How to sort a list? NOT a newbie question.

[Grant Griffin]

In article <mailman.1001096306.20291.python-list at python.org>, "Tim says...
>
>[Grant Griffin]
>> ...
>> Cartesian_cmp = lambda a,b: (a.real != b.real) * cmp(a.real, b.real) + \
>>                             (a.real == b.real) * cmp(a.imag, b.imag)
>
>Why lambda?  Surely
>
>    def Cartesian_cmp(a, b):
>        if a.real == b.real:
>            return cmp(a.imag, b.imag)
>        else:
>            return cmp(a.real, b.real)
>
>is clearer.

Like I said, I used lambda just for fun.  Also, I've been polishing my lambda
skills lately to make it easier to sort dictionaries by value <wink>.

>Simpler is
>
>    Cartesian_cmp = lambda a, b: cmp((a.real, a.imag), (b.real, b.imag))
>
>That is, sequences in Python are compared lexicographically, and tuples are
>a sequence type; you don't have to fake lexicographic comparison by hand.

Cool!--I hadn't thought of that!

I guess it's axiomatic that a one-liner that fits on one line is much better
than a one-liner that doesn't.

But waitaminute!  Are there two ways to do it?--or are they just not obvious?

>+ Python used to compare complex numbers that way.
>
>+ For technical implementation reasons, it used to be impossible to
>  raise an exception from a comparison function.  That's the real reason
>  Python defined comparison on any pair of objects of builtin types:
>  it had no choice.  

Even so, I think it's a good idea.

...

>+ Eventually, it became possible to raise exceptions from comparisons,
>  and, later, "rich comparisons" added the ability to know which
>  specific comparison was desired.  After both of those were in place,
>  Guido was keen to allow complex comparisons only of the == and !=
>  flavors.

Don't tell him, then, how my cold little DSP heart warmed to see that list
sorted in magnitude/phase order.

In all fairness, though, I've never had a need to sort a list of complex
numbers.

dictionaries-by-value-is-another-story-<wink>-ly y'rs,

=g2

_____________________________________________________________________

Grant R. Griffin                                       g2 at dspguru.com
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