in reply to Re: Control Structures
in thread Control Structures

The advantage to case statements is that each element is not mutually exclusive, as you have with a single if/elsif/else structure

So, in pseudocode, you might do something like:

switch (value) {
    case 'needs_slight_cleaning':
        &clean_up_values;
    case 'good value':
        &do_whatever_you_need_to;
        break;
    case 'totally unrelated';
        &do_something_else;
        break;
    default:
        &do_some_default_thing;
}

[download]

Note how there is no 'break' between the first two cases, so something that matches the first case will run '&clean_up_values' and '&do_whatever_you_need_to';

It's not an absolutely necessary control structure, but there are some times when it sure does come in handy. (most times when you have a giant if/elseif/else tree, where you're repeating large blocks of it).

Update: I forgot to answer the questions as they were asked:

What type of new control structures you would like to have in Perl?
I'm fine with the current structures
What would be your consideration for the design of control structures ?
Anything that helps me get my work done
Would that make your task easier?
switch/case statements, for the reason stated above.
Can you mimic them with current version (5.6+)?
Yes. With goto.

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Re^3: Control Structures
by Roy Johnson (Monsignor) on May 10, 2005 at 03:40 UTC
    You certainly don't need goto to do switch/case. One auxiliary function will give you the functionality you outlined above, and more. (I've included the ability to check regexen; you could do other things if you were inclined.) The syntax reads pretty well, IMO.
    Read more... (1408 Bytes)
    Writing these things has become something of a hobby for me. :-)
    Caution: Contents may have been coded under pressure.
[reply]
[d/l]

      Um...the goto bit was a joke. (although, it does come in handy for these sort of logic issues)

      But your example structure still doesn't behave like switch/case in C (okay, I did more LPC than I've ever done C, and that was all a long time ago). The important thing is that matching 'needs_slight_cleaning' does not mean that it matches 'good_value', or that it will match after &clean_up_values is called.

      Because I've been doing without switch for so long, it's difficult to think of examples when these structures are really useful. Once you see the logic, it makes perfect sense, though.

      Okay, here's a logic situation -- you have a program that's monitoring servers. You based on a given alert, you need to decide how to escallate.

      switch ($machine_name) { 
    # master LDAP server:
    case 'einstein':
        &alert_management();
    # LDAP replicas
    case 'joule':
    case 'boltzman':
    case 'hawking':
        &alert_ldap_sysadmin();
    # the mail systems.  (that authenticate off of LDAP)
    case 'feynman':
    case 'faraday';
    case 'curie':
    case 'newton':
        &alert_mail_sysadmin();
    # helpdesk gets notified of everything not development
    default:
        &alert_helpdesk();
    # development servers:
    case 'teller':
    case 'penn':
    case 'houdini':
    case 'copperfield':
        &log();
}

      [download]

      Blah...submitted too early...anyway, the point is, that in that example, if 'einstein' is true, we alert management, ldap sysadmin, mail sysadmin, helpdesk, and it gets logged. (okay, I could've moved '&log' outside of the switch statement). But flow continues until there's a break...and in this case, there's no break. It gets even tricker when you have multiple paths like that through the system. (this was just ldap and mail servers... imagine if I add in web servers, and some of those webservers are also LDAP authenticating... okay, well, at that point, switch might not be so good for that one.... but we could alert the web admins as part of the ldap replica section, and then have a seperate section for the web servers.

      I can do this in perl. (either using goto, or by using much more complicated lookup tables to tell how things propogate ... which may be needed, as the rules for notification can't be flattened to one dimension)

[reply]
[d/l]
        your example structure still doesn't behave like switch/case in C
        Yes, it does implement fall-through. Try it. The example program I gave demonstrated it. See the two lines of output for needs_slight_cleaning? It doesn't have to match 'good value' (how could it?) to get that second output line. 
        Checking testing...Default
Checking needs_slight_cleaning...Clean up values
Whatever you need
Checking good value...Whatever you need
Checking totally unrelated...Something else

        [download]
        If I didn't want to implement fall-through, I would have just had a bunch of ifs. If it doesn't encounter last, it's going to keep calling $case->(), which will short-circuit true if it has previously matched (that is the purpose of the $matched variable in the closure). last acts like break.

        Update: To stack up things like you did here, just have a series of calls to $case-() as separate statements (or you could join them up with ors), and follow the last one with && whatever.

        Caution: Contents may have been coded under pressure.
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[d/l]
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Re^3: Control Structures
by Roy Johnson (Monsignor) on May 10, 2005 at 00:58 UTC
[reply]

In Section Meditations