[racket] Implementing an equal? method for a class

From: Vincent St-Amour (stamourv at ccs.neu.edu)
Date: Sat Jul 28 22:42:43 EDT 2012

You could have `simple-grid%' implement the `equal<%>' interface.



At Sat, 28 Jul 2012 19:35:47 -0700,
Gregory Woodhouse wrote:
> Okay, this is really a newbie question, but here goes. I have a class that represents a Sudoku grid. Internally, the grid is stored as a vector of vectors of integers:
> (define simple-grid%
>   (class object%
>     (super-new)
>     ;b is an internal representation of the grid and is initialized to an empty grid
>     (define b(make-vector 9 (make-vector 9 0)))
>     (define/public (get-cell r c)
>       (let
>           ([row (vector-ref b (sub1 r))])
>         (vector-ref row (sub1 c))))
>      ;etc.
> I provide methods get-cell to retrieve the contents of a single cell, set-cell! to (destructively) update the contents of a cell and so forth. In principle, I could implement equal? by looping all 81 cells and comparing them one by one, but this seems awkward. It seems like I ought to be able to just check that the state vectors are equal? but I have no access to b outside the class. In Java, I'd add a protected method getStateVector that could be called from within a separate method named equalTo that tells me if a grid object g1 is equivalent to a g2. Or at least I could do this. I really don't want to expose how the internal representation of the grid to other classes. Then again, maybe that's Java thinking in the context of Racket.
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