Grasple has multiple check methods using the Computer Algebra System (CAS) which can be used within the editor to indicate when the answer of a student should be marked correct/incorrect. A full list of current options can be found in this article.
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This article covers the Diagonal Matrix check. There are three sections in this article:

If you want to check whether the student answer is a Diagonal Matrix you should use this check. This check only works on matrices of any size, but not on lists or expressions.

This method checks whether all elements on the matrix that are not on the diagonal are zero.

The CAS loops over all elements in the matrix which are not on the diagonal and verifies that these elements are zero (after simplification). Simplification is done using the mathematical rules within CAS.

Do you want to know more about this check, whether you should use it for your exercise or any of the other checks? Please let us know via the chat icon in the right bottom of the screen!