1. Perl Chapter 5 Hashes

2. 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