Converting number base

Question

Is there a platform function that will do the following?

convertBase :: (Num a, Num b) => Int -> Int -> [a] -> [b]

Convert a number from base 'a' to base 'b' where each list item is a digit in the number. for example:

convertBase 2 10 [1,1,0,1] = [1, 3]

I hope that makes sense, let me know if i can clear anything up

by platform function you mean part of the standard libraries? — j13r, Apr 05 '12 at 12:24
Why distinguish the types `a` and `b`, aren't they just some `Integral`? — Frerich Raabe, Apr 05 '12 at 12:49

score 15 · Accepted Answer · answered Apr 05 '12 at 12:45

15

Using the digits package from Hackage:

import Data.Digits (digits, unDigits)

convertBase :: Integral a => a -> a -> [a] -> [a]
convertBase from to = digits to . unDigits from

You can add a fromIntegral in there if you need the input and output types to be different. Also, the Integral constraint makes more sense than Num, since you probably don't want to deal with complex or floating-point digits.

answered Apr 05 '12 at 12:45

hammar

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The resulting function accepts a list of integrals. If I create a `decToBin = convertBase 10 2`, what does it mean to use `decToBin [10,10]`? – CMCDragonkai May 24 '15 at 10:53
Oh I found out that `decToBin [10, 10]` is the same as `decToBin [1010]`. It just concatenates all the numbers in the list. – CMCDragonkai May 24 '15 at 10:56

score 8 · Answer 2 · edited Jun 28 '18 at 18:23

8

The closest thing in the haskell platform is from module Numeric:

readInt :: Num a => a -> (Char -> Bool) -> (Char -> Int) -> ReadS a
showIntAtBase :: Integral a => a -> (Int -> Char) -> a -> ShowS

fromBase :: Int -> String -> Int
fromBase base = fst . head . readInt base ((<base).digitToInt) digitToInt

toBase :: Int -> Int -> String
toBase base num = showIntAtBase base intToDigit num ""

fromBaseToBase :: Int -> Int -> String -> String
fromBaseToBase from to = toBase to . fromBase from

edited Jun 28 '18 at 18:23

APerson

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answered Apr 05 '12 at 12:49

Riccardo T.

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Which is easily extensible if you provide your own "intToDigit" function supporting bases > 16 – Francesquini Nov 16 '16 at 14:34

score 2 · Answer 3 · answered Apr 05 '12 at 12:31

Couple of ideas:

use showIntAtBase or Text.printf to convert to a string, and convert back to a different base
write it yourself -- easier when one base is always a multiple of the other

Here is a link that might help you: http://rosettacode.org/wiki/Non-decimal_radices/Convert#Haskell -- Non-decimal radices/Convert

Converting number base

3 Answers3

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