Evaluation Control

[sharper47]

I teach algebra at the local community college and I'm trying to use Mathematica to display a set of homework or quiz problems, with an associated button for each problem to expose the answer (maybe someday to become part of a Mathematica-based textbook). These are quite simple problems (well, some of my students wouldn't agree...) and the problem I have is that Mathematica evaluates the problem and shows the answer before I can display the problem in unevaluated form. I've read the documentation on the topic pretty carefully, and have experimented quite a bit with functions like Unevaluated, Hold, HoldForm, Defer, etc, but so far haven't been able to come up with a satisfactory solution. I have also looked at the Demonstrations Project for similar stuff and studied some code from there, but still no luck. All of those focus on slightly more complex problems which don't immediately simplify (so they display as nice expressions like you want), but then you use Expand or Solve, and the numeric or symbolic answer comes out nicely.

Here's a prototypical problem (the formatting would be nicer in Mathematica, but I'm sure you get the idea):

Simplify 4x^2/(-3xy^(-3)). Answer: -4xy^3/3

As soon as I assign the problem expression to a symbol (or as a list element for a set of similar problems), Mathematica evaluates everything and all I have is the answers, when I wanted the students to do the work first... I really need a way to save the original expression and display it (nice formatted TraditionalForm), and then later force evaluation to display the answer under a button, or FlipView, or whatever. I've even tried saving the original form as a string and then using ToExpression, but there are spacing and other formatting problems that crop up when I do that...

What's the best way to do this?

Thanks,
Steve

[Ginger_McKee]

Below is a notebook that shows how to generate problems and an answer key. It shows problems with fractions and how to get them to display in original, not simplified, form. Let me know if you think this is helpful.
http://members.wolfram.com/mmorrison/Fa ... ogram.html

[sharper47]

Ginger,

Thanks for your response. The three code snippets in your link were all helpful and interesting to me, but unfortunately don't do anything to solve this particular problem. Indeed, the fraction quiz example in the last snippet actually suffers from the exact problem I've described, though in that particular case it's probably what the instructor prefers. That random integer approach will sometimes create fractions like 2/4, 2/6, 3/6, 2/8, 4/8, 6/8, 3/9, and 6/9 (and inverses of these) that are not in lowest terms. However, because the fraction is in the form of a math expression (rather than, say, a string expression), Mathematica evaluates it before it is displayed. This results in the fraction being reduced to lowest terms, whether you want the problem displayed that way or not. The only reason the sum or difference isn't evaluated also is that the "+" or "-" is displayed as a string, so the problem is really in separate pieces. To generate the answers, the StringTake expression then converts the "+" or "-" string to a "math" expression, and Mathematica does the evaluation of the entire expression for the answer. The only difference between this and my problem is that I'm dealing with symbolic expressions rather than numeric ones. Actually I've made some progress. After rereading the documentation several times, it looks like HoldForm is the right thing to do, though I'm still having difficulty extracting problems from a list.

Thanks Anyway,
Steve

[telefunkenvf14]

I teach algebra at the local community college and I'm trying to use Mathematica to display a set of homework or quiz problems, with an associated button for each problem to expose the answer (maybe someday to become part of a Mathematica-based textbook). These are quite simple problems (well, some of my students wouldn't agree...) and the problem I have is that Mathematica evaluates the problem and shows the answer before I can display the problem in unevaluated form. I've read the documentation on the topic pretty carefully, and have experimented quite a bit with functions like Unevaluated, Hold, HoldForm, Defer, etc, but so far haven't been able to come up with a satisfactory solution. I have also looked at the Demonstrations Project for similar stuff and studied some code from there, but still no luck. All of those focus on slightly more complex problems which don't immediately simplify (so they display as nice expressions like you want), but then you use Expand or Solve, and the numeric or symbolic answer comes out nicely.

Here's a prototypical problem (the formatting would be nicer in Mathematica, but I'm sure you get the idea):

Simplify 4x^2/(-3xy^(-3)). Answer: -4xy^3/3

As soon as I assign the problem expression to a symbol (or as a list element for a set of similar problems), Mathematica evaluates everything and all I have is the answers, when I wanted the students to do the work first... I really need a way to save the original expression and display it (nice formatted TraditionalForm), and then later force evaluation to display the answer under a button, or FlipView, or whatever. I've even tried saving the original form as a string and then using ToExpression, but there are spacing and other formatting problems that crop up when I do that...

What's the best way to do this?

Thanks,
Steve

Steve:

Here is something you should look at before putting too much work into your own approach. Go to http://home.comcast.net/~djmpark/Mathematica.html, then scroll down to the info on 'Two High School Tutorials' and 'Expression Manipulation' and grab the relevant notebooks and packages.

-RG

[telefunkenvf14]

Just playing around... don't know if this would be helpful or not.

Code: Select all

Labeled[TabView[{"Q:" -> HoldForm[ 4 x^2/(-3 xy^(-3))],
   "A:" -> 4 x^2/(-3 xy^(-3))},
  BaseStyle -> {48, FontFamily -> "Times", Italic},
  LabelStyle -> {24, FontFamily -> "Times", Italic},
  Alignment -> Center], "Simplifying Expressions", Top,
LabelStyle -> {24, FontFamily -> "Times", Bold}]

Preview:





-RG

[sharper47]

RG,

Thanks a bunch for your help. The code snippet you cooked up is a very pretty way of demonstrating what I've discovered about HoldForm. The link you provided in your previous post looks really interesting; I am going to spend some time studying that material. It looks like it should provide everything I need and then some.

Thanks,
Steve

[telefunkenvf14]

RG,

Thanks a bunch for your help. The code snippet you cooked up is a very pretty way of demonstrating what I've discovered about HoldForm. The link you provided in your previous post looks really interesting; I am going to spend some time studying that material. It looks like it should provide everything I need and then some.

Thanks,
Steve

Just so you know, David Park is a frequent poster on MathGroup (the Mathematica newsgroup), and a real nice guy as well. You might be able to get some help on using his package via that forum. To participate in the newsgroup go to: http://groups.google.com/group/comp.soft-sys.math.mathematica/topics

-RG