[Haskell-beginners] Convert bits to bytes
Alec Benzer
alecbenzer at gmail.com
Thu Sep 2 13:30:51 EDT 2010
Whoops, realized bitsToBytes [] = [] wasn't necessary.
On Thu, Sep 2, 2010 at 1:29 PM, Alec Benzer <alecbenzer at gmail.com> wrote:
> Thanks - I thought there would be some api for it but I see it's pretty
> simple to do on your own.
>
> I came up with this (dealing with non-multiple of 8 numbers of bits)
>
> bitsToBytes :: [Bool] -> [Word8]
> bitsToBytes [] = []
> bitsToBytes bits = map bitsToByte (getChunks bits)
> where bitsToByte = foldl' (\byte bit -> byte*2 + if bit then 1 else 0) 0
>
> getChunks :: [Bool] -> [[Bool]]
> getChunks [] = []
> getChunks xs
> | length xs < 8 = getChunks $ take 8 (xs ++ repeat False)
> | otherwise =
> let (these,rest) = splitAt 8 xs
> in these:getChunks rest
>
>
> On Thu, Sep 2, 2010 at 3:18 AM, David Virebayre <
> dav.vire+haskell at gmail.com <dav.vire%2Bhaskell at gmail.com>> wrote:
>
>> I could have been a bit more verbose.
>>
>> The way I see it, since you have an arbitrary long list of 'bits' that
>> you want to convert into bytes, the first thing to do is to group this
>> list into sublists of 8 bits.
>>
>> That's what chunk does: it splits the list at the 8th element, and
>> recursively does it for the rest of the list, until the list is empty.
>>
>> One problem with that is that if the length of the list isn't a
>> multiple of 8, then the last byte might be incorrect.
>>
>> > chunk :: [Bool] -> [[ Bool ]]
>> > chunk [] = []
>> > chunk l = a : chunk b
>> > where (a,b) = splitAt 8 l
>>
>> This one converts a list of 'bits' into a number. The head of the list
>> is assumed to be the most significant bit :
>>
>> > conv1 = foldl' (\a b -> a*2 + if b then 1 else 0) 0
>>
>> if we want the head of the list to be the least significant bit, then
>> you can convert with foldr :
>>
>> > conv1' = foldr (\b a -> a*2 + if b then 1 else 0) 0
>>
>> Now converting the whole list is just a matter converting the whole
>> list in groups, then converting each group :
>>
>> > convlist = map conv1 . chunk
>> >
>> > test = convlist (replicate 8 True ++ replicate 8 False :: [Bool] )
>>
>>
>> David.
>>
>
>
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