Path: csiph.com!eternal-september.org!feeder.eternal-september.org!mx02.eternal-september.org!.POSTED!not-for-mail
From: Marko Rauhamaa <marko@pacujo.net>
Newsgroups: comp.lang.python
Subject: Re: Case Statements
Date: Wed, 16 Mar 2016 16:31:51 +0200
Organization: A noiseless patient Spider
Lines: 78
Message-ID: <87mvpycx48.fsf@elektro.pacujo.net>
References: <f30500db-19cb-4de8-81f1-6eb77c28b3b4@googlegroups.com> <30502a2e-0bad-4b0f-a1e8-a2b40b0d7ab9@googlegroups.com> <mailman.181.1458102409.12893.python-list@python.org> <ncb4e0$op1$1@dont-email.me> <ncb6jk$rrm$1@ger.gmane.org> <56E928D4.3000701@rece.vub.ac.be> <ncbac2$qtk$1@ger.gmane.org> <56E93ADD.9040500@rece.vub.ac.be> <mailman.191.1458126480.12893.python-list@python.org> <ncbf5e$uq4$1@dont-email.me> <87vb4md362.fsf@elektro.pacujo.net> <ncbkr7$kj3$1@dont-email.me>
Mime-Version: 1.0
Content-Type: text/plain
Injection-Info: mx02.eternal-september.org; posting-host="b7cb1518d23ec19d482dcc9c31d30fdd"; logging-data="12629"; mail-complaints-to="abuse@eternal-september.org";	posting-account="U2FsdGVkX19BjXfWiqF8BPfHZkYTJ62U"
User-Agent: Gnus/5.13 (Gnus v5.13) Emacs/24.5 (gnu/linux)
Cancel-Lock: sha1:8bxIhKHxD4e8ZA+1or/JxKAUtow= sha1:P7R6blNLBpCz2aDnkwCUAYTZsww=
Xref: csiph.com comp.lang.python:105035

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?"