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. -- https://mail.python.org/mailman/listinfo/python-list
