Am hoping to prevail upon a fellow Schemer for help debugging my solution to one of the final HtDP exercises. So close, yet so far! The exercise adapts the checking-queens algorithm from Chapter 28 to state variables. It asks the student to do two things: 1) design two new functions, place-queen and unplace-queen, which serve to "set!" the chessboard as the algorithm backtracks into a solution, and; 2) implement those functions into a new version of 'placement.' Here's a link: <a href="http://www.htdp.org/2003-09-26/Book/curriculum-Z-H-53.html#node_sec_43.3">http://www.htdp.org/2003-09-26/Book/curriculum-Z-H-53.html#node_sec_43.3</a> I've done all that, but my solution fails in a very strange way. Specifically, it generates the correct solution to for all boards up to 5 x 5: ;(check-expect (placement 1) (vector (vector true))) ;; passes ;(check-expect (placement 2) false) ;; passes ;(check-expect (placement 3) false) ;; passes ;(check-expect (placement 4) (vector (vector false false true false) (vector true false false false) (vector false false false true) (vector false true false false))) ;; passes ; ;(check-expect (placement 5) (vector ;; passes ; (vector false false true false false) ; (vector true false false false false) ; (vector false false false true false) ; (vector false true false false false) ; (vector false false false false true))) On a 6 x 6 board, things start to get weird -- ;(check-expect (placement 6) (vector ;; fails ; (vector false false false true false false) ; (vector true false false false false false) ; (vector false false false false true false) ; (vector false true false false false false) ; (vector false false false false false true) ; (vector false false true false false false))) Here's what I get instead --- (vector (vector false false false true false false) (vector true false false false false false) (vector false false false false true false) (vector false true false false false false) (vector false false false false true true) (vector false false true false false false)) Compared to the correct solution, this result is off by one vector value, or "queen-position": "true" in 5th column, 5th row. That should be false -- the queen in column 6 should have "flipped" it after being placed. I tested extensively the place and unplace functions, and they seem to work fine. Looking for patterns, I found that if you trace the trajectories of the queens positioned in only vector-column-indexes 0-3, the resulting board is exactly the one I'm left with. As if the queens in vector-column-indexes 4 and 5 haven't been placed, even though the queen positions selected in columns 0-3 are made as though they had! Needless to say, things get progressively worse as the board dimension inputs get larger. All my efforts to debug have been fruitless. I've sat with the Stepper literally for hours trying to trace the origins of this result, but to no avail. I don't understand how I could get the correct solution for 5, but have it blow up at 6, get worse at 7, etc. I also don't understand why the backtracking strategy which succeeded in all cases in Chapter 28 is not working here. I know there are subtle differences, especially in the base case, but it seems the backtracking approach which worked in the first should translate to the second. It's making me a little crazy. I've posted two public ss files to Google docs for anyone who wants to take a look -- "placement-with-local" which is available here -- <a href="https://docs.google.com/leaf?id=0B_ZKElmhzsp-YzY2YzgxOTctYWFjOC00MDUwLWFlMTctYWMwNjk5MzE2MTgy&sort=name&layout=list&num=50">https://docs.google.com/leaf?id=0B_ZKElmhzsp-YzY2YzgxOTctYWFjOC00MDUwLWFlMTctYWMwNjk5MzE2MTgy&sort=name&layout=list&num=50</a> and "TestingFunctions" available here -- <a href="https://docs.google.com/leaf?id=0B_ZKElmhzsp-NDZiZWJiNzgtNjJhZC00NGRjLTlmNjAtNDE3ZTMwYzY4MWJl&sort=name&layout=list&num=50">https://docs.google.com/leaf?id=0B_ZKElmhzsp-NDZiZWJiNzgtNjJhZC00NGRjLTlmNjAtNDE3ZTMwYzY4MWJl&sort=name&layout=list&num=50</a> Placement-with-local contains the complete algorithm, while TestingFunctions contains all my tests for the auxiliary functions. Data definitions, written arguments for the algorithm (and a second alternative), function contracts, purpose statements, etc. are all included. This is a function with a lot of moving parts, so anyone who dives into with me to try and help would be doing me an enormous favor. The only thing I can offer in return is lots of gratitude.... and maybe my grandmother's not-really-secret-but-totally-yummie recipe for Spanish "tortilla patata." As always, any and all suggestions are welcome and appreciated. Many thanks! Dave Yrueta