I was wondering why my countdown timer is as I will describe - unity3d

Question

This is just a portion of my code, and I hope will be easy enough to understand. I found a way to "fix" it, but I still don't understand this:
I set my float countDownTime to 2f. In DisplayTime(), i thought the do-while loop would count from 2 down to 0, but instead starts counting down from 0 to negative numbers. I thought that it would count down and stop when countDownTime reaches 0, as assigned in the while(countDownTime >= 0) but it continues beyond that. Thanks for any feedback or assistance.
// Update is called once per frame
void Update()
{
if (Input.GetKeyDown(KeyCode.Space) && !hasStarted)
{
hasStarted = true;
StartGame();
}
DisplayTime();
}
void DisplayTime()
{
if (timerStart)
{
do
{
countDownTime -= Time.deltaTime;
} while (countDownTime >= 0);
timer2.text = Math.Round(countDownTime, 2).ToString();
}
}
I made changes to it, which is my fix:
// Update is called once per frame
void Update()
{
if (Input.GetKeyDown(KeyCode.Space) && !hasStarted)
{
hasStarted = true;
StartGame();
}
DisplayTime();
}
void DisplayTime()
{
if (timerStart && countDownTime >= 0)
{
do
{
countDownTime -= Time.deltaTime;
} while (countDownTime >= 2);
timer2.text = Math.Round(countDownTime, 2).ToString();
}
}

Answer 1

First off, I don't see where you're initially setting countDownTime to 2.
Second, countDownTime will more likely than not stop on a negative number rather than zero. This is because deltaTime is a measure of the ms that have passed since the last update, and varies quite a bit. So you should round it back up 0 if you don't want to be shown as negative.
Finally, I think you want to execute this code as a Coroutine, otherwise the entire loop will execute in a single frame, which is most likely not what you want.
IEnumerator DisplayTime() {
countDownTime = 2;
while (countDownTime > 0) {
countDownTime -= Time.deltaTime;
timer2.text = Math.Round(countDownTime, 2).ToString();
yield return new WaitForEndOfFrame();
}
}
void Start() {
// start coroutine
StartCoroutine(DisplayTime);
}
If you did not want a coroutine, and wanted to called it each frame from your Update method, you can simply drop the while loop.
void DisplayTime() {
if (timerStart) {
if (countDownTime > 0) {
countDownTime -= Time.deltaTime;
}
}
timer2.text = Math.Round(countDownTime, 2).ToString();
}
The benefit of the Coroutine method is it usually easier to manage. Rather than calling a method every time from Update, you can call it once and just let it run. Since the looping is confined to the method itself you can use DisplayTime to set the initial value of countDownTime to 2.

Answer 2

You need to consider 2 factors.
Update() Method is called on every frame. You are executing do-while loop in DisplayTime() which is called from Update() so do while loop will execute many time (once per frame).
do-while loop works differently from normal while loop. do-while loop first executes code inside do{} and only after checks condition and breaks loop if condition is false. so (even if condition is false) code inside do{} will run at least once once. In case of normal while if condition is false code inside loop will not run.
So in your case on first frame countDownTime is subscribed until it will become be less than 0. (So on screen you immediately see 0 instead of 2, because after first frame its
already 0).
after 1st frame on each frame when do-while is executed code inside do is executed only once and than loop immediately breaks because condition countDownTime >= is already false. So after 1st frame countDownTime -= Time.deltaTime;this code gets executed once per frame (without looping multiple times in do-while), that is why it works "correctly" afterwards.