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@shigemk2
Created Jun 10, 2015
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solveRPN :: String -> Double
-- solveRPN expression = head (foldl foldingFunction [] (words expression))
solveRPN = head . foldl foldingFunction [] . words
-- where foldingFunction stack item = ...
where foldingFunction (x:y:ys) "*" = (y * x):ys
foldingFunction (x:y:ys) "+" = (y + x):ys
foldingFunction (x:y:ys) "-" = (y - x):ys
foldingFunction (x:y:ys) "/" = (y / x):ys
foldingFunction (x:y:ys) "^" = (y ** x):ys
foldingFunction (x:xs) "ln" = log x:xs
foldingFunction xs "sum" = [sum xs]
foldingFunction xs numberString = read numberString:xs
main = do
print $ solveRPN "3 2 +"
print $ solveRPN "3 3.5 +"
print $ solveRPN "10 4 3 + 2 * -"
print $ solveRPN "90 34 12 33 55 66 + * - +"
print $ solveRPN "3 3 4 - +"
print $ solveRPN "10 10 10 10 sum 4 /"
print $ solveRPN "10 10 10 10 10 sum 4 /"
print $ solveRPN "10 4 ^"
print $ solveRPN "10000 2 ln"
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