Multiple "cmp"s chained one after another

vincent wehren vincent at visualtrans.de
Sat May 14 08:04:32 EDT 2005


"Volker Grabsch" <volker_grabsch at v.notjusthosting.com> schrieb im 
Newsbeitrag news:d64io9$s6o$05$1 at news.t-online.com...
| Hello!
|
| Ich just found a very nice 'pythonic' solution for an often appearing
| problem. I didn't find it documented anywhere, so I'm posting it here.
| If this isn't new in any way, I'd really like to get to know it.
|
| Example problem:
| I have some "datetime" objects and want to sort them, as here:
|
| birthdays = [d1,d2,d3,d4]
| birthdays.sort()
|
| However, I don't want to sort them the default way. These are birthdays,
| so only the month and day do matter, not the year. E.g.:
|
| 2003-01-01  should be smaller than  1984-05-01
|
| So I have to write the comparison on my own, e.g.
|
| def cmp_birthdays(d1,d2):
| if d1.month > d2.month: return 1
| if d1.month < d2.month: return -1
| if d1.day > d2.day: return 1
| if d1.day < d2.day: return -1
| return 0
|
| ...
| birthdays.sort(cmp_birthdays)


If you don't care about the year, why not just "normalize" the year
to all be the same using the replace method of the date instance?
Something like:

d1 = datetime.date(2004, 12, 2)
d2 = datetime.date(2001, 12, 3)
d3 = datetime.date(2002, 12, 6)
d4 = datetime.date(1977, 12, 7)
dates =[d1,d2,d3,d4]
datesNorm = [obj.replace(year=1900) for obj in (dates)]
datesNorm.sort()
print datesNorm # etcetera


HTH,

---
Vincent




|
| This implementation of cmp_birthdays is very ugly. Image you want to
| chain more than 2 values in that "cmp_birthdays". I also want to use the
| builtin "cmp" function, not ">" and "<".
|
| After thinking some minutes about it, I found a very nice solution:
| I have some "cmp"s one aftter another. If one if them return 1 oder -1,
| it sould be returned. If it returns 0, the next "cmp" is used. In other
| words: I have a sequence of numbers, and want to get the first one that
| is not 0. (or return 0, if all numbers were 0)
|
| But this is exactly what the "or" operator does, due to short-circuit
| evaluation. In this example, that means:
|
| def cmp_bithdays(d1,d2):
| return cmp(d1.month,d2.month) or cmp(d1.day,d2.day)
|
| The generic pattern is:
|
| return cmp(...) or cmp (...) or cmp(...) or ...
|
| I'm not sure whether this pattern is already a "common recipe", but
| I found it to be a very nice idea. :-)
|
| Any opinions?
|
|
| Greets,
|
| Volker
|
| -- 
| Volker Grabsch
| ---<<(())>>---
| \frac{\left|\vartheta_0\times\{\ell,\kappa\in\Re\}\right|}{\sqrt
| [G]{-\Gamma(\alpha)\cdot\mathcal{B}^{\left[\oint\!c_\hbar\right]}}} 





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