** Using WeBWorK **

If I set vectors to be displayed in ijk form by using:

$context = Context("Vector"); $context->flags->set(ijk=>1);

Then if I try to compute the cross product of two vectors by:

$a = Vector("1 i + 2 j +3 k"); $b = Vector("4 i + 5 j + 6 k");

$a_cross_b = Compute("$a >< $b");

I get the following error:

I get an Operands of '*' are not of compatible types;

Is there anything I can do to solve this?

Also - the resulting $a_cross_b object when displayed in the "correct" box isn't the calculated cross produce - the "><" operator is still present. Is there a way to have it display as the calculated value in the "correct" box?

(Sorry, I am not working with the most up to date version of Webwork. I hopefully will be soon but at the moment it is beyond my control)

Michal

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(I will fix the Parser so that it includes the parens when ijk mode is in effect, but then you will have to be sure to use Context()->texStrings when you use one of these vectors within the BEGIN_TEXT/END_TEXT block. But you would need to do that anyway to get the vectors to show up correctly.)

I'm not sure where you are getting the error about '*'; are you sure it's from this line?

As for seeing "><" in the answer, that's exactly what Compute() is supposed to do for you. One of the things it does is arrange that the string you used to compute the answer is the one shown for the correct answer. So if you don't want that, don't use Compute(). Just use

    $a_cross_b = $a x $b;

Finally, you don't really need to use Vector("1 i + 2 j + 3 k"), you could just use

    $a = i + 2*j + 3*k;

    $a = Vector(1,2,3);

Davide

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I should have said earlier: I tried this and it still came up with the same error.

> I'm not sure where you are getting the error about '*'; are you sure it's from this line?

Just checked, and yes.

> $a_cross_b = $a x $b; This works fine with the ijk flag, and is more convenient - thanks.

> Finally, you don't really need to use Vector("1 i + 2 j + 3 k") Thanks, I didn't know this.

Michal.

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Also, what version of WW are you using (and have you updated any of the files by hand)?

Davide

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