[Haskell-beginners] problem exercise 3 page 60 Programming in Haskell

[Roelof Wobben]

Hello, 

Some one else give me the answer but I still don't get it.
The answer is : concat [[(x,y) |  y <- [4,5,6]] | x <- [1..3]]

But the exercise says you have to use one generator and I see still two. 

Roelof


> Subject: Re: [Haskell-beginners] problem exercise 3 page 60 Programming in Haskell
> From: ds at iai.uni-bonn.de
> To: rwobben at hotmail.com
> CC: beginners at haskell.org
> Date: Fri, 22 Jul 2011 19:22:28 +0200
> 
> Hi Roelof,
> 
> I don't have a book, so I don't know the exercise exactly, but if I'm
> correct, you want the list 
> 
> [(1,4), (1,5), (1,6), (2,4), (2,5), (2,6), (3,4), (3,5), (3,6)]
> 
> So the basic idea is to nest one list comprehension into another:
> 
> You create a list [(x,4), (x,5), (x,6)] by the inner list comprehension
> with  y <- [4..6] as generator.
> It should be [(x,y) | y <- [4..6]], where x is just free.
> 
> The whole comprehension you stick as result into an outer comprehension
> whos generator creates the values for x (ie. 1,2 and 3).
> 
> This will return the list of lists
> [[(1,4), (1,5), (1,6)],[(2,4), (2,5), (2,6)], [(3,4), (3,5), (3,6)]]
> which you can flatten by concat.
> 
> I hope you manage the solution with the description above. I think it
> helps you more if I don't write the plain solution as working code.
> 
> 
> Cheers,
> 
> Daniel.
> 
>  
> 
> On Fri, 2011-07-22 at 16:48 +0000, Roelof Wobben wrote:
> > Hello, 
> > 
> > I don't see the answer here.
> > If I brake down the problem.
> > 
> > We have this : 
> > 
> > [(x,y) | x <- [1..3], y <- [4,5,6]]
> > 
> > I have to use one generator with two nested list compreshession and
> > make use of concat.
> > 
> > So the x generator  is [x | x <- [1..3]] 
> > and the Y genarator is [y| y <- [4..6]]
> > 
> > So if I put it together it uses the numbers [1..6] so I could do
> > something like this  in pseudo code as guard.
> > If generator smaller or equal 3 then its x else it's a y.
> > 
> > But if I do concat [x,y] then I get [1..6] and that not good.
> > If I use zip [x,y] then I get [ (1,4) (2,5) (3,6)] which is also not
> > good but better.
> > 
> > So im stuck now and im puzzeling the whole afternoon about this ?
> > 
> > Anyone who can give me a hint ?
> > 
> > Roelof
> > 
> > 
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> 
 		 	   		  
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