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Re: Case Statements

From Marko Rauhamaa <marko@pacujo.net>
Newsgroups comp.lang.python
Subject Re: Case Statements
Date 2016-03-16 16:31 +0200
Organization A noiseless patient Spider
Message-ID <87mvpycx48.fsf@elektro.pacujo.net> (permalink)
References (7 earlier) <56E93ADD.9040500@rece.vub.ac.be> <mailman.191.1458126480.12893.python-list@python.org> <ncbf5e$uq4$1@dont-email.me> <87vb4md362.fsf@elektro.pacujo.net> <ncbkr7$kj3$1@dont-email.me>

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BartC <bc@freeuk.com>:

> Yes, a few scripting languages can do interesting things with switch or
> case statements. Perl for example (where I think it is created out other
> language features, but it looks a regular part of the syntax).
>
> Even Ruby has one. It doesn't do anything 'sexy' with it, but it does
> have this:
>
>  case
>  when this
>     ....
>  when that
>     ....
>  when other
>     ...
>  end

That's a different topic.

> which is exactly equivalent to if this... elif that... (when the tests
> are ordered), with one difference:
>
> Each test starts with "when", instead of "if" for the first and "elif"
> for subsequent ones. That makes it easier to reorder tests, temporarily
> comment out the first test, copy a test from elsewhere, insert a new
> first test (you get the idea).

That is no different from a chained if/elif.

Scheme has this:

   (case (* 2 3)
     ((2 3 5 7)
      'prime)
     ((1 4 6 8 9)
      'composite)
     (else
      'unknown))

It has something better than C even:

   (case (die10)
     ((1 3 5 7 9)
      => (lambda (n)
           n))
     (else
      => (lambda (n)
           (/ n 2))))

which maps 1, 3, 5, 7 and 9 onto themselves but halves 2, 4, 6, 8 and
10.

As for a chained if/elif, Scheme as "cond:"

   (cond
    ((windy?)
     (fly-kite))
    ((shining? sun)
     (go-out))
    ((raining?)
     (play-soccer))
    (else
     (read-book)))

which also has a "=>" variant:

   (cond
    ((best-selling-book (this-year))
     => (lambda (book)
          (read book)))
    (else
     (play wii pes08)))


Marko

PS What is a "scripting language?"

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Case Statements jj0gen0info@gmail.com - 2016-03-15 13:46 -0700
  Re: Case Statements Mark Lawrence <breamoreboy@yahoo.co.uk> - 2016-03-15 23:11 +0000
  Re: Case Statements jj0gen0info@gmail.com - 2016-03-15 16:47 -0700
    Re: Case Statements Mark Lawrence <breamoreboy@yahoo.co.uk> - 2016-03-15 23:58 +0000
    Re: Case Statements BartC <bc@freeuk.com> - 2016-03-16 00:51 +0000
      Re: Case Statements Mark Lawrence <breamoreboy@yahoo.co.uk> - 2016-03-16 01:05 +0000
  Re: Case Statements jj0gen0info@gmail.com - 2016-03-15 18:55 -0700
    Re: Case Statements "Mario R. Osorio" <nimbiotics@gmail.com> - 2016-03-15 21:06 -0700
      Re: Case Statements BartC <bc@freeuk.com> - 2016-03-16 10:34 +0000
        Re: Case Statements Steven D'Aprano <steve@pearwood.info> - 2016-03-16 23:56 +1100
    Re: Case Statements Mark Lawrence <breamoreboy@yahoo.co.uk> - 2016-03-16 04:26 +0000
      Re: Case Statements Christian Gollwitzer <auriocus@gmx.de> - 2016-03-16 09:13 +0100
        Re: Case Statements Mark Lawrence <breamoreboy@yahoo.co.uk> - 2016-03-16 08:47 +0000
        Re: Case Statements Marko Rauhamaa <marko@pacujo.net> - 2016-03-16 11:16 +0200
        Re: Case Statements Antoon Pardon <antoon.pardon@rece.vub.ac.be> - 2016-03-16 10:35 +0100
        Re: Case Statements Mark Lawrence <breamoreboy@yahoo.co.uk> - 2016-03-16 09:51 +0000
          Re: Case Statements BartC <bc@freeuk.com> - 2016-03-16 19:41 +0000
            Re: Case Statements BartC <bc@freeuk.com> - 2016-03-16 22:30 +0000
        Re: Case Statements Antoon Pardon <antoon.pardon@rece.vub.ac.be> - 2016-03-16 11:52 +0100
        Re: Case Statements Mark Lawrence <breamoreboy@yahoo.co.uk> - 2016-03-16 11:07 +0000
          Re: Case Statements BartC <bc@freeuk.com> - 2016-03-16 11:16 +0000
            Re: Case Statements Mark Lawrence <breamoreboy@yahoo.co.uk> - 2016-03-16 11:33 +0000
            Re: Case Statements Marko Rauhamaa <marko@pacujo.net> - 2016-03-16 14:21 +0200
              Re: Case Statements BartC <bc@freeuk.com> - 2016-03-16 12:53 +0000
                Re: Case Statements Marko Rauhamaa <marko@pacujo.net> - 2016-03-16 16:31 +0200
                Re: Case Statements BartC <bc@freeuk.com> - 2016-03-16 15:00 +0000
                Re: Case Statements Marko Rauhamaa <marko@pacujo.net> - 2016-03-16 19:07 +0200
              Re: Case Statements Rustom Mody <rustompmody@gmail.com> - 2016-03-20 01:01 -0700
                Re: Case Statements Mark Lawrence <breamoreboy@yahoo.co.uk> - 2016-03-20 08:29 +0000
        Re: Case Statements Antoon Pardon <antoon.pardon@rece.vub.ac.be> - 2016-03-16 14:38 +0100
        Re: Case Statements Mark Lawrence <breamoreboy@yahoo.co.uk> - 2016-03-16 14:02 +0000
        Re: Case Statements Marko Rauhamaa <marko@pacujo.net> - 2016-03-16 21:27 +0200
          Re: Case Statements Antoon Pardon <antoon.pardon@rece.vub.ac.be> - 2016-03-17 09:22 +0100
            Re: Case Statements Marko Rauhamaa <marko@pacujo.net> - 2016-03-17 10:57 +0200
              Re: Case Statements Antoon Pardon <antoon.pardon@rece.vub.ac.be> - 2016-03-17 10:12 +0100
        Re: Case Statements Steven D'Aprano <steve@pearwood.info> - 2016-03-17 11:19 +1100
          Re: Case Statements Steven D'Aprano <steve@pearwood.info> - 2016-03-17 12:54 +1100
            Re: Case Statements Gregory Ewing <greg.ewing@canterbury.ac.nz> - 2016-03-17 18:45 +1300
              Re: Case Statements Chris Angelico <rosuav@gmail.com> - 2016-03-17 17:30 +1100
                Re: Case Statements Gregory Ewing <greg.ewing@canterbury.ac.nz> - 2016-03-18 18:59 +1300
              Re: Case Statements Steven D'Aprano <steve+comp.lang.python@pearwood.info> - 2016-03-17 17:29 +1100
                Re: Case Statements Chris Angelico <rosuav@gmail.com> - 2016-03-17 17:48 +1100
                Re: Case Statements Steven D'Aprano <steve@pearwood.info> - 2016-03-17 21:53 +1100
                Re: Case Statements Chris Angelico <rosuav@gmail.com> - 2016-03-17 22:49 +1100
            Re: Case Statements Antoon Pardon <antoon.pardon@rece.vub.ac.be> - 2016-03-17 10:23 +0100
            Re: Case Statements Chris Angelico <rosuav@gmail.com> - 2016-03-17 20:55 +1100
          Re: Case Statements Antoon Pardon <antoon.pardon@rece.vub.ac.be> - 2016-03-17 09:36 +0100
        Re: Case Statements Antoon Pardon <antoon.pardon@rece.vub.ac.be> - 2016-03-17 09:38 +0100
  Re: Case Statements l0r0m0a0i0l@gmail.com - 2016-03-16 06:15 -0700
    Re: Case Statements Steven D'Aprano <steve@pearwood.info> - 2016-03-17 00:27 +1100
      Re: Case Statements Mark Lawrence <breamoreboy@yahoo.co.uk> - 2016-03-16 14:00 +0000
    Re: Case Statements Mark Lawrence <breamoreboy@yahoo.co.uk> - 2016-03-16 13:59 +0000
  Re: Case Statements l0r0m0a0i0l@gmail.com - 2016-03-16 07:21 -0700
    Re: Case Statements Marko Rauhamaa <marko@pacujo.net> - 2016-03-16 16:33 +0200

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