# Haskell Quiz/Internal Rate of Return/Solution Dolio

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< Haskell Quiz | Internal Rate of Return(Difference between revisions)

m (category) |
(use until instead of iterate) |
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Line 9: | Line 9: | ||

secant :: (Double -> Double) -> Double -> Double |
secant :: (Double -> Double) -> Double -> Double |
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− | secant f delta = fst . head . dropWhile err . iterate update $ (0,1) |
+ | secant f delta = fst $ until err update (0,1) |

where |
where |
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update (x,y) = (x - (x - y)*(f x)/(f x - f y), x) |
update (x,y) = (x - (x - y)*(f x)/(f x - f y), x) |
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− | err (x,y) = abs (x - y) > delta |
+ | err (x,y) = abs (x - y) < delta |

npv :: Double -> [Double] -> Double |
npv :: Double -> [Double] -> Double |

## Latest revision as of 13:41, 9 February 2008

My solution for this quiz uses the secant method, which is quite easy to implement.

import Data.Function import Numeric import System.Environment secant :: (Double -> Double) -> Double -> Double secant f delta = fst $ until err update (0,1) where update (x,y) = (x - (x - y)*(f x)/(f x - f y), x) err (x,y) = abs (x - y) < delta npv :: Double -> [Double] -> Double npv i = sum . zipWith (\t c -> c / (1 + i)**t) [0..] main = do (s:t) <- getArgs let sig = read s cs = map read t putStrLn . ($"") . showFFloat (Just sig) $ secant (flip npv cs) (0.1^sig)

The resulting program expects the first argument to be the number of digits to be displayed after the decimal point, while the rest are the yearly income. For instance:

./IRR 4 -100 30 35 40 45 0.1709