Foundations

Hashes

Hashes store values by key, which is useful for lookups.

hash lookup A hash key selects a stored value, such as `$prices{$item}`.

Hashes

item
hashes.pl
Replay: real traced execution (multi-file project)
use strict;
use warnings;

my %prices = (
    apple => 2,
    banana => 1,
    pear => 3,
);

my $item = "apple";
my $price = $prices{$item};

print "item=$item\n";
print "price=$price\n";

use strict;
use warnings;

my %prices = (
    apple => 2,
    banana => 1,
    pear => 3,
);

my $item = "banana";
my $price = $prices{$item};

print "item=$item\n";
print "price=$price\n";

use strict;
use warnings;

my %prices = (
    apple => 2,
    banana => 1,
    pear => 3,
);

my $item = "pear";
my $price = $prices{$item};

print "item=$item\n";
print "price=$price\n";

  1. %prices ← 3, $item ← apple, $price ← 2

    4my %prices→ 3 = (5    apple => 2,6    banana => 1,7    pear => 3,8);910my $item→ apple = "apple"; #@item="banana", "pear"11my $price→ 2 = $prices{$item}2;1213print "item=$itemapple\n";14print "price=$price2\n";
    outputitem=apple
price=2

  1. %prices ← 3, $item ← banana, $price ← 1

    4my %prices→ 3 = (5    apple => 2,6    banana => 1,7    pear => 3,8);910my $item→ banana = "banana";11my $price→ 1 = $prices{$item}1;1213print "item=$itembanana\n";14print "price=$price1\n";
    outputitem=banana
price=1

  1. %prices ← 3, $item ← pear, $price ← 3

    4my %prices→ 3 = (5    apple => 2,6    banana => 1,7    pear => 3,8);910my $item→ pear = "pear";11my $price→ 3 = $prices{$item}3;1213print "item=$itempear\n";14print "price=$price3\n";
    outputitem=pear
price=3

What Happens

  1. %prices stores apple => 2, banana => 1, and pear => 3.
  2. $item starts as "apple".
  3. $prices{$item} looks up the value for apple.
  4. $price becomes 2.
  5. The program prints item=apple and price=2.

Hash Picture

| item | lookup | price | | --- | --- | --- | | apple | $prices{"apple"} | 2 | | banana | $prices{"banana"} | 1 | | pear | $prices{"pear"} | 3 |

Try It

Exercise: hashes.pl

Reproduce item=apple and price=2, then use the pinned items banana and pear to predict each price.