I have a list like this:
[[1,0,0,0,1,1,1,0,0],[1,1,1,0,1,1,1,0,0],[1,0,1,0,0,1,1,0,0],[0,1,0,0,1,1,1,0,0],[0,0,1,0,1,1,1,0,0],[1,0,0,1,1,1,1,0,1]] The 'inner' list is a list of 9 items. I want to process the list so that a list of lists is returned but the 9th element in each inner list is dropped. So the function type would be [[a]] -> [[a]] So to get started I wrote a function like this: discardparitybyte :: [[Bit]] -> [[Bit]] But then not sure how to transform the inner list. I know I can access the first inner element using take 1 list. But how do I then access/manipulate this inner list? discardparitybyte (x:xs) = take 9 ??? (take 1 xs) : discardparitybyte ??? -------------- next part -------------- An HTML attachment was scrubbed... URL: <http://www.haskell.org/pipermail/beginners/attachments/20140102/02343f1f/attachment.html> |
Hi Angus,
You map the drop function over the list. Something like this will do foo xs = map (take 8) xs On Thu, Jan 2, 2014 at 5:08 PM, Angus Comber <anguscomber at gmail.com> wrote: > I have a list like this: > > > [[1,0,0,0,1,1,1,0,0],[1,1,1,0,1,1,1,0,0],[1,0,1,0,0,1,1,0,0],[0,1,0,0,1,1,1,0,0],[0,0,1,0,1,1,1,0,0],[1,0,0,1,1,1,1,0,1]] > > The 'inner' list is a list of 9 items. I want to process the list so that > a list of lists is returned but the 9th element in each inner list is > dropped. > > So the function type would be [[a]] -> [[a]] > > So to get started I wrote a function like this: > > discardparitybyte :: [[Bit]] -> [[Bit]] > > But then not sure how to transform the inner list. > > I know I can access the first inner element using take 1 list. But how do > I then access/manipulate this inner list? > > discardparitybyte (x:xs) = take 9 ??? (take 1 xs) : discardparitybyte ??? > > _______________________________________________ > Beginners mailing list > Beginners at haskell.org > http://www.haskell.org/mailman/listinfo/beginners > > An HTML attachment was scrubbed... URL: <http://www.haskell.org/pipermail/beginners/attachments/20140102/18669ed8/attachment.html> |
In reply to this post by acomber
It's usually much simpler and better design to think about the simple case
first and then piece things together. So start with just modifying one inner list: discardparitybyte :: [Bit] -> [Bit] discardparitybyte xs = take 9 xs When you're happy that this works, you can apply it to all the elements of your data list: discardallparitybytes :: [[Bit]] -> [[Bit]] discardallparitybytes xs = map discardparitybye xs And, if you want to, you can make it shorted by making the functions point-free: discardparitybyte = take 9 discardallparitybytes = map discardparitybye Peter On 2 January 2014 11:38, Angus Comber <anguscomber at gmail.com> wrote: > I have a list like this: > > > [[1,0,0,0,1,1,1,0,0],[1,1,1,0,1,1,1,0,0],[1,0,1,0,0,1,1,0,0],[0,1,0,0,1,1,1,0,0],[0,0,1,0,1,1,1,0,0],[1,0,0,1,1,1,1,0,1]] > > The 'inner' list is a list of 9 items. I want to process the list so that > a list of lists is returned but the 9th element in each inner list is > dropped. > > So the function type would be [[a]] -> [[a]] > > So to get started I wrote a function like this: > > discardparitybyte :: [[Bit]] -> [[Bit]] > > But then not sure how to transform the inner list. > > I know I can access the first inner element using take 1 list. But how do > I then access/manipulate this inner list? > > discardparitybyte (x:xs) = take 9 ??? (take 1 xs) : discardparitybyte ??? > > _______________________________________________ > Beginners mailing list > Beginners at haskell.org > http://www.haskell.org/mailman/listinfo/beginners > > An HTML attachment was scrubbed... URL: <http://www.haskell.org/pipermail/beginners/attachments/20140102/c2a905a4/attachment.html> |
In reply to this post by acomber
The operation you are trying to is abstracted in Haskell using the
higher-order function *map. *Commonly, we want to take a list or other sequence of data and perform an operation (function) on each element of the list. In non-functional languages, we would need to explicitly write a for-loop through each element (ie for (int i = 0; i < array.length; i++) { array[i] = doSomething(array[i]); } Of course in Haskell and other functional languages use of mutable state and explicit iteration aren't used in most circumstances so new functional programmers go "Iteration is the same as recursion so I'll write a recursive function instead something like: recursiveIteration :: [a] -> [a] recursiveIteration [] = [] recursiveIteration x:xs = $$ Lots of operations using x $$ : recursiveIteration xs Of course, the lazy intermediate functional programmer after writing this function a few times realizes that "I'm using a functional language, why not abstract the operations using x using a function and pass the function as a parameter. Something like this: recursiveIterationAbstractOperations :: (a -> b) -> [a] -> [b] recursiveIterationsAbstractOperations _ [] = [] recursiveIterationsAbstractOperations func x:xs = func x : recursiveIterationsAbstractOperations func xs This is exactly the definition of *map* in the GHC source code<http://hackage.haskell.org/package/base-4.6.0.1/docs/src/GHC-Base.html#map> ! I would write it as a map over the outer list and then process each inner element. Something like this: discardparitybyte :: [[Bit]] -> [[Bit]] discardparitybyte = map (take 8) Learning to use higher-order functions like map, foldl, and filter making functional programming great. In my mind as an intermediate Haskell programmer, when I start using direct recursion especially when using lists, I stop myself and think about a better way to structure my code so that I can write it as a combination of these and other higher order functions because they allow for more modulatrity in the code. -David On Thu, Jan 2, 2014 at 10:38 PM, Angus Comber <anguscomber at gmail.com> wrote: > I have a list like this: > > > [[1,0,0,0,1,1,1,0,0],[1,1,1,0,1,1,1,0,0],[1,0,1,0,0,1,1,0,0],[0,1,0,0,1,1,1,0,0],[0,0,1,0,1,1,1,0,0],[1,0,0,1,1,1,1,0,1]] > > The 'inner' list is a list of 9 items. I want to process the list so that > a list of lists is returned but the 9th element in each inner list is > dropped. > > So the function type would be [[a]] -> [[a]] > > So to get started I wrote a function like this: > > discardparitybyte :: [[Bit]] -> [[Bit]] > > But then not sure how to transform the inner list. > > I know I can access the first inner element using take 1 list. But how do > I then access/manipulate this inner list? > > discardparitybyte (x:xs) = take 9 ??? (take 1 xs) : discardparitybyte ??? > > _______________________________________________ > Beginners mailing list > Beginners at haskell.org > http://www.haskell.org/mailman/listinfo/beginners > > An HTML attachment was scrubbed... URL: <http://www.haskell.org/pipermail/beginners/attachments/20140102/da2a8e98/attachment.html> |
Perhaps a glance at SICP will be helpful. As David and Peter have rightly
pointed out, higher order functions make it easier to abstract common patterns. A glance at the SICP chapter on this will be useful, me thinks. It helped me a lot. <a href="http://mitpress.mit.edu/sicp/full-text/book/book-Z-H-12.html#%_sec_1.3">http://mitpress.mit.edu/sicp/full-text/book/book-Z-H-12.html#%_sec_1.3 On Thu, Jan 2, 2014 at 5:44 PM, David Flicker <dtflicker at gmail.com> wrote: > The operation you are trying to is abstracted in Haskell using the > higher-order function *map. *Commonly, we want to take a list or other > sequence of data and perform an operation (function) on each element of the > list. In non-functional languages, we would need to explicitly write a > for-loop through each element (ie > for (int i = 0; i < array.length; i++) { > array[i] = doSomething(array[i]); > } > > Of course in Haskell and other functional languages use of mutable state > and explicit iteration aren't used in most circumstances so new functional > programmers go "Iteration is the same as recursion so I'll write a > recursive function instead something like: > recursiveIteration :: [a] -> [a] > recursiveIteration [] = [] > recursiveIteration x:xs = $$ Lots of operations using x $$ : > recursiveIteration xs > > Of course, the lazy intermediate functional programmer after writing this > function a few times realizes that "I'm using a functional language, why > not abstract the operations using x using a function and pass the function > as a parameter. Something like this: > recursiveIterationAbstractOperations :: (a -> b) -> [a] -> [b] > recursiveIterationsAbstractOperations _ [] = [] > recursiveIterationsAbstractOperations func x:xs = func x : > recursiveIterationsAbstractOperations func xs > > This is exactly the definition of *map* in the GHC source code<http://hackage.haskell.org/package/base-4.6.0.1/docs/src/GHC-Base.html#map> > ! > > I would write it as a map over the outer list and then process each inner > element. Something like this: > > discardparitybyte :: [[Bit]] -> [[Bit]] > discardparitybyte = map (take 8) > > Learning to use higher-order functions like map, foldl, and filter making > functional programming great. In my mind as an intermediate Haskell > programmer, when I start using direct recursion especially when using > lists, I stop myself and think about a better way to structure my code so > that I can write it as a combination of these and other higher order > functions because they allow for more modulatrity in the code. > > -David > > > On Thu, Jan 2, 2014 at 10:38 PM, Angus Comber <anguscomber at gmail.com>wrote: > >> I have a list like this: >> >> >> [[1,0,0,0,1,1,1,0,0],[1,1,1,0,1,1,1,0,0],[1,0,1,0,0,1,1,0,0],[0,1,0,0,1,1,1,0,0],[0,0,1,0,1,1,1,0,0],[1,0,0,1,1,1,1,0,1]] >> >> The 'inner' list is a list of 9 items. I want to process the list so >> that a list of lists is returned but the 9th element in each inner list is >> dropped. >> >> So the function type would be [[a]] -> [[a]] >> >> So to get started I wrote a function like this: >> >> discardparitybyte :: [[Bit]] -> [[Bit]] >> >> But then not sure how to transform the inner list. >> >> I know I can access the first inner element using take 1 list. But how >> do I then access/manipulate this inner list? >> >> discardparitybyte (x:xs) = take 9 ??? (take 1 xs) : discardparitybyte ??? >> >> _______________________________________________ >> Beginners mailing list >> Beginners at haskell.org >> http://www.haskell.org/mailman/listinfo/beginners >> >> > > _______________________________________________ > Beginners mailing list > Beginners at haskell.org > http://www.haskell.org/mailman/listinfo/beginners > > An HTML attachment was scrubbed... URL: <http://www.haskell.org/pipermail/beginners/attachments/20140102/c24f9639/attachment-0001.html> |
In reply to this post by acomber
Hi Angus! Take a look at the following function:
discardParityByte :: [[Integer]] -> [[Integer]] discardParityByte = map init It returns a list constructed by applying "init" to all items in the list you pass in. "init" accepts a list and returns the list without its last item. Stavros Mekesis. Quoting Angus Comber <anguscomber at gmail.com>: > I have a list like this: > > [[1,0,0,0,1,1,1,0,0],[1,1,1,0,1,1,1,0,0],[1,0,1,0,0,1,1,0,0],[0,1,0,0,1,1,1,0,0],[0,0,1,0,1,1,1,0,0],[1,0,0,1,1,1,1,0,1]] > > The 'inner' list is a list of 9 items. I want to process the list so that > a list of lists is returned but the 9th element in each inner list is > dropped. > > So the function type would be [[a]] -> [[a]] > > So to get started I wrote a function like this: > > discardparitybyte :: [[Bit]] -> [[Bit]] > > But then not sure how to transform the inner list. > > I know I can access the first inner element using take 1 list. But how do > I then access/manipulate this inner list? > > discardparitybyte (x:xs) = take 9 ??? (take 1 xs) : discardparitybyte ??? |
I like that :)
On 2 January 2014 15:31, <smekesis at csd.auth.gr> wrote: > Hi Angus! Take a look at the following function: > > discardParityByte :: [[Integer]] -> [[Integer]] > discardParityByte = map init > > It returns a list constructed by applying "init" to all items in the list > you pass in. "init" accepts a list and returns the list without its last > item. > > Stavros Mekesis. > > > > Quoting Angus Comber <anguscomber at gmail.com>: > > I have a list like this: >> >> [[1,0,0,0,1,1,1,0,0],[1,1,1,0,1,1,1,0,0],[1,0,1,0,0,1,1,0,0] >> ,[0,1,0,0,1,1,1,0,0],[0,0,1,0,1,1,1,0,0],[1,0,0,1,1,1,1,0,1]] >> >> The 'inner' list is a list of 9 items. I want to process the list so that >> a list of lists is returned but the 9th element in each inner list is >> dropped. >> >> So the function type would be [[a]] -> [[a]] >> >> So to get started I wrote a function like this: >> >> discardparitybyte :: [[Bit]] -> [[Bit]] >> >> But then not sure how to transform the inner list. >> >> I know I can access the first inner element using take 1 list. But how do >> I then access/manipulate this inner list? >> >> discardparitybyte (x:xs) = take 9 ??? (take 1 xs) : discardparitybyte ??? >> > > > > _______________________________________________ > Beginners mailing list > Beginners at haskell.org > http://www.haskell.org/mailman/listinfo/beginners > An HTML attachment was scrubbed... URL: <http://www.haskell.org/pipermail/beginners/attachments/20140102/1ef8889b/attachment.html> |
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