[Haskell-beginners] Get max element of a list using foldl or foldr

[Rein Henrichs]

The parens around max are unnecessary:

myMax xs = foldl1 max xs

On Fri, Sep 25, 2015 at 5:13 PM <jamb at hinojosa.com> wrote:

> El 2015-09-25 18:57, Francesco Ariis escribió:
> > On Fri, Sep 25, 2015 at 06:18:08PM -0500, jamb at hinojosa.com wrote:
> >> Hello,
> >>
> >> I am a complete Haskell beginner and I am doing some exercises of my
> >> book
> >> but I am stuck with the following:
> >>
> >> Define
> >> myMax :: Ord a => [a] -> a
> >> which returns the maximum element of a list.
> >>
> >> I must use foldl1 or foldr1 and I am given the hint to use max which
> >> gets
> >> the maximum of 2 elements.
> >>
> >> I will very much appreciate if you help me solve it.
> >
> > Let's say you have
> >
> >     foldl1 f [1,7,2,5]
> >
> > where f is a binary operator (a function that 'takes two parameters').
> > `foldl1` will apply 1 and 7 to f, obtaining X, so we have:
> >
> >     X and [2,5]
> >
> > then it will apply X and 2 to f, obtaining Y, so we're left with
> >
> >     Y and 5
> >
> > and finally `f Y 5`, leading to your final result Z.
> >
> > Now, if `f a b = a + b`, we would have
> >
> >        [1,7,2,5]  -- 1+7
> >     8  [2,5]      -- 8+2
> >     10 [5]        -- 10+5
> >     15 <-- final result
> >
> > But you aren't interested in the sum of the list, but its maximum.
> > Which operation could you use instead of (+) to achieve your goal?
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>
>
>
>
>
> I´ve got it!
> I tried the following successfully:
>
> myMax [] = error "List is empty."
> myMax xs = foldl1 (max) xs
>
> I see that my problem was in understanding the way max was applied to a
> list.
>
>
> Thanks for your kind support.
>
>
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