99 questions/46 to 50
m (minor syntactical/semantical fixes to solution 46)
m (solution for problem 48)
Revision as of 03:39, 13 December 2006
These are Haskell translations of Ninety Nine Lisp Problems.
If you want to work on one of these, put your name in the block so we know someone's working on it. Then, change n in your block to the appropriate problem number, and fill in the <Problem description>,<example in lisp>,<example in Haskell>,<solution in haskell> and <description of implementation> fields.
1 Logic and Codes
2 Problem 46
Define predicates and/2, or/2, nand/2, nor/2, xor/2, impl/2 and equ/2 (for logical equivalence) which succeed or fail according to the result of their respective operations; e.g. and(A,B) will succeed, if and only if both A and B succeed.
A logical expression in two variables can then be written as in the following example: and(or(A,B),nand(A,B)).
Now, write a predicate table/3 which prints the truth table of a given logical expression in two variables.
Example: (table A B (and A (or A B))) true true true true fail true fail true fail fail fail fail Example in Haskell: > table2 (\a b -> (and' a (or' a b)) True True True True False True False True False False False False
not' :: Bool -> Bool not' True = False not' False = True and',or',nor',nand',xor',impl',equ' :: Bool -> Bool -> Bool and' True True = True and' _ _ = False or' True _ = True or' _ True = True or' _ _ = False nor' a b = not' $ or' a b nand' a b = not' $ and' a b xor' True False = True xor' False True = True xor' _ _ = False impl' a b = (not' a) `or'` b equ' True True = True equ' False False = True equ' _ _ = False table2 :: (Bool -> Bool -> Bool) -> IO () table2 f = putStrLn . unlines $ [show a ++ " " ++ show b ++ " " ++ show (f a b) | a <- [True, False], b <- [True, False]]
The implementations of the logic functions are quite verbose and can be shortened in places (like "equ' = (==)").
The table function in Lisp supposedly uses Lisp's symbol handling to substitute variables on the fly in the expression. I chose passing a binary function instead because parsing an expression would be more verbose in haskell than it is in Lisp. Template Haskell could also be used :)
3 Problem 47
Truth tables for logical expressions (2).
Continue problem P46 by defining and/2, or/2, etc as being operators. This allows to write the logical expression in the more natural way, as in the example: A and (A or not B). Define operator precedence as usual; i.e. as in Java.
Example: * (table A B (A and (A or not B))) true true true true fail true fail true fail fail fail fail Example in Haskell: > table2 (\a b -> a `and'` (a `or'` not b)) True True True True False True False True False False False False
-- functions as in solution 46 infixl 4 `or'` infixl 6 `and'` -- "not" has fixity 9 by default
Java operator precedence (descending) as far as I could fathom it:
logical not equality and xor or
Using "not" as a non-operator is a little evil, but then again these problems were designed for languages other than haskell :)
4 Problem 48
Truth tables for logical expressions (3).
Generalize problem P47 in such a way that the logical expression may contain any number of logical variables. Define table/2 in a way that table(List,Expr) prints the truth table for the expression Expr, which contains the logical variables enumerated in List.
Example: * (table (A,B,C) (A and (B or C) equ A and B or A and C)) true true true true true true fail true true fail true true true fail fail true fail true true true fail true fail true fail fail true true fail fail fail true Example in Haskell: > table3 (\a b c -> a `and'` (b `or'` c) `equ'` a `and'` b `or'` a `and'` c) True True True True True True False True True False True True True False False True False True True True False True False True False False True True False False False True
-- functions as in solution 46 infixl 4 `or'` infixl 4 `nor'` infixl 5 `xor'` infixl 6 `and'` infixl 6 `nand'` infixl 3 `equ'` -- was 7, changing it to 3 got me the same results as in the original question :( table3 :: (Bool -> Bool -> Bool -> Bool) -> IO () table3 f = putStrLn . unlines $ [show a ++ " " ++ show b ++ " " ++ show c ++ " " ++ show (f a b c) | a <- [True, False], b <- [True, False], c <- [True, False]]
Using individual table functions for different numbers of variables is even more ugly, but anything else would be a bit of a pain in haskell AFAIK.
5 Problem 49
An n-bit Gray code is a sequence of n-bit strings constructed according to certain rules. For example,
n = 1: C(1) = ['0','1']. n = 2: C(2) = ['00','01','11','10']. n = 3: C(3) = ['000','001','011','010',´110´,´111´,´101´,´100´].
Find out the construction rules and write a predicate with the following specification:
% gray(N,C) :- C is the N-bit Gray code
Can you apply the method of "result caching" in order to make the predicate more efficient, when it is to be used repeatedly?
Example in Haskell: P49> gray 3 ["000","001","011","010","110","111","101","100"]
gray :: Int -> [String] gray 1 = ["0", "1"] gray (n+1) = let xs = gray n in map ('0':) xs ++ map ('1':) (reverse xs)
It seems that the gray code can be recursively defined in the way that for determining the gray code of n we take the gray code of n-1, prepend a 0 to each word, take the gray code for n-1 again, reverse it and perpend a 1 to each word. At last we have to append these two lists. (Wikipedia seems to approve this.)Instead of the equation for
gray 0 = [""]
what leads to the same results.
6 Problem 50
Example: <example in lisp> Example in Haskell: <example in Haskell>
<solution in haskell>
<description of implementation>