1 / 16

Perl Chapter 5

Perl Chapter 5. Hashes. Hashes. Outside of world of Perl, know as associative arrays Also called hash tables Perl one of few languages that has hashes built-in. Structure of Hashes. Hashes are lists of scalar values BUT have string indices (called keys) keys also stored in structure

filipina
Download Presentation

Perl Chapter 5

An Image/Link below is provided (as is) to download presentation Download Policy: Content on the Website is provided to you AS IS for your information and personal use and may not be sold / licensed / shared on other websites without getting consent from its author. Content is provided to you AS IS for your information and personal use only. Download presentation by click this link. While downloading, if for some reason you are not able to download a presentation, the publisher may have deleted the file from their server. During download, if you can't get a presentation, the file might be deleted by the publisher.

E N D

Presentation Transcript

  1. Perl Chapter 5 Hashes

  2. Hashes • Outside of world of Perl, know as associative arrays • Also called hash tables • Perl one of few languages that has hashes built-in

  3. Structure of Hashes • Hashes are lists of scalar values BUT have string indices (called keys) • keys also stored in structure • variable name starts with % • have their own namespace (like arrays) • need not be declared, grow and shrink • no way to determine order • internal has function to store and retrieve

  4. literals • no hash literals (use list literals) (“bob”, 42, “carol”, 40, …) • or use => instead of comma (“bob” => 42, “carol” => 40, …) or (bob => 42, carol => 40, …) • do not need “ “’s, left of => implicitly quotes barewords

  5. first is actually a list, odd subscripted elements of array  keys of the hash @list = (Bob, 42, Carol, 40); %ages = @list; same as %ages =(“bob” => 42, “carol” => 40); must be even length! %salaries = (“Bob” => 79_500, “Carol” => 43_000);

  6. accessed by “subscripting” with key $salaries{“Bob”}  79500 • insert new values $salaries{“Mike”} = 51_950; • if Mike not in table, adds it • if Mike is in table, changes value • set to empty %salaries = (); undef %salaries • NOT %salaries = undef;  (1 element, undef)

  7. printing • hash variables not interpolated in double-quoted strings • print “%salaries\n”; • prints %salaries • print %salaries; • prints keys and values, no spaces

  8. slice of hash • gives us a list or array @some_salaries = @salaries{“Bob”, “Mike”}; • @some_salaries  (79500, 51950) • note the @ form of the variable • since slice of a hash is an array, can be interpolated in double quoted strings

  9. operators delete $salaries {“Billie”}; • key and salary deleted from %salaries if (exists $salaries{“Billie”}) … • to find out if in hash

  10. keys and values operators • keys and values of a hash are arrays • keys operator  list of keys • values operator  list of values %highs=(“mon”=>64, “tue”=>66, “wed”=>72, “thu”=>55, “fri”=>35); @days = keys %highs; #array context • @days is (“mon”, “tue”, “wed”, “thu”, “fri”)

  11. foreach foreach $day (@days) { … } • or foreach $day (keys %highs){ print “on $day,the temp was $highs{$day}\n”; } ^hashing • of course, can sort (sort (keys %highs)) • keys in scalar context $length = keys %highs;

  12. values operator @temps = values %highs; foreach $temp (values %highs){ print “$tep\n”; }

  13. Process pairs • use each operator to return next element ($day, $temp) = each %highs; • usually iterate on it while (($day, $temp)= each %highs){ print “On $day, the high temp was $temp.\n”; } • cannot add to hash in loop body, if keys or each used in loop

  14. in boolean expression if (%highs) … • scalar context –> boolean expression, true if hash not empty

  15. Predefined hashes • %ENV in first example • When to use array vs. hash • when you have many accesses to specific elements

  16. Examples • freq.pl • FindFiles.pl

More Related