Wolfgang Maier於 2013年3月13日星期三UTC+8下午6時43分38秒寫道： > Steven D'Aprano <steve+comp.lang.python <at> pearwood.info> writes: > > > > > > > > On Tue, 12 Mar 2013 17:03:08 +0000, Norah Jones wrote: > > > > > > > For example: > > > > a=[-15,-30,-10,1,3,5] > > > > > > > > I want to find a negative and a positive minimum. > > > > > > > > example: negative > > > > print(min(a)) = -30 > > > > > > > > positive > > > > print(min(a)) = 1 > > > > > > Thank you for providing examples, but they don't really cover all the > > > possibilities. For example, if you had: > > > > > > a = [-1, -2, -3, 100, 200, 300] > > > > > > I can see that you consider -3 to be the "negative minimum". Do you > > > consider the "positive minimum" to be 100, or 1? > > > > > > If you expect it to be 100, then the solution is: > > > > > > min([item for item in a if item > 0]) > > > > > > If you expect it to be 1, then the solution is: > > > > > > min([abs(item) for item in a]) > > > > > > which could also be written as: > > > > > > min(map(abs, a)) > > > > > > A third alternative is in Python 3.3: > > > > > > min(a, key=abs) > > > > > > which will return -1. > > > > > > > thinking again about the question, then the min() solutions suggested so far > > certainly do the job and they are easy to understand. > > However, if you need to run the function repeatedly on larger lists, using > min() > > is suboptimal because its performance is an O(n) one. > > It's faster, though less intuitive, to sort your list first, then use bisect > on > > it to find the zero position in it. Two manipulations running at O(log(n)). > > > > compare these two functions: > > > > def with_min(x): > > return (min(n for n in a if n<0), min(n for n in a if n>=0)) > > > > def with_bisect(x): > > b=sorted(x) > > return (b[0] if b[0]<0 else None, b[bisect.bisect_left(b,0)]) > > > > then either time them for small lists or try: > > > > a=range(-10000000,10000000) > > with_min(a) > > with_bisect(a) > > > > of course, the disadvantage is that you create a huge sorted list in memory > and > > that it's less readable. > > > > Best, > > Wolfgang
Sorting numbers of such range M in a list of length N by radix sort is faster but requires more memory. -- http://mail.python.org/mailman/listinfo/python-list
