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From: Howard Hinnant <howard.hinnant@gmail.com>
Newsgroups: comp.std.c++
Subject: Re: for-else: adding else to for loops
Date: Fri,  7 Aug 2009 16:24:23 CST
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On Aug 7, 1:56 pm, Jared Grubb <jared.gr...@gmail.com> wrote:
> 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";
>
> }
>
> 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
>
> }
>
> Being able to insert some code that runs when the loop condition is
> false can be useful, even if it is somewhat rare. I've seen examples
> in live code where developers solve this by:
> 1) re-testing the loop condition,
> 2) using an extra flag-variable declared outside of the for loop that
> can be tested after the for loop finished (e.g., put "bool
> success=false" outside the loop in the second example),
> 3) by encapsulating the for-loop in a separate function (the "break"
> becomes "return" and the extra code follows the for loop).
>
> The first two solutions result in code that is not as readable and
> even awkward (once you know how "for-else" works -- it's a little
> strange at first glance). The third solution is probably the best way
> to do it, but sometimes encapsulating the for-loop out is not easy or
> nice.
>
> I tried to find if this has ever been discussed, and I haven't been
> able to find any previous discussion on this construct. So, I was
> curious if anyone else had an opinion on whether this kind of
> construct would be useful.

Yes, I think this (and more) would be useful as well.  However I do
not expect to see it in C++0x.  Here is the syntax/functionality I've
toyed with:

for (auto i : seq)
{
   cout << ", " << i;
   for one   {cout << "The only number is " << i << '\n';}
   for first {cout << "The numbers are " << i;}
   for last  {cout << " and " << i << '\n';}
}
for none {cout << "There are no numbers\n";}

Example outputs:

{}:
There are no numbers

{0}
The only number is 0

{0, 1}
The numbers are 0 and 1

{0, 1, 2}
The numbers are 0, 1 and 2

{0, 1, 2, 3}
The numbers are 0, 1, 2 and 3

-Howard


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