langref.org - haskell

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Numbers

Find the distance between two points

haskell

data Floating n => Point2 n = Point2 n n

distance :: Floating n => Point2 n -> Point2 n -> n
distance (Point2 x1 y1) (Point2 x2 y2) = sqrt (x'*x' + y'*y')
where
x' = x1 - x2
y' = y1 - y2

-- > distance (Point2 5 10) (Point2 3 5)
-- 5.385...
-- > distance (Point2 1 1) (Point2 2 2)
-- 1.414...

Output

Zero pad a number

Given the number 42, pad it to 8 characters like 00000042

haskell

import Text.Printf

printf "%08d" 42

Right Space pad a number

Given the number 1024 right pad it to 6 characters "1024 "

haskell

let s = show 1024
p = 6
in s ++ (replicate (p - length s) ' ')

import Text.Printf

main = do
putStrLn $ printf "%-6d" (1024::Int)

Format a decimal number

Format the number 7/8 as a decimal with 2 places: 0.88

haskell

import Text.Printf

printf "%3.2f" (7/8)

main = putStrLn $ Numeric.showFFloat (Just 2) (7/8) ""

Left Space pad a number

Given the number 73 left pad it to 10 characters " 73"

haskell

import Text.Printf

formatted :: String
formatted = printf "%10d" 73

Random

Generate a random integer in a given range

Produce a random integer between 100 and 200 inclusive

haskell

import System.Random

randInRange :: Int -> Int -> IO Int
randInRange a b = getStdRandom $ randomR (a, b)

main = randInRange 100 200 >>= print

import System.Random

main = randomRIO (1,100) >>= print

Generate a repeatable random number sequence

Initialise a random number generator with a seed and generate five decimal values. Reset the seed and produce the same values.

haskell

import System.Random
import Control.Monad (forM_)

main = do
printRands
printRands

where printRands = forM_ [1..5] (\i -> print (randInt i))
randInt i = fst $ randomR (100, 200) (mkStdGen i) :: Int

import System.Random

gen1 = mkStdGen 12345
gen2 = mkStdGen 12345

main = do
print $ take 5 (randoms gen1 :: [Float])
print $ take 5 (randoms gen2 :: [Float])