** Using WeBWorK **

Hi,

I would like to know how to use the swap_rows subroutine defined in PGmorematrixmacros.pl

What is wrong with the following code?

DOCUMENT();

loadMacros(
"PGbasicmacros.pl",
"PGanswermacros.pl",
"PGmatrixmacros.pl",
"PGmorematrixmacros.pl",
);

TEXT(beginproblem());

$A = random_inv_matrix(3,3);

$B=swap_rows($A,1,2);

BEGIN_TEXT

\{ mbox(display_matrix($B)) \}

END_TEXT

ENDDOCUMENT();

I obtain the error:

Can't use an undefined value as an ARRAY reference at /opt/webwork/webwork2/lib/WeBWorK/ContentGenerator/Problem.pm line 1218.

Thanks in advance.