COS 381

1. COS 381 Day 22

2. Agenda • Questions?? • Resources • Source Code Available for examples in Text Book in Blackboard • Also @ http://perleybrook.umfk.maine.edu/SourceCode/ • In Class Work • http://perleybrook.umfk.maine.edu/SourceCode/inclassWork/ • Assignment 5 is posted • Due April 21 at 11:05 AM • 3rd and final Capstone progress report due April 25 • Final Capstone presentation • May 5 or May 9 @ 10AM • Continue Discussion on Perl • Perl Documentation • http://perldoc.perl.org/ • Perl Tutorial • http://www.sthomas.net/roberts-perl-tutorial.htm

3. ETS testing • ETS Field Test in Business for • Wednesday, April 23, in Nadeau 109. • the morning; 9:00 a.m. • afternoon 1:00 PM • Juniors and Seniors in the accredited Professional Management Programs (ecommerce) should take this test. • If a student took the test last year, they can take it again, if they want to.

4. Chapter 8 The Basics of Perl

5. 8.9 Basics of Pattern Matching • Perl has powerful pattern matching facilities built in • These have been imitated in a number of other systems • Regular expressions were described in the JavaScript chapter • The m operator indicates a pattern matching • This is used with delimiters like q and qq but the enclosed characters form a pattern • If the delimiter is / then the m is not required • m/ddd-dd-dddd/ is the same as /ddd-dd-dddd/ • A match is indicated by the =~ operator with a string on the left and a pattern on the right returns true if there is a match • A pattern alone is matched by default to $_ • /rabbit/ is the same as $_ =~ /rabbitt/ • The split function can take a pattern as the first argument rather than a character • The pattern specifies the pattern of characters used to split the string apart • @words = split /[ .,]\s*/, $str

6. 8.9 An Example • Example word_table.pl uses a pattern to split a text into words • A hash table is used to count the frequency of each word • The keys are the words, the corresponding values are the counts of the words in the text • The exists function is used to tell if a word is already entered into the hash • http://perldoc.perl.org/perlre.html

7. 8.9 Remembering Matches • Parts of a pattern can be parenthesized /(reg_ex)(reg_ex)/ • If the pattern matches a string, the variables $1, $2, … refer to the parts of the string matched by the parenthesized sub-patterns • “4 July 1776” =~ /(\d+) (\w+) (d+)/; • print “$2 $1, $3”  July 4 , 1776 • If a match is successful on a string, three strings are available to give the context of the match • $& is the part that actually matched the pattern • $` is the part of the string before the part that matched • $’ is the part of the string after the part that matched "Tony Gauvin" =~ /au/; print "$& $' $`. \n;«  perl\reg_expession_implict.pl

8. 8.9 Substitutions • The s operator specifies a substitution • s/pattern/new-string/ • s/ there / their / • The new-string will replace the part of a string matched by the pattern • The =~ operator is used to apply the substitution to a string • If the operator is not used, $_ is operated on by default • $str = “they fixed there car”; • $str =~ s/ there / their /; • print “ $str “; • A g modifier on the substitution causes all substrings matching the pattern to be replaced, otherwise only the first match is changed • If the operator is not used, $_ is operated on by default • $str = “they fixed there car in there garage”; • $str =~ s/ there / their /g; • print “ $str “; • The i modifier cause the pattern match to be case insensitive

9. 8.9 The Transliterate Operator • This is written tr/char-list1/char-list2/ • When applied to a string it causes each character of the string that appears in the first list to be replaced by the corresponding character in the second list • If the second list is empty, the characters from the first list are deleted from the string • The =~ operator is used to apply the transliteration • If the operator is not used, $_ is operated on by default • $test-str = “I Love Perl Programming”; • tr/P/p/; same as s/P/p/g; • tr/aeiou/i;

10. 8.10 File Input and Output • http://www.troubleshooters.com/codecorn/littperl/perlfile.htm • To carry out file input and output, a filehandle must be created for each file • The open function is used to create a file handle • The first parameter to open is the name of a file handle • By convention the name is all capital letters • The second parameter to open is a string value naming the file and, optionally, including a character to indicate the mode of opening the file • < indicates open for input (default) • > indicates open for output, deleting the content of an existing file • >> indicates open for output, appending to a file that already exists • Read a file open(DAFILE, “<read.txt”) or die “whoops -$1”; • Write a file open(DAFILE, “>write.txt”); • Append to a file open(DAFILE, “>>append.txt”); • Read and write to a file open(DAFILE, “+>readandwrite.txt”);

11. 8.10 Input and Output Operations • The print function is used to send output to a filehandle • print OUTHANDLE “data”, “more data”; • Note that there is no comma after the OUTHANDLE • This is important, otherwise the value of the handle will be displayed on the output console • The input operator <> can be used on an input file handle • $next_line = <INFILE>; • The read function reads a number of characters into a given array • read(FILE, $store, 255, 0); • The file, the storage, the # of chars, the offest • The function returns actual number of characters read • The function parameters can indicate that characters are to be stored in the array are somewhere other than at the beginning of the file • The seek function can be used to position the filehandle cursor at a different position in the file • seek(FILE, offset, base~[0,1,2]) • offset is the numbers of linesto move (positive moves toward the EOF) • 0 = beginning of file • 1 =current cursor position • 2 is EOF • Examples • seek(FILE, 0 ,0); • seek(FILE, -1, 1);

12. 8.11 Example • The example wages.pl illustrates many of the features of Perl • An input file contains lines of data with fields separated by colons • The split function can be used to separate the fields • Pattern matches are used on names • A hash is used to store employees and their salaries • More in class work • Help on assignments

13. 3 card shuffle • Put 3 cards in order • If card b < card a swap card a and card b • If card c < card b swap card c and card b • If card b < card a swap card a and card b • Print a, b, c