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==STACK Question Variables==
This field is for variables that are used later in the question. The values can be quite general: integers, floating point numbers, symbols or expressions, or random selections of any of the preceding elements.  The variables can be access in other fields of the question by including them in [[question/type/stack/doc/doc.php/Authoring/CAStext.md|CAStext]].  The assignment operator is the colon (:), as is consistent with the maxima program which processes expressions in STACK.  These assignments can be used to build up complicated expressions from simple ones using maxima syntax.
===Simple Assignments===
An example of a simple variable assignment is

p:5;

This assigns the value of 5 to the variable p.  In your question text, but example the appearance of @p@, will be replaced by 5.  The line need not end in a semicolon, but its good practice in case you want to cut and past the assignments into a maxima session for testing.

q:x^2+2;

This assigns the symbolic expression representing 'x<sup>2</sup>+2' to the variable q.  This can be used as a model answer, for instance or part or a later expression.

===Random Assignments===
The significant power of question variables becomes evident when they are used to generate randomly chosen values for a question by means of the rand operator.

'''IMPORTANT NOTE''': If you use random variables in your Question Variables you need to be sure and include them in the [[question/type/stack/doc/doc.php/Authoring/Question note.md|Question Note]] as well.  Otherwise, you (and Moodle) won't be able distinguish between the various question variants when analyzing the questions and reporting results.

Using rand with an integer argument produces a random integer between 0 and one less than that number.  In other words, rand(4) would randomly return either 0, 1, 2, or 3.  An assignment like

x:rand(6)+1;

would produce a random number between 1 and 6 inclusive that could be used in other parts of the question or answer.  A random floating point number is generated by including a number with a decimal point as an argument.  It returns a floating point number between 0 and the given value.  Thus,

y:rand(10.0)-5;

would return a random floating point number between -5.0 and 5.0.

The rand function can also return a random element in a list.  So, for instance,

q:rand([a,b,c,d,e]);

would return a random selection of a, b, c, d, or e.  Using this facility, one could build random symbolic expressions in the questions for each student.

====Variations on rand====
There are a couple of specialized rand functions you can use for unique situations.  The function rand_with_step takes three arguments:
* the lower bound of the number
* the upper bound of the number 
* the step size
Thus rand(1,5,2) would randomly choose 1, 3, or 5.  A related function is rand_with_prohib.  It also take three arugments:
* the lower bound of the numbers
* the upper bound of the numbers
* a list of values to be excluded
For instance, rand_with_prohib(-5,5,[0]) would generate random integers between -5 and 5 inclusive, but not including 0.