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 );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; 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> ');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> <tr><td>Symbol</td><td>Name</td><td>Atomic Number</td><td>Electron Configuration</td><tr> </thead> <tbody> <tr><td><_></td><td><_></td><td><_></td><td><_></td></tr> <tr><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}> Display to Students 