Use Perl Array Variables

Perl arrays let you use tabular data or iterate over a list of values, for example, a set of data points for a question. In addition, the Question Editor supports a special array called @dat.

Perl array variables are prefixed by the at sign (@). 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.

One-Dimensional Arrays

One-dimensional arrays are like grocery lists. For example:

wine
cheese
bread

In Perl, you enclose the list in parentheses and separate individual list items with commas. For example:

@groceries = ('wine', 'cheese', 'bread');

To refer to a single element in the array, specify the array variable as a scalar with an index indicating which element you are referring to. The index starts with 0 and follows the variable name in brackets. For example:

The second item on my grocery list is <eqn $groceries[1]>.

You can mix numeric and text data in an array.

Tip: To specify a sequential range of integers, you can list the first value, two periods, and the last value. For example:
@one_to_ten = (1..10);

Two-Dimensional Arrays

Two-dimensional arrays let you work with tabular data. For example:

Mercury0.39
Venus0.72
Earth1
Mars1.52
Jupiter5.20
Saturn9.54
Uranus19.18
Neptune30.06

In Perl, you enclose the entire table in parentheses, enclose each row in brackets, and separate both rows and table cells with commas. For example:

@dat = ( ['Mercury' , 0.39 ],
         ['Venus'   , 0.72 ],
         ['Earth'   , 1    ], 
         ['Mars'    , 1.52 ],
         ['Jupiter' , 5.20 ],
         ['Saturn'  , 9.54 ],
         ['Uranus'  , 19.18],
         ['Neptune' , 30.06] );
Note: Technically, two-dimensional arrays in Perl are arrays of arrays. Each "row" is itself a reference to the anonymous array in brackets.

To refer to an element in a two-dimensional array, specify the array variable as a scalar with two indexes indicating the row and column of the element you are referring to. The index values start with 0 and follow the variable name in brackets. For example:

The average distance from the Sun to <eqn $dat[4][0]> is <eqn $dat[4][1]> AU.

The @dat Array

WebAssign provides support for viewing one- and two-dimensional array values in the Question Previewer when you use an array named @dat. If your question contains the @dat array, the Question Previewer displays an additional tab named Array, which displays the contents of the array in addition to the question code and a preview of your question.

image of Array tab in the Question Previewer
Note: The letters in the row headings are not meaningful index values for the elements in @dat. Use numeric index values for both axes.

For example, in the array shown above, $dat[2][0] is "Earth" but $dat[C][0] would be parsed as $dat[0][0] and return "Mercury."

In all other respects, the @dat array is a normal array and can contain whatever values you want.

To use an array in your question:

  1. In an <EQN> or <eqn> tag, set the values for elements of the array using assignment statements like the following examples.
    @planets = ('Mercury', 'Venus', 'Earth', 'Mars', 'Jupiter', 'Saturn', 'Uranus', 'Neptune');
    @mean_au = (0.39, 0.72, 1, 1.52, 5.20, 9.54, 19.18, 30.06);
    @dat = ( ['Mercury' , 0.39 ],
             ['Venus'   , 0.72 ],
             ['Earth'   , 1    ], 
             ['Mars'    , 1.52 ],
             ['Jupiter' , 5.20 ],
             ['Saturn'  , 9.54 ],
             ['Uranus'  , 19.18],
             ['Neptune' , 30.06] );
    $dwarf_planets[0] = 'Pluto';
    $dwarf_planets[1] = 'Eris';
    
    Best Practice: Initialize arrays and other variables at the very beginning of Question.
  2. In an <EQN> or <eqn> tag, reference specific array element values by specifying the array variable as a scalar with an index indicating which element you are referring to, as in the following examples.
    $thisplanet = $planets[7];         # sets $thisplanet = Neptune
    $thismean_au = $mean_au[7];        # sets $thismean_au = 30.06
    $firstplanet = $dat[0][0];         # sets $firstplanet = Mercury
    $firstorbit = $dat[0][1];          # sets $firstorbit = 0.39

Example Question Using Array to Define Possible Answers

The following table summarizes an actual question.

QID
Name
ModeFill-in-the-Blank
Question
<eqn>
@dat= ('John Lennon', 'Paul McCartney', 'George Harrison', 'Ringo Starr');
''
</eqn>
Name one of the Beatles: <_>
Answer
<EQN join("\t",@dat)>
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Example Question Using Array and Randomization

The following table summarizes an actual question.

QID
1950001
Name
Template2 RAND.ARRAY
ModeMultiple-Choice
Question
<eqn>
# weight in Newtons
$weight = randnum(495,940,1);

# calculate weight on other planets
@dat = ( ['Mercury' , decform($weight * 0.348, 0) ],
         ['Venus'   , decform($weight * 0.907, 0) ],
         ['Mars'    , decform($weight * 0.377, 0) ],
         ['Jupiter' , decform($weight * 2.364, 0) ],
         ['Saturn'  , decform($weight * 1.064, 0) ],
         ['Uranus'  , decform($weight * 0.889, 0) ],
         ['Neptune' , decform($weight * 1.125, 0) ] );

# pick three indices for @dat
@selected = pick(3, 0..$#dat);
''
</eqn>
If you weighed <eqn $weight> N on Earth:<br>
<ul>
<li>Your weight on <eqn $dat[$selected[0]][0]> would be <eqn $dat[$selected[0]][1]> N</li>
<li>Your weight on <eqn $dat[$selected[1]][0]> would be <eqn $dat[$selected[1]][1]> N</li>
<li>Your weight on <eqn $dat[$selected[2]][0]> would be <eqn $dat[$selected[2]][1]> N</li>
</ul>
On which planet would your mass be the least? <_>
Answer
<EQN $ORDERED=5; 'Earth'>
<EQN $dat[$selected[0]][0]>
<EQN $dat[$selected[1]][0]>
<EQN $dat[$selected[2]][0]>
Your mass would be the same
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