Get max element of a list using foldl or foldr
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. Thanks in advance, JAMB
On Fri, Sep 25, 2015 at 06:18:08PM -0500, jamb@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?
El 2015-09-25 18:57, Francesco Ariis escribió:
On Fri, Sep 25, 2015 at 06:18:08PM -0500, jamb@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? _______________________________________________ Beginners mailing list Beginners@haskell.org http://mail.haskell.org/cgi-bin/mailman/listinfo/beginners
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.
The parens around max are unnecessary:
myMax xs = foldl1 max xs
On Fri, Sep 25, 2015 at 5:13 PM
El 2015-09-25 18:57, Francesco Ariis escribió:
On Fri, Sep 25, 2015 at 06:18:08PM -0500, jamb@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? _______________________________________________ Beginners mailing list Beginners@haskell.org http://mail.haskell.org/cgi-bin/mailman/listinfo/beginners
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.
_______________________________________________ Beginners mailing list Beginners@haskell.org http://mail.haskell.org/cgi-bin/mailman/listinfo/beginners
Hi JAMB, If I just show an answer if defeats the purpose of the exercise, but just use max in a lambda function for the fold. But, you need to consider what the maximum of an empty list is. Since it is hard to say what it may be, may I suggest you return a Maybe value for your function, and don’t forget to cover the case of the empty list []. Andrew
participants (4)
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Andrew Bernard -
Francesco Ariis -
jamb＠hinojosa.com -
Rein Henrichs