Haskell Program to Round a Number to n Decimal Places

In Haskell, we can use round, printf and truncate functions to round a number to n decimal places. In the first example, we are going to use (roundTo n x = (fromInteger $ round $ x * (10^n)) / (10.0^^n)) function and in the second example, we are going to use (roundTo n x = read $ printf ("%." ++ show n ++ "f") x) function.In the third example, we are going to use (roundTo n x = fromIntegral (truncate $ x * 10^n) / 10^n).

Algorithm

• Step 1 ? The roundTo function is defined using round function

• Step 2 ? The program execution will be started from main function. The main() function has whole control of the program. It is written as main = do.

• Step 3 ? The variable named, ?x? is defined that will hold the decimal number value that is to be rounded to n decimal places.

• Step 4 ? The roundTo function is called and the number and up to which decimal place it should be rounded of is passed as argument to it.

• Step 5 ? The resultant rounded number value up to n decimal places is printed to the console, once the function is being called.

Example 1

In this example, Number is rounded to n decimal places using round function.

roundTo :: Int -> Double -> Double
roundTo n x = (fromInteger $ round $ x * (10^n)) / (10.0^^n)

main :: IO ()
main = do
   let x = 3.14159265358979323846264338327950288419716939937510
   let rounded = roundTo 4 x
   putStrLn $ "Original number: " ++ show x
   putStrLn $ "Number rounded to 4 decimal places: " ++ show rounded

Output

[1 of 1] Compiling Main             ( main.hs, main.o )
Linking main ...
Original number: 3.141592653589793
Number rounded to 4 decimal places: 3.1416

Example 2

In this example, Number is rounded to n decimal places using printf function.

import Text.Printf (printf)

roundTo :: Int -> Double -> Double
roundTo n x = read $ printf ("%." ++ show n ++ "f") x

main :: IO ()
main = do
   let x = 3.14159265358979323846264338327950288419716939937510
   let rounded = roundTo 4 x
   putStrLn $ "Original number: " ++ show x
   putStrLn $ "Number rounded to 4 decimal places: " ++ show rounded

Output

[1 of 1] Compiling Main             ( main.hs, main.o )
Linking main ...
Original number: 3.141592653589793
Number rounded to 4 decimal places: 3.1416

Example 3

In this example, Number is rounded to n decimal places using truncate function.

roundTo :: Int -> Double -> Double
roundTo n x = fromIntegral (truncate $ x * 10^n) / 10^n

main :: IO ()
main = do
   let x = 3.14159265358979323846264338327950288419716939937510
   let rounded = roundTo 4 x
   putStrLn $ "Original number: " ++ show x
   putStrLn $ "Number rounded to 4 decimal places: " ++ show rounded

Output

[1 of 1] Compiling Main             ( main.hs, main.o )
Linking main ...
Original number: 3.141592653589793
Number rounded to 4 decimal places: 3.1415

Conclusion

Rounding a number to n decimal places in Haskell means that you want to round a given number to a specific number of decimal places, where n is the number of decimal places you want to round to. We can implement this by using round, printf and truncate function.