On Nov 22, 2010, at 1:17 PM, Ken Hegeland wrote: > The problem is 27.5.4 can be found at > http://htdp.org/2003-09-26/Book/curriculum-Z-H-34.html#node_sec_27.5 > > I am still on the beginning of this extended exercise and I am entirely lost. > I know how it works but can't get anything even remotely close. I was able to > create subtract from 27.5.2, but triangulate eludes me after a few days of > thinking. My first thought is to make something that takes either a > (listof(listof numbers)), or 3 (listof numbers), and have it output the first > part of triangulation. > > So for the lolon of '((2 2 3 10)(2 5 12 31)(4 1 -2 1)) I want to produce an > answer of > '((2 2 3 10)(3 9 21)(-3 -8 -19)) > > I feel like applying subtract to first and second in lolon produces (3 9 21), > but I am not sure how to make it continue on the rest of the list. I tried > (subtract2(subtract(first lolon)(second lolon))), which really doesn't work > and it seems obvious to me just from glancing at it. > > I am thinking back to how I did merge sort, create a function that merges 2 > lists, apply it to the first and second items in a lolon, then recursively > apply it to the rest of the list. > > In subtract2 my final goal is to apply subtract to the first and second item, > then recursively apply it to first and rest of the list, but I can't figure > out how to get this. > > I know the real question has to do with applying this over and over to any > lists of equal length, until you drop the first position to 0, and maybe I am > heading in the wrong direction, but it seems easier to break it into smaller > problems. I really want to complete this problem before moving on with the > book but I can't find anything that works, everything I think of causes > infinite loops and freezes my computer for 10 minutes.
10 minutes!? Are you familiar with control-k / command-k shortcut for halting a computation? All the best, John Clements
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