"Natural" use of cmp= in sort

Paddy paddy3118 at gmail.com
Mon Nov 10 19:45:00 CET 2014


Hi, I do agree with                                                                          Raymond H. about the relative merits of cmp= and key= in sort/sorted, but I decided to also not let natural uses of cmp= pass silently.

In answering this question, http://stackoverflow.com/a/26850434/10562 about ordering subject to inequalities it seemed natural to use the cmp= argument of sort rather than key=.

The question is about merging given inequalities to make 1 inequality such that the inequalities also stays true.


Here is a copy of my code:

Python 2.7.5 (default, May 15 2013, 22:43:36) [MSC v.1500 32 bit (Intel)] on win32
Type "copyright", "credits" or "license()" for more information.
>>> ineq = """f4 > f2 > f3
f4 > f1 > f3
f4 > f2 > f1
f2 > f1 > f3"""
>>> print(ineq)
f4 > f2 > f3
f4 > f1 > f3
f4 > f2 > f1
f2 > f1 > f3
>>> greater_thans, all_f = set(), set()
>>> for line in ineq.split('\n'):
....tokens = line.strip().split()[::2]
....for n, t1 in enumerate(tokens[:-1]):
........for t2 in tokens[n+1:]:
............greater_thans.add((t1, t2))
............all_f.add(t1)
........all_f.add(t2)


>>> sorted(all_f, cmp=lambda t1, t2: 0 if t1==t2 else 
...........(1 if (t1, t2) not in greater_thans else -1))
['f4', 'f2', 'f1', 'f3']
>>> 




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