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digitalmars.D - Re: Proposal of a general do-while loop

Eh... I like it, and I can see where it'd be useful, but I don't think it
justifies the additional complexity. I could go either way though, so... votes
= votes < 0 ? -abs(votes) : abs(votes);

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.
 

Jul 17 2007