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From: "Alf P. Steinbach" <alfps@start.no>
Newsgroups: comp.std.c++
Subject: Re: for-else: adding else to for loops
Date: Sat,  8 Aug 2009 09:53:43 CST
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* Jared Grubb:
> I am both a Python and C++ developer, and there is one feature in
> Python that I've grown to appreciate, and that I wish C/C++ had. In
> Python, loops can have an else clause; the else clause gets executed
> when the loop condition fails (but not when the loop is exited via a
> break, return, or exception). For a dummy example:
> 
> auto iter = my_list.begin();
> for ( ; iter != lst.end(); ++iter)
> {
>      if (*iter == NEEDLE) break;
> }
> else
> {
>     cout << "needle not in the list";
> }

Oh my.

    struct Success {};
    auto iter = myList.begin();
    try
    {
        for( ; iter != myList.end();  ++iter )
        {
            if( *iter == needle ) { throw Success(); }
        }
        cout << "needle not in the list\n";
    }
    catch( Success const& ) {}
    cout << "tell me, did I just say something? or not?\n";

But, well, that sort of turns the usual meaning of an exception upside down.

So.

Perhaps :-) ...

    auto iter = myList.begin();
    for( ; iter != myList.end();  ++iter )
    {
        if( *iter == needle ) { goto later; }
    }
    cout << "needle not in the list\n";
    later:
    cout << "tell me, did I just say something? or not?\n";

But, wait, if I'm not mistaken that's a so called "goto". It's infamous for
allowing efficient and terse code. Can't have that, can we?

So...

    auto iter = myList.begin();
    for(
        ;
        (iter == myList.end()? (cout << "needle not in the list\n", 0) : 1);
        ++iter )
    {
        if( *iter == needle ) { break; }
    }
    cout << "tell me, did I just say something? or not?\n";

But again  --  it's not quite 100%...

For, the failure handling code can conceivably be unsuitable for being placed
inline in the loop head, yet not something that one would want to define a
separate routine for.

Therefore, combining two of the above approaches,

    struct LocalFailure { static bool x() { throw LocalFailure(); } };

    ...

    auto iter = myList.begin();
    try
    {
        for( ; (iter != myList.end() || LocalFailure::x()); ++iter )
        {
            if( *iter == needle ) { break; }
        }
    }
    catch( LocalFailure const& )
    {
        cout << "needle not in the list\n";
    }
    cout << "tell me, did I just say something? or not?\n";

This is so verbose and possibly inefficient, compared to the "goto", that no
doubt it will in a very short time be established as the idiomatic way to do
this thing. For what's "try" and "catch" /for/, if not for use for for loops?


> For a second example:
> 
> for (unsigned try = 0; try < MAX_TRIES; ++try)
> {
>      bool success = did_it_work();
>      if (success) break;
>      sleep(POLL_PERIOD);
> }
> else
> {
>      // Never worked
> }

Well, this sort of reminds of the oft repeated request for /multi-level/ 'break'.

It would break out of any labeled control construct, to following code.

E.g.

    // Example of Alf's preferred language extension for above problem.
    // Because it solves a lot more than just that problem.
    // And is very simple both syntactically and semantically.

    tryMaxTimes:
    {
        for( int t = 0; t < maxTries; ++t )
        {
            bool const success = didItWork();
            if( success ) { break tryMaxTimes; }  // <- Here. That's it.
            sleep( pollPeriod );
        }
        cout << "timeout, sorry\n";
    }
    cout << "tell me, did I just say something? or not?\n";

However, it may be too practical for standardization?


Cheers & hth.,

- Alf

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