Cannot understand haskell arrays

Question

I cannot understand haskell arrays.

For example I want to create and store an array in variable bsd but what goes in .... if I want an array of size eg 10 and of type Bool.

bsd :: Array Int Bool --is this correct?
bsd = .... --what comes here?

Please help me understand...

and what if I want to change a value in bsd at e.g. index 5 what is the syntax

and how can i access a index in bsd ??

please help

An Array is a pure value. You can't change it. There are operations for creating new arrays representing modifications of old ones, but those are too slow for any non-small arrays. If your arrays really have just ten or so elements, then that's likely fine, but otherwise you might want to use mutable arrays or pure sequences. — dfeuer
– dfeuer, Commented Sep 25, 2017 at 4:09

clay · Accepted Answer · 2017-09-25 02:43:51Z

2

Using https://hackage.haskell.org/package/array-0.5.2.0/docs/Data-Array-IArray.html

This constructs an array of bools from a list. There are tons of other options and functions to use as well

import Data.Array.IArray
let bsd = listArray (0, 3) [False, True, True, False] :: Array Int Bool
elems bsd -- [False,True,True,False]
bsd -- array (0,3) [(0,False),(1,True),(2,True),(3,False)]
bsd ! 0 -- Get element at index 0, which is False
-- Create new array with element 0 changed to True.
let bsd2 = bsd // [(0, True)]
bsd2 -- array (0,3) [(0,True),(1,True),(2,True),(3,False)]

answered Sep 25, 2017 at 2:43

clay

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4 Comments

Jhoy Over a year ago

how can i say get the size of bsd? and how can i assign to say index 2 ?

Stéphane Laurent Over a year ago

@Jhoy bsd // [(2,False)] to assign at index 2 ?

Stéphane Laurent Over a year ago

For the size you could do indices bsd which returns the list of indices and then take the length of this list (maybe there's a better way, I don't know).

clay Over a year ago

bounds bsd gives you min+max index, so you can get length from that. Also MArray or Data.Array.ST are better for scenarios involving lots of mutation.

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