median.py

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Return the median of a list of numbers. By default, assumes the list is unordered and needs sorting (and this default can be over-loaded at the command line).

Whether or not to sort by default sounds like a big issue. However, after several timing studies, I can report that unless lists are large (10,000 items or more), in absolute terms, it makes very little difference to the runtimes (since Python's built in sort functions can sort thousands of numbers in ten-thousands of a second).

Programmer's Guide

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If ordered is False, do not sort lst

def median(lst,ordered=False):
  assert lst,"median needs a non-empty list"
  n  = len(lst)
  p  = q  = n//2
  if n < 3:
    p,q = 0, n-1
  else:
    lst = lst if ordered else sorted(lst)
    if not n % 2: # for even-length lists, use mean of mid 2 nums
      q = p -1
  return lst[p] if p==q else (lst[p]+lst[q])/2

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Copyleft

Permission is hereby granted, free of charge, to any person obtaining a copy of this software and associated documentation files (the "Software"), to deal in the Software without restriction, including without limitation the rights to use, copy, modify, merge, publish, distribute, sublicense, and/or sell copies of the Software, and to permit persons to whom the Software is furnished to do so, subject to the following conditions:

The above copyright notice and this permission notice shall be included in all copies or substantial portions of the Software.

THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY, FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM, OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE SOFTWARE.

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median.py

Programmer's Guide

Copyleft

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