this quote
• the word in occurs once
• the string in occurs twice
• a string is a sequence of characters

3
The need for a string counter

• grep Powell ap.stories
• asked Powell. Powell replied that he would
• grep returns the line with multiple Powells
• tr -cs A-z0-9 1 sort uniq -c
• 43 Powell
• gives the answer but with a lot of extra
  unnecessary computing

4
A Perl Counter

• Perl allows a scalar variable that can hold a
  numerical value lets call it ct
• We can start ct with a value of 0
• Each time our program finds Powell, it can add
  1 to the value of ct
• At the end of the program, we can print the final
  value of ct, which will be the number of times
  it found the string Powell

5
Writing a Perl Program

• Writing a program around the counter involves
  more than using the counter.
• Lets step through a program that counts the
  number of instances of in in the Gettysburg
  Address

6
Filehandles

• allow you to grab data
• weve already used ltSTDINgt as a filehandle in
  name ltSTDINgt
• now we want to associate a filename with a
  variable called FH (filehandle)
• (FH, ARGV0)

7
Filehandles (2)

• (FH, ARGV0)
• we are writing a program called findwd.pl
• the command line for this program will be
• findwd.pl gettysburg
• ARGV0 is the file gettysburg

8
Opening a file

• In most programming languages, the program has to
  be told how to treat a file.
• Is the file to be written to, read from, or
  opened for processing?
• open (FH,"ARGV0")

9
Non-existent files

• If Perl cant find the file that is to be opened
  we want to
• stop the program from running
• die
• get a failure report
• cannot open
• Putting it together
• die cannot open

10
How Perl reads from a file

• Like sed, grep, etc., Perl reads a file line by
  line
• So, ltFHgt will have the value of the current line
• If we want to do text processing a line at a
  time, we can bind a variable in our program to
  the current value of ltFHgt
• line ltFHgt

11
Getting a Perl script to read from a file

• open (FH,"ARGV0") die "cannot open"
• while (line ltFHgt)

• open the file thats listed first on the command
  line and bind it to FH
• or stop the program and report failure
• bind the incoming line to the variable line
• Q1,2

12
The operator

• The equal sign binds a value to a variable
• name ltSTDINgt
• a fred
• The match operator matches a pattern somewhere
  in a string
• line /in/
• line /Jjohn/ Q3,4

13
Counters in Perl

• variables begin life with a dollar sign ct
• numerical variables begin life with the value 1
• an increment operator automatically adds 1 each
  time it is encountered ct
• Q5

14
Lecture 12

• Perl Arrays
• Control-flow statements
• Control flow foreach
• Associative Arrays

15
Arrays in Perl

• an array is a variable (.e.g, _at_line) that can
  hold a list
• _at_line (four, score, and, seven)
• each element in the list is a scalar variable
  that can be referenced by its place in the list
• _at_line (four, score, and, seven)
• line0 line1 line2 line3

16
Array Functions

• _at_line(four,score,and,seven,
  years)
• push(_at_line,ago)
• xpop(_at_line)
• unshift(_at_line,address)
• xshift(_at_line)
• _at_xreverse(_at_line)
• _at_xsort(_at_line) Q6

17
The for statement

• used to repeatedly execute a segment of code
• for loop syntax
• for (set_counter test_counter
  increment_counter)
• action

18
The for loop counters

• set_counter sets initial value for counter
• test_counter states a condition tested at top
• of loop
• increment_counter
• increments counter at
• bottom of loop, just before
• testing test_counter

19
Execution of a for loop

• 1. the counter is set this is done once and only
  once
• 2. test_counter is tested for a True/False answer
• 3. if the result of test_counter is False
• the for loop terminates
• and the program drops to the action after the for
  loop
• 4. if the result of test_counter is True
• we execute the action
• we increment the counter
• we move to the next record

20
a for loop

• !/usr/bin/perl -w
• for (i1ilt5ii1)
• print "i "
• ii2
• Q7

21
Control Structures in Perl

• the if statement
• if (name kermit
• print Hello, name
• the while statement
• while (line ltFHgt
• chop line
• print line

22
Control Structures in Perl (2)

• the for statement
• for (i2ilt8ii2)
• print i
• (to increment by 1 on each loop, use the
  shorthand i)

23
Control Structures (3)

• the foreach statement loops over each index in an
  array
• _at_a (3,5,7,9)
• foreach one (_at_a)
• one 3
• print one prints 9,15,21,27
• Q7

24
Control Structures (3)

• the foreach statement loops over each element in
  an array
• !/usr/bin/perl -w
• foreach.pl
• _at_sentence (Here,we,go,again)
• foreach word (_at_sentence)
• print "word\n"

25
foreach.pl output

• Here
• we
• go
• again
• Q8

26
Tabular Data

• Name SSN Lang Sec
• Gutierrez 095973624 Spanish 2A
• Kim 497860227 Korean 3B
• Boyer 853626857 French 3A
• Masaka 353864257 Japanese 4C

27
Tabular Data in Linux

• NameSSNLangLevel
• Gutierrez095973624Spanish2A
• Kim497860227Korean3B
• Boyer853626857French3A
• Masaka353864257Japanese4C

28
Accessing info from table-like data in Perl

• in Perl
• the notion of a row is captured by an array
• _at_data
• Gutierrez095973624Spanish2A
• the notion of a table is captured by a hash
  data
• Gutierrez095973624Spanish2A
• Kim497860227Korean3B

29
Accessing info from table-like data in Perl (2)

• info in the hash is accessed through a column, or
  key, in the data
• _at_fields split(//,line)
• key student0
• the key is the 1st element of each row (the
  students last name in our data)

30
Accessing info from table-like data in Perl (3)

• Other columns in the table are referenced with
  respect to this key, e.g.
• langkey fields2
• the language associated with a given key is in
  the 3rd field.
• Q9,10