Copyright © 2006 The McGraw-Hill Companies, Inc. Programming Languages 2nd edition Tucker and Noonan Chapter 12 Imperative Programming I really hate this darn machine; I wish they would sell it; It won’t do what I want it to, but only what I tell it. Programmer’s lament (anonymous)

Copyright © 2006 The McGraw-Hill Companies, Inc. Contents 12.1 What Makes a Language Imperative? 12.2 Procedural Abstraction 12.3 Expressions and Assignment 12.4 Library Support for Data Structures 12.5 C 12.6 Ada 12.7 Perl

Copyright © 2006 The McGraw-Hill Companies, Inc. Perl is: widely used a scripting language (originally for Unix) dynamically typed encourages a variety of styles supports regular expression pattern matching

Copyright © 2006 The McGraw-Hill Companies, Inc. “Larry Wall... created Perl when he was trying to produce some reports from a Usenet-news-like hierarch of files for a bug reporting system, and awk ran out of steam. Larry, being the lazy programmer that he is, decided to over-kill the problem with a general purpose tool that he could use in at least one other place. The result was the first version of Perl.”

Copyright © 2006 The McGraw-Hill Companies, Inc. Scripting Languages “glue” take output from one application and reformat into desired input format for a different application. most time is spent in the underlying applications. also used for Web applications

Copyright © 2006 The McGraw-Hill Companies, Inc. General Characteristics dynamically typed default conversion from one type to another (vs. Python) result is distinct operators; ex:. for string concatenation types: numbers, strings, regular expressions dynamic arrays: indexed and associative

Copyright © 2006 The McGraw-Hill Companies, Inc. String vs. numeric comparisons: 10 < 2 # false - numeric 10 < "2" # false "10" lt "2" # true - string 10 lt "2" # true

Copyright © 2006 The McGraw-Hill Companies, Inc. Indexed = (2, 3, 5, 7); # size is 4... $a[7] = 17; # size is 8; # $a[4:6] are undef

Copyright © 2006 The McGraw-Hill Companies, Inc. Associative Arrays %d = (“bob” => “3465”, “allen” => “3131”, “rebecca” => “2912”); print $d{“bob”};# prints 3465

Copyright © 2006 The McGraw-Hill Companies, Inc. Many different ways of saying the same thing Much of the syntax is optional; ex: ( ) Perl 5 added support for classes and objects Great strengths: support for regular expressions Many irregularities in Perl

Copyright © 2006 The McGraw-Hill Companies, Inc. #! /usr/bin/perl die "Usage mygrep string \n" < 1; use strict; my $string = shift; my $ct = 0; while (<>) { $ct++; print "$ct:\t$_" if /$string/; } exit;

Copyright © 2006 The McGraw-Hill Companies, Inc. Scalar variables start with a $ Indexed arrays with Hash arrays with % Otherwise: bare word syntax error

Copyright © 2006 The McGraw-Hill Companies, Inc. use strict forces declaration of variables –local : dynamic scoping –my : static scoping –NB: only 1 $_

Copyright © 2006 The McGraw-Hill Companies, Inc. Strings Double quotes: special characters interpreted –ex: “$a \n” –forms: “ “, qq{ }, qq/ / Single quotes: special characters uninterpreted –forms: ‘ ‘, q{ }, q/ /

Copyright © 2006 The McGraw-Hill Companies, Inc. while (<>) {... } is same as: while ($_ = ) {... } where: <> is read a line returns undef at end of file; undef interpreted as false no subject: $_ if $_ =~ m/pattern/ # implied subject, operator

Copyright © 2006 The McGraw-Hill Companies, Inc. #! /usr/bin/perl if < 1) { die "Usage mygrep string \n" ; } use strict; my $string = my $ct = 0; my $line; while ($line = ) { $ct++; if ($line =~ m/$string/) { print STDOUT $ct, ":\t", $line; } exit;

Copyright © 2006 The McGraw-Hill Companies, Inc. Ex: Mailing Grades typical glue program student grades kept in a spreadsheet after each project/test, mail each student her grades include averages export data in comma-separated value format (CSV) CSV format varies by spreadsheet

Copyright © 2006 The McGraw-Hill Companies, Inc. Fig ::Proj1:Test1:::::Total:Average ::50:100::::::150: Average::88:91:::::135:90

Copyright © 2006 The McGraw-Hill Companies, Inc. Main program - part 1 retrieves class designation from command line opens CSV file for input or die is a common idiom declares some useful constants (some should be command line options)

Copyright © 2006 The McGraw-Hill Companies, Inc. #! /usr/bin/perl use strict; my $class = shift; my $suf = ".csv"; open(IN, "<$class$suf") || die "Cannot read: ". "$class$suf\n"; my $sep = ":"; my $tab = 8;

Copyright © 2006 The McGraw-Hill Companies, Inc. Main program - part 2 reads grade column names reads maximum points for each column adds 100% array

Copyright © 2006 The McGraw-Hill Companies, Inc. # read header lines: titles, max grades = &readSplit(); = &readSplit(); '100%');

Copyright © 2006 The McGraw-Hill Companies, Inc. Main program - part 3 loop reads 1 student per iteration (line) chomp() idiom; irregular tr deletes “, ' tokenizer for Perl last line pops averages from student array and splits values into columns (printed with each student)

Copyright © 2006 The McGraw-Hill Companies, Inc. # read students while ( ) { chomp; tr /"'//d; $_); } = split(/$sep/,

Copyright © 2006 The McGraw-Hill Companies, Inc. Main - part 4 for loop generates mail, 1 student per iteration student grades split into columns subroutine call; & optional mails averages to script invoker prints number of student s generated

Copyright © 2006 The McGraw-Hill Companies, Inc. # gen mail for each student my $ct = 0; foreach { = split(/$sep/); $ct += } $ave[1] = $ENV{"USER"}; print " s sent: $ct\n"; exit;

Copyright © 2006 The McGraw-Hill Companies, Inc. sub readSplit reads a line; note $_ is global chomp idiom tr deletes quotes splits line and returns array of columns

Copyright © 2006 The McGraw-Hill Companies, Inc. sub readSplit { $_ = ; chomp; tr /"'//d; = split(/$sep/); }

Copyright © 2006 The McGraw-Hill Companies, Inc. sub sendMail no formal parameters shift -- call by array reference -- call by reference return if student has no address open pseudo file or die –MAIL is a pipe to Berkeley mail command –s option is subject –$ is mail address

Copyright © 2006 The McGraw-Hill Companies, Inc. sub sendMail { my $name = shift; my $ = shift; return 0 unless $ ; open(MAIL, "| mail -s '$class Grades' $ ") || die "Cannot fork mail: $!\n"; print MAIL "GRADE\t\tYOUR\tMAX\tCLASS\n", "NAME\t\tSCORE\tSCORE\tAVE\n\n";

Copyright © 2006 The McGraw-Hill Companies, Inc. sub sendMail -- for loop for each column skip column if empty header -- not a grade otherwise print header if column value starts with a digit, round value to an integer; otherwise print value { } required; reason for backwards if/unless print maximum points print average (rounded)

Copyright © 2006 The McGraw-Hill Companies, Inc. my $ct = 1; foreach { $ct++; next unless $hdr[$ct]; print MAIL "$hdr[$ct]\t"; print MAIL "\t" if length($hdr[$ct]) < $tab; if (/^\d/) { print MAIL int($_ + 0.5); } else { print MAIL $_; }

Copyright © 2006 The McGraw-Hill Companies, Inc. print MAIL "\t$max[$ct]\t"; if ($ave[$ct] =~ /^\d/) { print MAIL int($ave[$ct] + 0.5); } else { print MAIL $ave[$ct];} print MAIL "\n"; } # foreach return 1; } # sub sendMail

Copyright © 2006 The McGraw-Hill Companies, Inc. $_ : implied object/subject $ARG : default input $. : input line number $/ : input record separator : default \n –undef $/; $_ = ; # reads entire file $, : output field separator in print $\ : output record separator $" : list separator : default space $[ : starting array index : default zero 10 pages of these; also names

Copyright © 2006 The McGraw-Hill Companies, Inc. Regular Expressions Operators m// -- match, m optional s/// -- substitute split -- array returning function

Copyright © 2006 The McGraw-Hill Companies, Inc. Modifiers i -- case insensitive m -- treat string as multiple lines s -- treat string as a single line x -- extend with whitespace and comments

Copyright © 2006 The McGraw-Hill Companies, Inc. Most characters match themselves Meta characters: \ | ( ) [ { ^ $ * + ?. ^ if first -- match from beginning of string $ if last -- match until end of string. -- single character wildcard * + ? -- Kleene star Questions –Empty string? –Entire string? –First x to end of string?

Copyright © 2006 The McGraw-Hill Companies, Inc. [ ] -- set –letters, digits, vowels [^ ] -- not in set | -- or ( ) -- grouping predefined sets (lc) and their inverses (uc) –\d : digits –\w : letters plus digits –\s : whitespace