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Implement C's Switch in Python 3


Hi,
On Sat, Feb 2, 2019 at 8:54 PM Avi Gross <avigross at verizon.net> wrote:
>
> I may be missing something, but the focus seems to be only on the rightmost
> digit. You can get that with
>
>
>         str(day)[-1]
> or with
>         day % 10
>
> Problem: print 1..31 using suffixes such as 1st, 2nd, 3rd, 4th ...
>
> So your dictionary needs entries for "1" or 1 and "2" or "2" and of course 3
> becoming 3rd.
>
> And, frankly, you don't even need a dictionary as a one-liner will do.
>
> Here is an example using a helper function:
>
> """ Use last digit to determine suffix """
>
> def nthSuffix(day):
>     nth = day % 10
>     suffix = "st" if nth == 1 else ("nd" if nth == 2 else ("rd" if nth == 3
> else "th"))
>     return str(day) + suffix
>
>
> Output:
>
> >>> for day in range(1, 32):
>         print( nthSuffix(day))
>
> 1st
> 2nd
> 3rd
> 4th
> 5th
> 6th
> 7th
> 8th
> 9th
> 10th
> 11st

Shouldn't this be 11th? And then 12th and 13th?

Thank you.

> 12nd
> 13rd
> 14th
> 15th
> 16th
> 17th
> 18th
> 19th
> 20th
> 21st
> 22nd
> 23rd
> 24th
> 25th
> 26th
> 27th
> 28th
> 29th
> 30th
> 31st
>
> -----Original Message-----
> From: Python-list <python-list-bounces+avigross=verizon.net at python.org> On
> Behalf Of Sayth Renshaw
> Sent: Saturday, February 2, 2019 8:53 PM
> To: python-list at python.org
> Subject: Re: Implement C's Switch in Python 3
>
>
> > >I am trying to convert a switch statement from C into Python. (why?
> > >practising).
> > >
> > >This is the C code.
> > >
> > >printf("Dated this %d", day);
> > >  switch (day) {
> > >    case 1: case 21: case 31:
> > >        printf("st"); break;
> > >    case 2: case 22:
> > >        printf("nd"); break;
> > >    case 3: case 23:
> > >        printf("rd"); break;
> > >    default: printf("th"); break;
> > >
> > >  }
> > >  printf(" day of ");
> > >
> > >#Premise if the use enter an int as the date 21 for example it would
> print 21st. It appends the correct suffix onto a date.
> > >Reading and trying to implement a function that uses a dictionary.
> > >Not sure how to supply list into it to keep it brief and with default
> > >case of 'th'.
> > >
> > >This is my current code.
> > >
> > >def f(x):
> > >    return {
> > >        [1, 21, 31]: "st",
> > >        [2, 22]: "nd",
> > >        [3, 23]: "rd",
> > >    }.get(x, "th")
> > >
> > >
> > >print(f(21))
> > >
> > >I have an unhashable type list. Whats the best way to go?
> >
> > Skip has commented on lists being unhashable. We can elaborate on that
> > if you like.
> >
> > However, even if you went to tuples (which would let you construct the
> > dict you lay out above), there is another problem.
> >
> > You're looking up "x" in the dict. But the keys of the dict are not
> > integers, they are lists (or tuples) or integers, so they won't match.
> >
> > You _could_ do this:
> >
> >   return {
> >     1: "st", 21: "st", 31: "st",
> >     2: "nd", 22: "nd",
> >     3: "rd", 23: "rd",
> >   }.get(x, "th")
> >
> > which makes distinct entries for each integer value of interest.
> >
> > The conventional approach would normally be:
> >
> >   if x in (1, 21, 31):
> >     return "st"
> >   if x in (2, 22):
> >     return "nd"
> >   if x in (3, 23):
> >     return "rd"
> >   return "th"
> >
> > While this works for a small number of choices, if you had a huge dict
> > with lots of possible values you could return to your
> > dict-keyed-on-tuples approach. You would need to try each tuple in turn:
> >
> >   mapping = {
> >     (1, 21, 31): "st",
> >     (2, 22): "nd",
> >     (3, 23): "rd",
> >   }
> >   for key, suffix in mapping.items():
> >     if x in key:
> >       return suffix
> >   return "th"
> >
> > However, for big dictionaries (with many keys) you loose a key
> > strength of dicts: constant time lookup. You can see the above code
> > (and the earlier "if" code) are rather linear, with run time going up
> > linearly with the number of keys. You're better with the int->string
> > single value dict version.
> >
> > Cheers,
> > Cameron Simpson
>
> It seems odd with C having switch that its cleaner and more efficient than
> python where we are having to implement our own functions to recreate switch
> everytime.
>
> Or perhaps use a 3rd party library like
> https://github.com/mikeckennedy/python-switch
>
> You have both given good options, it seems there are no standard approaches
> in this case.
>
> Cheers
>
> Sayth
>
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