Use Perl Hash Variables

Perl hashes are special kinds of arrays consisting of named key/value pairs. You can return a value by specifying the corresponding key.

Perl hash variables are prefixed by the percent sign (%) character. In addition, all Perl variable names must follow these rules:

Variable names must contain only letters (a-z, A-Z), underscores (_), and numeric digits (0-9).
The first character of a variable name must be a letter (a-z, A-Z) or underscore (_).
Variable names are case-sensitive, so myvariable is not the same as MyVariable.

Some variable names are used by WebAssign. These variables are listed in the documentation.

Hashes

Hashes are like a lookup table with a key column and a value column. Instead of specifying a numeric index, you use the key as an index to find the corresponding value.

Key	Value
Mercury	0.39
Venus	0.72
Earth	1
Mars	1.52
Jupiter	5.20
Saturn	9.54
Uranus	19.18
Neptune	30.06

In Perl, you enclose the entire table in parentheses, separate rows with commas, and use the => operator between the key and the value. For example:

%planets = ( 'Mercury' => 0.39 ,
             'Venus'   => 0.72 ,
             'Earth'   => 1    , 
             'Mars'    => 1.52 ,
             'Jupiter' => 5.20 ,
             'Saturn'  => 9.54 ,
             'Uranus'  => 19.18,
             'Neptune' => 30.06 );

Best Practice Initialize hashes and other variables at the very beginning of Question.

You can add a single key/value pair to your hash by specifying the hash variable as a scalar followed by the key name in braces. For example:

$planets{'Neptune'} = 30.06;

Note Because this is a simple assignment statement, you use = and not =>.

To look up a value in a hash variable, you also specify the hash variable as a scalar followed by the key name in braces. For example:

The average orbital distance of Venus is <eqn $planets{'Venus'}> AU.

In an <EQN> or <eqn> tag, set the keys and values for the hash as in the following examples.

%elements = ( 'H'  => 'Hydrogen', 
              'He' => 'Helium', 
              'Li' => 'Lithium', 
              'Be' => 'Beryllium', 
              'B'  => 'Boron',
              'C'  => 'Carbon',
              'N'  => 'Nitrogen',
              'O'  => 'Oxygen',
              'F'  => 'Fluorine',
              'Ne' => 'Neon' );
$elements{'Na'} = 'Sodium';
%oxygen = ( 'symbol' => 'O', 
            'number' => 8, 
            'weight' => 15.9994, 
            'econfig' => '1s<sup>2</sup> 2s<sup>2</sup> 2p<sup>4</sup> ');

Best Practice Initialize arrays and other variables at the very beginning of Question.

In an <EQN> or <eqn> tag, look up hash values by specifying the hash variable as a scalar followed by the key name in braces, as in the following examples.
```
$element_name = $elements{'He'};
# sets $element_name = Helium

if ($this_weight == $oxygen{'weight'}) {$my_element = 'Oxygen'}; 
# compares $this_weight and 15.9994
```

Example Question Using Hash Variables

The following table summarizes an actual question.

QID
Name
Mode	Fill-in-the-Blank
Question	<eqn> $keyA = "He\tNe\tAr\tKr\tXe\tRn"; # \t creates a {tab} in the answer key. $keyB = $keyA; %names = ( 'He' => 'Helium', 'Ne' => 'Neon', 'Ar' => 'Argon', 'Kr' => 'Krypton', 'Xe' => 'Xenon', 'Rn' => 'Radon'); %numbers = ( 'He' => 2, 'Ne' => 10, 'Ar' => 18, 'Kr' => 36, 'Xe' => 54, 'Rn' => 86); %econfig = ( 'He' => '1s^{2}', 'Ne' => '1s^{2} 2s^{2} 2p^{6}', 'Ar' => '[Ne] 3s^{2} 3p^{6}', 'Kr' => '[Ar] 3d^{10} 4s^{2} 4p^{6}', 'Xe' => '[Kr] 5s^{2} 4d^{10} 5p^{6}', 'Rn' => '[Xe] 4f^{14} 5d^{10} 6s^{2} 6p^{6}'); '' </eqn> Provide the following information for any two of the six naturally-occurring noble gases:<br><br> <table border=1 rules="all"> <thead style="background-color: #9DC2DF;"> <tr><td>Symbol</td><td>Name</td><td>Atomic Number</td><td>Electron Configuration</td><tr> </thead> <tbody> <tr style="vertical-align: top;"><td><_></td><td><_></td><td><_></td><td><_></td></tr> <tr style="vertical-align: top;"><td><_></td><td><_></td><td><_></td><td><_></td></tr> </tbody> </table>
Answer	`<EQN $CASE=1; $A=$thisresponse; $keyA> <EQN $names{$A}> <EQN $numbers{$A}> <EQN $PAD='chem'; $CHEM='econf,either'; $econfig{$A}> <EQN $CASE=1; $B=$thisresponse; $keyB =~ s/($A)//; $keyB> <EQN ($A eq $B) ? '' : $names{$B}> <EQN ($A eq $B) ? '' : $numbers{$B}> <EQN $PAD='chem'; $CHEM='econf,either'; ($A eq $B) ? '' : $econfig{$B}>`
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