Tinkering with Perl

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Hashes

Hashes are a little more complicated than either lists or scalars, but they are very useful.

Let's say that you know some people, and you want to keep track of what kind of pets they have. Suppose that you know John, Sue, and Mary. Let's also suppose that John has a dog, Sue has a cat, and Mary has a goldfish. What would be good would be a single variable, called Pet: if you plugged in John, you would get dog; if you plugged in Sue, you would get cat; if you plugged in Mary, you would get goldfish.

A hash is a variable that can do that. We refer to a hash in slightly different ways, depending on what you want to do. If you refer to the whole hash, you put a percent sign ('%') in front of the hash. So the hash with the pets might be:

%pet

But if you want to refer to a specific pet -- say, you want to find out what pet Mary has -- you would do it like this:

$pet{"Mary"}

(We put the "Mary" in quotes, because it is a href="../../definitions/string.html">string, and the computer will get confused if it sees the letters M, a, r, and y all by themselves.)

The expression

$pet{"Mary"}

should be read as, "Mary's pet".

Tinkering with Perl

Hashes

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