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List: python-list
Subject: Re: How is max supposed to work, especially key.
From: Peter Otten <__peter__ () web ! de>
Date: 2014-11-27 18:01:21
Message-ID: m57ou2$lom$1 () ger ! gmane ! org
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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.
--
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