Proposal of a general do-while loop

[Stewart Gordon]

"Taro Kawagishi" <tarok at acm.org> wrote in message 
news:f7hplb$1ovc$1 at digitalmars.com...
<snip>
> I can write the same logic as in listing 2 and 3 below, but their meanings 
> would be less clear than listing 1, because the looping condition is in 
> the if statement together with the break statement in it, and you need to 
> spot the if statement in the while body to understand it.
<snip>

On seeing a while (true), I think it goes without saying that you need to 
look for a break or return statement somewhere to find the terminating 
condition.

BTW here are two other ways of doing your particular case:

    // listing 5
    size_t pos = 0;
    while ((pos = text.find(pattern, 0)) != string::npos) {
        cout << "pattern found at " << pos << "\n";
        ++pos;
    }

    // listing 6
    for (size_t pos = 0; (pos = text.find(pattern, 0)) != string::npos; 
++pos) {
        cout << "pattern found at " << pos << "\n";
    }

but I realise this is no use in the more general case when you've got more 
to do in the portion before the break.

> I think a more natural way to express the logic is to write the code as in 
> listing 4.
>
>    // listing 4
>    size_t pos = 0;
>    do {
>        pos = text.find(pattern, pos);
>    } while (pos != string::npos) {
>        cout << "pattern found at " << pos << "\n";
>        ++pos;
>    }
<snip>

I'm not sure I like this notation.  There are always going to be people who 
prefer to put curly brackets on a line of their own.  And then, given a 
snippet of code

        ...
    }
    while (cond)
    {
        ...

it'll be necessary to scroll, possibly through several screens of code, just 
to find out whether the block above the while is part of the loop as well.

Moreover, I don't know if it's a common pitfall to write

    do {
        ...
    }

(with no while clause) intending an infinite loop (or one that must be 
broken out of).  But should somebody fall into this trap and follow it 
immediately with a while loop, it would take on a whole new meaning.

> The current while loop and do-while loop will be specialized forms of this 
> general do-while loop.
>
> The advantage of the new construct will be seen if you have more complex 
> statements within do and while blocks.
> I believe allowing this extended construct will be smooth since it will 
> not break the existing code.
<snip>

I'm still not sure it gains anything practically over the if-break method. 
Moreover, what if you want multiple terminating conditions at different 
points in the loop?  You can put any number of breaks in a loop; your idea 
OTOH doesn't easily extend to multiple exit points.

(For the record, Beta Basic on the ZX Spectrum had an "EXIT IF" statement, 
really the equivalent of if-break though it also looked a bit like your 
idea, especially as the automatic indentation would show that particular 
statement at the same level as the DO and LOOP.  If you like, you _could_ 
outdent your if-break statements to achieve the same effect....)

Stewart.