## WeBWorK Problems

### Can't convert an empty value to a real number on incorrect interval response

Number of replies: 2
Hi again -

When using MathObjects, if a student responds to a question asking for an interval with an incorrect response, they receive the message "Can't convert an Empty Value to a Real Number"

Am I coding something incorrect? Is it possible to coerce Webwork into giving a more meaningful message (to me, it would be preferable to simply say "incorrect").

I've copied below sample code which exhibits the problem.

Thanks,

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########################################################################

DOCUMENT();

"PGstandard.pl", # Standard macros for PG language
"MathObjects.pl",
#"source.pl", # allows code to be displayed on certain sites.
#"PGcourse.pl", # Customization file for the course
);

# Print problem number and point value (weight) for the problem
TEXT(beginproblem());

# Show which answers are correct and which ones are incorrect
$showPartialCorrectAnswers = 1; ############################################################## # # Setup # # Context("Numeric");$pi = Real("pi");

##############################################################
#
# Text
#
#

Context()->texStrings;
BEGIN_TEXT

Let $$r(t)$$ be the signed distance of a point from the origin at time $$t$$ seconds. Suppose that $$r(t) = 1 + \sin(2t)$$. Suppose that the object begin movement at time $$t = 0$$ and ceases movement at time $$t = 2\pi$$. On what interval is the object's distance from the origin at least $$\frac32$$? Express your answer as an interval. If necessary, use "U" for union.
$BR \{ans_rule(40)\} END_TEXT Context()->normalStrings; ############################################################## # # Answers # # Context("Interval");$int = Compute("[pi/12, 5pi/12] U [13pi/12, 18pi/12]");
ANS($int->cmp()); ENDDOCUMENT(); In reply to Adam Weyhaupt ### Re: Can't convert an empty value to a real number on incorrect interval response by Michael Gage - Try using Context("Interval"); from the beginning of the problem. I think that the fact that pi was defined when you are in numeric context might be causing the response you indicate. Context("Interval");$int = Compute("[pi/12, 5pi/12] U [13pi/12, 18pi/12]");
TEXT(\$int->cmp->evaluate("45")->pretty_print);

Gave me a sensible error message. (pi is automatically defined
in Interval context.):

"Your answer isn't an interval, set or union (it looks like a real number)"