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Today's Topics:
1. Re: hanoi 4 problem
(Sumit Sahrawat, Maths & Computing, IIT (BHU))
2. Re: function to read file and return list of contents not
compiling (Sumit Sahrawat, Maths & Computing, IIT (BHU))
3. Re: hanoi 4 problem (Roelof Wobben)
4. Re: function to read file and return list of contents not
compiling (Geoffrey Bays)
5. Re: tower hanoi problem (Doug McIlroy)
----------------------------------------------------------------------
Message: 1
Date: Thu, 19 Feb 2015 22:57:54 +0530
From: "Sumit Sahrawat, Maths & Computing, IIT (BHU)"
<[email protected]>
To: The Haskell-Beginners Mailing List - Discussion of primarily
beginner-level topics related to Haskell <[email protected]>
Subject: Re: [Haskell-beginners] hanoi 4 problem
Message-ID:
<cajbew8n4wosupcehj4hh74htyfovepk2hpgxpaeffckiphe...@mail.gmail.com>
Content-Type: text/plain; charset="utf-8"
What you did recursively is ...
- Move (n - 1) from peg1 to peg3
- Move 1 from peg1 to peg3
- Move (n - 1) from peg1 to peg3
.. which is not what you want. Keep thinking, you will get it.
Some things to consider:
- Distribute the discs on two intermediates.
- Keep track of the order in which discs are assembled on the
intermediates.
- Move 1 disc from peg1 to peg4.
- Then, move the distributed discs back in the reverse order from how
you put them there.
By keeping track, I mean keep track of which peg is filled first.
Hope this helps.
On 19 February 2015 at 19:20, Roelof Wobben <[email protected]> wrote:
> Hello,
>
> I try now to make the optional exercise hanoi with 4 pegs,
>
> So I did on paper this :
>
> start temp1 temp2 end
>
> 3
> 2
> 1
>
> first move : start -> temp2
>
> start temp1 temp2 end
> 2 3
> 1
>
> second move : start -> temp1
>
> start temp1 temp2 end
> 1 2 3
>
> now I tried to make this piece in code
>
> So I did :
>
> hanoi4 1 start _ _ end = "move 1 disk from " ++ [start] ++ " to " ++
> [end] ++ ".\n"
> hanoi4 n start temp1 temp2 end = hanoi4 (n-1) start end temp1 temp2 ++
> hanoi4 1 start end temp1 temp2 ++ hanoi4 (n-1) start temp1 end temp2
>
> because on the first step start must be start and end must be temp1
> and on the second step start must be start and end must be temp2
>
> but when I run in I see this output :
>
> move 1 disk from a to b.
> move 1 disk from a to b.
> move 1 disk from a to b.
> move 1 disk from a to c.
> move 1 disk from a to d.
> move 1 disk from a to d.
> move 1 disk from a to d.
>
> Can anyone explain or let me see where my thinking took the wrong turn ?
>
> Roelof
>
>
>
> _______________________________________________
> Beginners mailing list
> [email protected]
> http://mail.haskell.org/cgi-bin/mailman/listinfo/beginners
>
--
Regards
Sumit Sahrawat
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Message: 2
Date: Thu, 19 Feb 2015 22:59:00 +0530
From: "Sumit Sahrawat, Maths & Computing, IIT (BHU)"
<[email protected]>
To: The Haskell-Beginners Mailing List - Discussion of primarily
beginner-level topics related to Haskell <[email protected]>
Subject: Re: [Haskell-beginners] function to read file and return list
of contents not compiling
Message-ID:
<cajbew8ov_zeaa6fruf1twdtpgatf9rhndkrvjxq02huzuzn...@mail.gmail.com>
Content-Type: text/plain; charset="utf-8"
Thus,
return (lines contents)
will work.
On 19 February 2015 at 22:55, Sumit Sahrawat, Maths & Computing, IIT (BHU) <
[email protected]> wrote:
> The type should be IO [String]. You do IO, and it results in a list of
> Strings.
> Also, take a look at the following types:
>
> contents :: String
> lines :: String -> [String]
>
> Which means that,
>
> lines contents :: [String]
>
> But because you're dealing with a monad (IO in this case), this will not
> typecheck. You can convert a pure value to a monadic value using the return
> function:
>
> return :: Monad m => a -> m a
>
> Not specific to IO, but all monads.
> Hope this helps.
>
> On 19 February 2015 at 22:47, Geoffrey Bays <[email protected]> wrote:
>
>> Haskellers:
>>
>> I want to write a function that takes a class name and reads from a file
>> and then returns the
>> list of String of the file lines. I have experimented with various return
>> types, but GHC does not like any of them. Here is my function:
>>
>> readData :: String -> IO String -- what type here??
>> readData classNameP = do
>> let fileName = classNameP ++ ".txt"
>> contents <- readFile fileName
>> lines contents
>>
>> Not to complain, but this would not be difficult in any other language I
>> have tried, so I could use
>> some explanation. I suspect it has to do with understanding the IO Monad.
>>
>> Many Thanks
>>
>> Geoffrey
>>
>> _______________________________________________
>> Beginners mailing list
>> [email protected]
>> http://mail.haskell.org/cgi-bin/mailman/listinfo/beginners
>>
>>
>
>
> --
> Regards
>
> Sumit Sahrawat
>
--
Regards
Sumit Sahrawat
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Message: 3
Date: Thu, 19 Feb 2015 18:30:53 +0100
From: Roelof Wobben <[email protected]>
To: [email protected], The Haskell-Beginners Mailing
List - Discussion of primarily beginner-level topics related to
Haskell <[email protected]>
Subject: Re: [Haskell-beginners] hanoi 4 problem
Message-ID: <[email protected]>
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Message: 4
Date: Thu, 19 Feb 2015 13:00:08 -0500
From: Geoffrey Bays <[email protected]>
To: [email protected], The Haskell-Beginners Mailing
List - Discussion of primarily beginner-level topics related to
Haskell <[email protected]>
Subject: Re: [Haskell-beginners] function to read file and return list
of contents not compiling
Message-ID:
<cad8ukx6wjqcbpo-o2e32ven3dnmhasjf7yo_tungx1hczh4...@mail.gmail.com>
Content-Type: text/plain; charset="utf-8"
Thanks. I played around some more with return and came up with this which
compiles fine:
readData :: String -> IO [String]
readData classNameP = do
let fileName = classNameP ++ ".txt"
contents <- readFile fileName
return $ lines contents
Geoffrey
On Thu, Feb 19, 2015 at 12:25 PM, Sumit Sahrawat, Maths & Computing, IIT
(BHU) <[email protected]> wrote:
> The type should be IO [String]. You do IO, and it results in a list of
> Strings.
> Also, take a look at the following types:
>
> contents :: String
> lines :: String -> [String]
>
> Which means that,
>
> lines contents :: [String]
>
> But because you're dealing with a monad (IO in this case), this will not
> typecheck. You can convert a pure value to a monadic value using the return
> function:
>
> return :: Monad m => a -> m a
>
> Not specific to IO, but all monads.
> Hope this helps.
>
> On 19 February 2015 at 22:47, Geoffrey Bays <[email protected]> wrote:
>
>> Haskellers:
>>
>> I want to write a function that takes a class name and reads from a file
>> and then returns the
>> list of String of the file lines. I have experimented with various return
>> types, but GHC does not like any of them. Here is my function:
>>
>> readData :: String -> IO String -- what type here??
>> readData classNameP = do
>> let fileName = classNameP ++ ".txt"
>> contents <- readFile fileName
>> lines contents
>>
>> Not to complain, but this would not be difficult in any other language I
>> have tried, so I could use
>> some explanation. I suspect it has to do with understanding the IO Monad.
>>
>> Many Thanks
>>
>> Geoffrey
>>
>> _______________________________________________
>> Beginners mailing list
>> [email protected]
>> http://mail.haskell.org/cgi-bin/mailman/listinfo/beginners
>>
>>
>
>
> --
> Regards
>
> Sumit Sahrawat
>
> _______________________________________________
> Beginners mailing list
> [email protected]
> http://mail.haskell.org/cgi-bin/mailman/listinfo/beginners
>
>
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------------------------------
Message: 5
Date: Thu, 19 Feb 2015 21:27:10 -0500
From: Doug McIlroy <[email protected]>
To: [email protected]
Subject: Re: [Haskell-beginners] tower hanoi problem
Message-ID: <[email protected]>
Content-Type: text/plain; charset=us-ascii
On 2/19/15 4:47 AM, Joel Neely wrote:
> Incidentally, I also find it interesting to see the subtle effects
> that our terminology has on the way we approach problems. Thinking of
> a list as "it may be empty or not" takes my thoughts in a different
> direction than if I think "it may have a head or not".
A telling aphorism. It will stick with me for a long time.
Regarding the problem that sparked this discussion, Hanoi: if you
think first about the base case, it's likely to be this: to transfer
a oone-disc pile from A to B using C as an intermediate, move A to B.
If you think first about the recursive step, you'll likely come
up with a different base case: to transfer zero discs, do nothing.
With this insight, you've generalized your original understanding
of the problem.
Doug
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