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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

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Perl Chapter 5

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  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

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