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Why should I learn/use Perl? • Perl is fun • Perl is useful for… • Text processing • Web programming • System administration • Game programming • Scientific research • bio-informatics, linguistics • Open Source
How to ‘get’ Perl – Windows • Strawberry Perl • For Windows only • Just the same as Perl elsewhere • http://strawberryperl.com/ • Recommended!! • ActivePerl • For Windows, Linux, Mac OS X, Solaris, AIX and HP-UX • http://www.activestate.com/activeperl/
Scalars • Numbers • Decimal floating point • (Can be made integer, octal, hexadecimal) • Strings • Can contain any character • Can be null: “” • Can be undef
Strings • Single-quoted • characters are as shown with only two exceptions. • single-quote in a single-quoted string requires \’ • backslash in a single-quoted string requires \\ • Double-quoted • it will interpolate– calculate variables or control sequences. • For example • $foo = “myfile”; • $datafile = “$foo.txt”; • will result in the variable $datafile holding the string “myfile.txt” • Another example • print ‘Howdy\n’; will print: • Howdy\n • print “Howdy\n”; will print • Howdy • (\n is a control sequence, standing for “new line”).
Scalar operators • Math, just like C/C++ • *, /, %, **, etc. • Strings • x to repeat the thing on the left • “hi” x 5 gives “hihihihihi” • . concatenates strings • “Hello” . “ World!” • Perl knows to convert when mixing these two types: • “3”*7 gives 21 • “3”.7 gives “37”
Comparing Scalars Comparison Numeric String • Equal == eq • Not equal != ne • Less than < lt • Greater than > gt • Less or equal <= le • Greater or equal >= ge 8 < 25 TRUE! “8” lt “25” FALSE!
Variables • A sign, followed by a letter, followed by pretty much whatever. • Sign determines the type: • $foo is a scalar • @foo is a array hold a list of scalars • %foo is a hash • Variables default to global (they apply in all parts of your program). This can be problematic. • local $var will make the variable active only for the current “block” of code. • my $var does the same, and dies when out the current “block”
Using Arrays • Elements are indexed, from 0. • my@animals = (“frog”, “bear”, “elephant”); • $animals[2]; # elephant • Note: element is a scalar, so $ rather than @ • Subsections are “slices”. • my@mammals = @animals[1,2]; • Lots of functions for • push, pop … • splitting a scalar string into an array • my$sentence = “元智大學 資訊工程系”; • my@words = split(“ “, $sentence); # @words contains (“元智大學”, “資訊工程系”);
Control flow • Control structures (generally like C/C++) • if / then / elsif / else • while • do {} while • do {} until • for () / foreach() # loops over a list • switch (NOTE: After perl 5.10) • … • Errors / warnings • die“message” terminates program and output “message”. • warn“message” give you a warning and keeps going.
Hashes • “Associative arrays” • A set of • values (any scalar), indexed by • keys (strings) • Example • my%info; • $info{ “name” } = “宋政隆”; • $info{ “age” } = 32; • $info{ “future” } = undef; • hashes 與 arrays 可以摻在一起用 • arrays of arrays, arrays of hashes, hashes of arrays …
Matching string patterns using regular expressions • m/pattern/ will check the last stored variable ($_) for pattern. • $var =~ m/pattern/; will check $var for pattern. • If the pattern is in $var, then • $var =~ m/pattern/ is TRUE. • If you “group” part of the pattern and it is present, • $var =~ m/(pattern)/ is true, AND, now a variable names $1 contains the first match it found. • Group more pieces of the pattern and the matches are stored in $2, $3, etc. • This only grabs the *first* match. To grab all, say • my @matches = ($var =~ m/(pattern)/g); • This will store every match in the array @matches.
What’s a “regular expression”? . any single character * zero or more of the previous + one or more of the previous ? zero or one of the previous {n} match exact n times {n,} match at least n times {n,m} match at least n but not more than m times [] character class ^ beginning of the line $ end of the line \b word boundary \d \D digit / non-digit \s \S space / non-space \w \W word character / non-word character | or – match this or that () grouping
Google Page Rank via WWW::Google::PageRank use WWW::Google::PageRank; my$url = shift; my$gpr = WWW::Google::PageRank->new(); printscalar($gpr->get ($url)); % perl mpr.pl www.cse.yzu.edu.tw 6
What is CPAN • Since 1995-10-26 • Collection of Perl software/class/documentation • DRY (Don’t Repeat Yourself) • 8000+ authors • ~18000 modules • Testers report for ALL module
