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Verifying Programs

Although EM was not designed as an assessor program, but rather for self-testing in a computer-based learning environment, reliable checking of the answers is a necessary feature to provide feedback for students. It is important to analyze what went wrong in a given answer. For instance, EM should be able to report that the dimensions of the matrices are not correct, or that a special type of matrix is expected. So, we have implemented the checking algorithm separately for each exercise as a verifying program, which is designed to report error messages relevant to that exercise. Obviously, there must be a mechanism to make a proposed answer available to the verifying program. This is accomplished by translating the current state of all objects comprising the answer template into expressions that are passed to the kernel just before the program is evaluated. The verifying program can access those expressions because EM assigns them to be values of symbols identical to the object-tags of the objects. Therefore, the strings given as object-tags in the object definitions should be different from any other symbols used in the generating and verifying programs. Each object-type has its own scheme for translating the state of the objects into expressions. For instance, consider the box of a MatrixEntry object with theMatrix as its object-tag. If we type

[Graphics:../Images/index_gr_10.gif]

EM will pass the following expression when the checking is initiated.

[Graphics:../Images/index_gr_11.gif]

Similarly, the state of a CheckBoxes or RadioControls object is presented as a list of True/False symbols. One possible way to structure the verifying program, which we have used throughout all the exercises on matrix algebra, is as follows.

[Graphics:../Images/index_gr_12.gif]

Various conditions can be tested in turn by the Which operator. If one of them fails, the corresponding error message, kept as a string in errMsgs, is returned. If all conditions hold, "OK" is returned; this is the only string that EM will recognize as a correctly answered problem. As an example, the verifying program of the "LU Decomposition" exercise is given in Listing 2. Students are asked to compute L and U for a given square matrix A such that

[Graphics:../Images/index_gr_13.gif]

where L is lower-triangular, U is upper-triangular, and P is any permutation matrix. The answer template for this exercise is comprised of two objects of type MatrixEntry. Since the strings theL and theU are their object tags, EM will assign the proposed answer to theL and theU symbols just before the verifying program is evaluated. The checking in this case is not trivial, since the decomposition is not unique; L and U are not the same for all possible P.

This example well justifies our choice of Mathematica as a computational engine. Even though we have generated the problem starting from the answer (first we generate l and u appropriately and then we compute a = l.u), we cannot carry out the checking by testing SameQ[theL,l] and SameQ[theU,u]. Instead, after making sure that theL and theU have the proper dimensions and that they are lower- and upper-triangular, respectively, we compute theL.theU. Inverse[a] and check to see if it is a permutation matrix. In this example, MatrixQ, LoMatrixQ, UpMatrixQ, and PermutMatrixQ are predicates indicating whether their argument is a general, lower-triangular, upper-triangular, or permutation matrix, respectively. They are defined in the utility package.

[Graphics:../Images/index_gr_14.gif]
Listing 2. Verifying program for "LU decomposition."


Converted by Mathematica      October 5, 1999

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