How is max supposed to work, especially key.

[Albert van der Horst]

In article <mailman.16378.1417111312.18130.python-list at python.org>,
Peter Otten  <__peter__ at web.de> wrote:
>Albert van der Horst wrote:
>
>> In the Rosetta code I come across this part of
>> LU-decomposition.
>>
>> def pivotize(m):
>>     """Creates the pivoting matrix for m."""
>>     n = len(m)
>>     ID = [[float(i == j) for i in xrange(n)] for j in xrange(n)]
>>     for j in xrange(n):
>>         row = max(xrange(j, n), key=lambda i: abs(m[i][j]))
>>         if j != row:
>>             ID[j], ID[row] = ID[row], ID[j]
>>     return ID
>>
>> That it's using a cast from boolean to float and using
>> at the other moment a float as a boolean, suggest that this
>> code is a bit too clever for its own good, but anyway.
>>
>> My problem is with the max. I never saw a max with a key.
>>
>> In my python help(max) doesn't explain the key. It says that
>> max can handle an iterator (I didn't know that), and you can
>> pass and optional "key=func", but that's all.
>>
>> I expect it to be something like
>>   elements in the iterator are taken into account only if the
>>   key applied to the iterator evaluates to a True value.
>>
>> However that doesn't pan out:
>> "
>> max(xrange(100,200), key=lambda i: i%17==0 )
>> 102
>> "
>>
>> I expect the maximum number that is divisible by 17 in the
>> range, not the minimum.
>>
>> Can anyone shed light on this?
>
>Given a function f() max(items, key=f) returns the element of the `items`
>sequence with the greatest f(element), e. g. for
>
>max(["a", "bcd", "ef"], key=len)
>
>the values 1, 3, 2 are calculated and the longest string in the list is
>returned:
>
>>>> max(["a", "bcd", "ef"], key=len)
>'bcd'
>
>If there is more than one item with the maximum calculated the first is
>given, so for your attempt
>
>max(xrange(100,200), key=lambda i: i%17==0 )

>
>the values False, False, True, False, ... are calculated and because
>
>>>> True > False
>True
>
>the first one with a True result is returned.
>

So in that case max doesn't return the maximum (True), but instead
something else.

Useful as that function may be, it shouldn't have been called max.

I don't blame myself for being misled.

Groetjes Albert
-- 
Albert van der Horst, UTRECHT,THE NETHERLANDS
Economic growth -- being exponential -- ultimately falters.
albert at spe&ar&c.xs4all.nl &=n http://home.hccnet.nl/a.w.m.van.der.horst