While and condition tests

Projects Programming
1

When using a while loop to check a variable within a function, when exactly is the variable tested?
From what I can see, it is only tested a the end of each iteration of the function. So only in position #3.
Is that correct?

bool flag = false;

void loop()
{
  while (!flag) doStuff();
  Serial.println("done");
}

void doStuff()
{
  flag = true;                  // #1
  // someCode
  flag = true;                  // #2
  // someMoreCode
  flag = true;                  // #3
}
2

Sort of. More clearly, it's done within the context of the while() structure, but after return from the function call. So not quite where you're describing.

3

Not exactly. doStuff must complete in it's entirety and return before the while test happens again, so #3 is the line that is effective, but the others would have had the same effect if you had omitted it.

4

In a 'while' the condition is tested before each time the body is executed:

while (!flag) doStuff();
acts like:

  while (true) // Repeat forever
  {
    if (flag) break;  // Leave the loop
    doStuff();
  }

If you want the test AFTER the body, use 'do-while':

  do
  {
    doStuff();
  } while (!flag);

acts like:

  while (true) // Repeat forever
  {
    doStuff();
    if (flag) break;  // Leave the loop
  }

The code inside 'doStuff()' makes no difference at all. All that matters is the value of 'flag' when the test is done.

5

In my case, the code in doStuff() does make a difference as flag can be changed in one of two places - either by the flag = true; statement or within // someCode.
I was trying to figure out why the sketch didn't do what I wanted, and the while test seemed to be the culprit. So it's good to have that confirmed.

Thanks everyone!

6

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