Maximum value in a list

[sashan]

Hi

I was reading through the tutorial Yet Another Haskell Tutorial and
doing chapter 3 ex 5, write a function using foldr and max to return the
maximum value in a list.

I came up with the following that will work for positive numbers.

maxInList :: [Int]->Int
maxInList [] = 0
maxInList l = foldr max 0 l

Is there an identity for the max function? Because currently if the list
[-1,-3,-4] is passed to maxInList it will return 0.

[Liyang HU]

Post by sashan
I came up with the following that will work for positive numbers.
maxInList :: [Int]->Int
maxInList [] = 0
maxInList l = foldr max 0 l

Given that you've already taken care of the empty case, (and assuming you
think the maximum of an empty list is indeed zero) can't you just use foldr1
instead of foldr? (foldr1 f l is equivalent to foldr f (head l) (tail l))

Have fun in your adventures with Haskell, :)
/Liyang

[sashan]

Post by Liyang HU
Given that you've already taken care of the empty case, (and assuming you
think the maximum of an empty list is indeed zero) can't you just use foldr1
instead of foldr? (foldr1 f l is equivalent to foldr f (head l) (tail l))

Thanks. Didn't know about foldr1 but should've thought of the definition.

Post by Liyang HU
Have fun in your adventures with Haskell, :)
/Liyang

Yeah...I thought I'd try it again after doing a basic course on it a few
years ago and liking the language. Now I want to do something practical
with it.

[Garner, Robin]

You'll find in the Prelude that Int is an instance of Bounded, so minBound
and maxBound are defined, and are the identities you need.

cheers

-----Original Message-----
From: sashan [mailto:***@ihug.co.nz]
Sent: Monday, May 05, 2003 12:30 PM
To: haskell-***@haskell.org
Subject: Maximum value in a list


Hi

I was reading through the tutorial Yet Another Haskell Tutorial and
doing chapter 3 ex 5, write a function using foldr and max to return the
maximum value in a list.

I came up with the following that will work for positive numbers.

maxInList :: [Int]->Int
maxInList [] = 0
maxInList l = foldr max 0 l

Is there an identity for the max function? Because currently if the list
[-1,-3,-4] is passed to maxInList it will return 0.

[Hamilton Richards]

Post by sashan
Hi
I was reading through the tutorial Yet Another Haskell Tutorial and
doing chapter 3 ex 5, write a function using foldr and max to return
the maximum value in a list.
I came up with the following that will work for positive numbers.
maxInList :: [Int]->Int
maxInList [] = 0
maxInList l = foldr max 0 l
Is there an identity for the max function? Because currently if the
list [-1,-3,-4] is passed to maxInList it will return 0.
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No, max has no identity, and so maxInList has no correct definition
for an empty list. The shortest lists for which it is defined are
lists of one element.

The Standard Prelude contains a close relative of foldr called
foldr1. It's defined for just this situation.

--HR

[Dean Herington]

Post by sashan
Hi
I was reading through the tutorial Yet Another Haskell Tutorial and
doing chapter 3 ex 5, write a function using foldr and max to return the
maximum value in a list.
I came up with the following that will work for positive numbers.
maxInList :: [Int]->Int
maxInList [] = 0
maxInList l = foldr max 0 l
Is there an identity for the max function? Because currently if the list
[-1,-3,-4] is passed to maxInList it will return 0.

Hint: Find a better value than `0` to give as the second argument to
`foldr`.

Dean