## WeBWorK Problems

### Matrices with variables in the -> using operations

by Murphy Waggoner -
Number of replies: 1
Hi,

I'm just trying to write a simple problem where the students perform matrix operations.  I put variable names in the matrices.  But I continue to run into problems with this.

a)  I tried to have WebWork calculate the transpose of a matrix, but I got the error "Can't locate object method "transpose" via package "Value::Formula"" which makes it sound as if I can't use matrix operations with variable names in them.

b)  So I punted and just entered the transpose myself, but now when I have WebWork display the matrices so the students can enter them, the ones with variables in them don't display as a matrix, but simply as a single blank.  See attached image.

I have tried looking to see if I need to include some other pl file so that I can do this, but the documentation doesn't mention it.

c)  Is there a better way to do the 'impossible' case?

Thanks, as always, for the help on this.  I'm sure it is just something I am not seeing, but after 45 minutes of debugging this problem (I had other issues before I could get this far) I just need to take a break.

DOCUMENT();

"PGstandard.pl",
"PGcourse.pl",
"PGessaymacros.pl",
"MathObjects.pl",
);

TEXT(beginproblem());
$showPartialCorrectAnswers = 1; Context('Matrix')->variables->are(c=>'Real',d=>'Real',x=>'Real',y=>'Real', 'm'=>'Real','p'=>'Real', 'r'=>'Real','u'=>'Real', 'v'=>'Real','w'=>'Real');$A = Matrix(  [1,2],    ["c", "d"]  );

$B = Matrix( [3, "x", "y"], [1, 4, 0] );$C =    Matrix([   [2,5], ["m", "p"], ["r", "u"]]);
$D = Matrix(["u", 2, 0], [1, "v", 0], [0, 1, "w"]);$Imp = Matrix([[-1,-1], [-1, -1], [-1, -1]]);

$D2 = Matrix("$D * $D");$AB = Matrix("$A*$B");
$CT = Matrix([ [2, "m", "r"], [5, "p", "u"]]);$BmCT = Matrix("$B -$CT");

Context()->texStrings;
BEGIN_TEXT
Let $A = A,\ B = B,\ C = C \text{ and } D = D.$

$BR$BR
a)  $$D^2 =$$ \{ $D2->ans_array \}$BR
$BR b) $$AB =$$ \{$AB->ans_array \}
$BR$BR
c)  $$B^TC =$$ \{ $Imp->ans_array \}$BR
$BR d) $$B - C^T =$$ \{$BmCT->ans_array \}
$BR$BR
e)  If any of the calculations were impossible, explain why.
\{essay_box(8,48)\}
$BR$BR
These last question will be graded by the instructor.  You will not get immediate feedback.

END_TEXT
Context()->normalStrings;

ANS($D2->cmp); ANS($AB->cmp);
ANS($Imp->cmp); ANS($BmCT->cmp);
ANS(essay_cmp());

COMMENT('MathObject version');
ENDDOCUMENT();

### Re: Matrices with variables in the -> using operations

by Christopher Heckman -
a) I tried to have WebWork calculate the transpose of a matrix, but I got the error "Can't locate object method "transpose" via package "Value::Formula"" which makes it sound as if I can't use matrix operations with variable names in them.

Bingo. Most Matrix operations require you have numbers as entries; not even fractions work. I think this is on a lot of peoples' wish lists.

b)  So I punted and just entered the transpose myself, but now when I have WebWork display the matrices so the students can enter them, the ones with variables in them don't display as a matrix, but simply as a single blank.  See attached image.

Have you tried taking out the quotation marks? Unless WeBWorK went through a major revision recently, you shouldn't need them.

c)  Is there a better way to do the 'impossible' case?

I'm going to punt on that one; I've never done essay questions before in WeBWorK.