Re: [Haskell-cafe] List all multiply/add combinations

It might be rare that a "real world" problem can be formulated as such a simple mathematical challenge, so I can't blame you for thinking about home work. I did too. Actually it's part of a logic puzzle I'm implementing. Attacking the problem textually, I can treat the list of infix operators as char, and insert parentheses in all possible configurations. Then remove the illegal and unnecessary one. To get the result, I then need to parse the string. That seems slow, and awkward. I need to do hundreds of thousands of these calculations. I could then work directly with parsing trees, and generate all binary trees of fixed lengths. But most of them would be unnecessary, so it seems like I'm attacking it from the wrong angle. I get that using permutations you can get rid of the need for parentheses, but you also get a lot of impossible calculations (like a*c+b in the example before). It's also not straight forward to go back to the original representation with parentheses. But maybe I'm missing something obvious here? On Sat, Nov 17, 2012 at 4:18 PM, wrote:

This smells like homework to me, which isn't a bad thing, it will however change the way I answer you.

Please look at http://hackage.haskell.org/packages/archive/base/latest/doc/html/Data-List.h... and show us your attempts to use this function.

Timothy

---------- Původní zpráva ---------- Od: Rune Harder Bak Datum: 17. 11. 2012 Předmět: [Haskell-cafe] List all multiply/add combinations

Given a list of numbers of fixed length I need to list all possible values (and the associated computation) you get by inserting +,-,*,/ between the numbers, and also set parentheses where you please. It shouldn't list computations with unnecessary parentheses. Example list of length 3 [a,b,c] and only with + and *: a*b+c, a*(b+c),a*b*c,a+b*c,(a+b)*c,a+b+c

What would be a good way to do this, and a good representation in Haskell?

Best, Rune

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