how to write this loop better?

Question

i have a code that does something like this:

while (doorIsLocked()) {
  knockOnDoor();
}
openDoor();

but i want to be polite and always knock on the door before i open it.

i can write something like this:

knockOnDoor();
while (doorIsLocked()) {
  knockOnDoor();
}
openDoor();

but i'm just wondering if there's a better idiom that doesn't repeat a statement.

thanks.

Answer 1

You can use a do-while instead of a while-do loop:

do {

  knockOnDoor();

} while (doorIsLocked());

openDoor();

Unlike a while-do loop, the do-while executes the body once before checking the termination condition.

See also

• Wikipedia/Do while loop

---

Pre-test and post-test loops

The do-while loop -- sometimes called just a do statement -- in C/C++/C#/Java/some others is what is known as a "post-test" loop: the terminating condition is checked after each iteration. It loops while a condition is true, terminating immediately once it's false.

Pascal has a repeat-until loop, which is also a "post-test" loop; it loops while a condition is false, terminating immediately once it's true.

Fortran's do-while loop on the other hand is a "pre-test" loop: the terminating condition is checked before each iteration. In C/C++/Java/C#/some others, while-do and for loops are also "pre-test" loops.

Always check language reference when in doubt.

References

• MSDN: C++ Language Reference: The do-while Statement
• MSDN: C# Language Reference: do
• Java Language Specification §14.13: The do Statement

Related questions

• Test loops at the top or bottom? (while vs. do while)
• Is there ever a need for a do {…} while ( ) loop?
• When is a do-while appropriate?

Answer 2

You could do a recursive call:

void openLockedDoor(){
    knockOnDoor();
    if( doorIsLocked() ){
        return openLockedDoor();
    }
    return openDoor();
}

but the do-while above works just as well (or even better depending on your compiler.)

Answer 3

@polygenelubricants already posted the obvious best solution to this.

In some cases though, it may be simpler to remove the condition from the loop entirely, and do something like this:

for (;;) {
  knockOnDoor();
  if (!doorIsLocked()) { break; }
}

since it gives you full control over what to do before and after the loop condition.

But when a do-while does what you need, definitely prefer that.

Answer 4

You forgot to Wait...If you keep on knocking it would also be impolite...so best way is to do it like this:

#define WAITING_TIME 2000 //2 Seconds are enough waiting time for another knock
while(doorIsLocked()){
knockOnDoor();
//Now wait
Sleep(WAITING_TIME);
}
openDoor();

Answer 5

Here you are:

while(knockOnDoor(),doorIsLocked());