[ww-bugs] Bug 4092: Correct answer is a formula, but any number is also accepted

[bugzilla-daemon at webwork.maa.org]

http://bugs.webwork.maa.org/show_bug.cgi?id=4092


Davide P. Cervone <dpvc at union.edu> changed:

           What    |Removed                     |Added
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                 CC|                            |dpvc at union.edu




--- Comment #1 from Davide P. Cervone <dpvc at union.edu>  2018-03-08 14:12:11 ---
I suspect the error is in the line

    return ($student->D("x") == a*($student*$student + $ksq));

because the "a" in this is not the same as the "a" in the context; here, it is
just a perl string "a".  When the student answer is a number, then you get that
string a times a MathObject Real, and the Real tries to convert the "a" to a
number.  It gets 0 in that case (that should probably be viewed as a bug in
MathObjects).  So the right-hand side is 0.  The left is the derivative of the
constant, so is also 0.  So that means the value is "correct".

When the student answer is a formula, the right hand side turns the string "a"
into a Formula not a Real, and since "a" is a variable in the Context, it is a
formula "a" times the other stuff, and that produces a formula result, and
everything checks as expected.  But is it still bad practice to use a literal
"a" as a string, and it would be better as

   return ($student->D("x") == Formula("a*($student*$student + $ksq)"));

for example.

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