C++ - back to start of loop without checking the condition

Question

Programming on perl, we can use a smart function named 'redo' - we can go back to the start of the loop, without looking at the condition. It's useful when, for example, we create a table in which we must to set a expected values/characters (e.g. "a-b-c", nothing else). I would like to ask if exist in C++ function like that. I would be grateful for your help.

I cannot imagine a use case for that which cannot be served with the usual constructs (`if`, `while`, etc). Please provide a minimal example — Walter, Jun 18 '15 at 01:05
For example: we have an array we want to fill in specific characters (only "a" or "b"). If we wouldn't use 'goto' after writing incorrect value, the condition would be rechecked (or... I don't know better option). As a result - our counter would be increased (instead of write tab[i] again, wrote badly before, we would write tab[i+1]) — Plusce, Jun 18 '15 at 01:16
what's wrong with a `while` loop? as in `char x=0; while(is_wrong(x=obtain())); table[i]=x;` — Walter, Jun 18 '15 at 09:11
We can do it in this way too, but isn't using a 'goto' more intuitive in this situation? (knowing, that this command isn't recommended usually) — Plusce, Jun 19 '15 at 15:18
`goto` is **never** intuitive, since the label and the `goto` statement are in general not related in a structural way. With `do` there is always a `while`, an associated block of execution and a condition. — Walter, Jun 19 '15 at 17:25

score 6 · Accepted Answer · answered Jun 18 '15 at 00:45

6

There is no redo keyword for going back to the start of a loop, but there's no reason you couldn't do it with goto. (Oh I feel so dirty recommending goto...)

int tries = 0;
for(int i=0; i<10; ++i) {
  loop_start:
  bool ok = ((i+tries)%3==0);
  if(ok) {
     ++tries;
     goto loop_start;
  }
}

answered Jun 18 '15 at 00:45

Michael Anderson

70,661
7
134
187

I used the the same operation before the moment :). Works good. So, we couldn't do it shortly? – Plusce Jun 18 '15 at 01:01

score 3 · Answer 2 · answered Jun 18 '15 at 03:38

3

I think a more reasonable solution is

int trie=0;
for(int i=0; i<10; i++) {
    while((i+tries)%3==0 {
         ++tries;
    }
}

answered Jun 18 '15 at 03:38

john zhao

986
1
6
8

mpapec · Answer 3 · 2015-06-18T07:10:26.487

1

If you want to avoid goto at any cost,

int tries = 0;
for (int i=0; i<10; ++i) {do{

  bool redo = ((i+tries)%3 == 0);

  if (redo) { ++tries; }
  else      { break; }

} while(1); }

edited Jun 18 '15 at 07:10

answered Jun 18 '15 at 06:50

mpapec

50,217
8
67
127

Walter · Answer 4 · 2015-06-18T09:24:11.280

Why can you not use a simple while loop?

auto is_okay = [](char x) { return x=='a' || x=='b'; };
container C;
for(std::size_t i=0; i!=C.size(); ++i) {
  char x;
  while(!is_okay(x=obtain_new_character()));  // get new x until it's okay
  C[i]=x;
}

Or, equivalently, a do while loop

container C;
for(std::size_t i=0; i!=C.size(); ++i) {
  char x;
  do {
    x=obtain_new_character();
  } while(x!='a' && x!='b');
  C[i]=x;
}

Or even a for loop

container C;
for(std::size_t i=0; i!=C.size(); ++i) {
  char x=0;
  for(; x!='a' && a!='b'; x=obtain_new_character());
  C[i]=x;
}

C++ - back to start of loop without checking the condition

4 Answers4