Proposal of a general do-while loop

[Tomas Lindquist Olsen]

On Tue, 17 Jul 2007 03:04:11 -0400, Taro Kawagishi wrote:

> Hello all,
> 
> every once in a while I feel uneasy when I find I can't fit my logic into
> a do-while or while loop in a concise way. Here is a C++ example:
> 
> void
> find_string_occurrences(const string& text, const string& pattern) {
> 
>     // listing 1
>     size_t pos = text.find(pattern, 0);
>     while (pos != string::npos) {
>         cout << "pattern found at " << pos << "\n"; ++pos;
>         pos = text.find(pattern, pos);
>     }
>     }
> 
> }
> The way the code is written might look redundant in calling find() twice,
> but I think it is reasonable because you can test the loop condition only
> after you run function find() but here you can't use a do-while loop which
> doesn't allow you to place other statements after the condition statement.
> 
> 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.
> 
>     // listing 2
>     size_t pos = 0;
>     while (true) {
>         pos = text.find(pattern, pos);
>         if (pos == string::npos) {
>             break;
>         }
>         cout << "pattern found at " << pos << "\n"; ++pos;
>     }
>     }
>     // listing 3
>     size_t pos = 0;
>     do {
>         pos = text.find(pattern, pos);
>         if (pos == string::npos) {
>             break;
>         }
>         cout << "pattern found at " << pos << "\n"; ++pos;
>     } while (true);
> 
> 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;
>     }
>     }
> The meaning of
> 
>     do {
>         aa;
>     } while (bb) {
>         cc;
>     }
>     }
> is
> 
>     while (true) {
>         aa;
>         if (not bb) {
>             break;
>         }
>         cc;
>     }
>     }
> and is a natural extension to both of
> 
>     do {
>         aa;
>     } while (bb);
> 
> and
> 
>     while (bb) {
>         cc;
>     }
>     }
> 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. I
> think D language would be a great fit to have this feature because the
> language seems to be still evolving.

I dont see any reason to further complicate the language for this. As you
already pointed out yourself it can be easily accomplished with the
current language features.

Here's mine:

for(;;)
{
	aa;
	if (!cond) break;
	bb;
}

- Tomas