CODEWITHSUNDEEP
listhaskelllist-comprehension
Pigna
Pigna

Reputation: 2924

list of n zeros in Haskell

This is probably very easy, but I cannot figure out how to do the equivalent of Python's

[0]*n

in Haskell, in order to get a list with n zeros.

[0]*n

doesn't work. Am I obliged to do something like: [0 | x <-[1..5]] ?

Upvotes: 4

Views: 6929

Answers (3)

Ingo
Ingo

Reputation: 36349

Another possibility:

import Data.Monoid

zeroes = 5 * [0] where (*) = mtimes

Upvotes: 0

Sibi
Sibi

Reputation: 48746

You can do this:

λ> take 5 (repeat 0)
[0,0,0,0,0]

Or as @obadz points out, this is even more concise:

λ> replicate 5 0
[0,0,0,0,0]

I personally don't like the python syntax. * means multiplication but it does something else in your case. But that's just my opinion :).

Upvotes: 10

karakfa
karakfa

Reputation: 67507

you can define it yourself (as a toy example)

(*) :: [a] -> Int -> [a]                     
(*) xs n = take n $ cycle xs

so you can have

Prelude> [0]*5                                                      
[0,0,0,0,0]

Prelude> [0,1]*5
[0,1,0,1,0]

obviously, by redefining existing operators have "side effects"

Prelude> 4*4

<interactive>:169:1:
    Non type-variable argument in the constraint: Num [a]
    (Use FlexibleContexts to permit this)
    When checking that `it' has the inferred type
      it :: forall a. Num [a] => [a]

or perhaps python interpretation is replicate the list n times (instead of picking n elements from the cycled list elements). In that case you can define it as

(*) :: [a] -> Int -> [a]                                                                                 
(*) xs n = concat $ replicate n xs                                                                       


Prelude> [0]*4                                                                                                        
[0,0,0,0]

Prelude> [0,1]*4                                                                                                      
[0,1,0,1,0,1,0,1]

obviously, this is a toy example and you may want to define another operator or simply a function for this.

Upvotes: -1

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