Loops in Coroutines

A loop with a yield return inside runs over many frames, because every pass pauses before the next. With WaitForSeconds the loop repeats something on a beat. This siren flashes 3 times:

IEnumerator Flash()
{
    for (int i = 0; i < 3; i++)
    {
        lamp.enabled = true;
        yield return new WaitForSeconds(0.2f);
        lamp.enabled = false;
        yield return new WaitForSeconds(0.2f);
    }
}

With yield return null the loop does a little every frame until its condition is met, which is how a coroutine animates something. This balloon, at a scale of 1, grows by 0.5 a second until its scale reaches 2, then reports:

IEnumerator Inflate()
{
    while (transform.localScale.x < 2f)
    {
        transform.localScale += Vector3.one * 0.5f * Time.deltaTime;
        yield return null;
    }
    Debug.Log("Full");
}

Time.deltaTime works here as in Update(): the coroutine resumes once per frame, and the line after the loop runs only when the loop is done. Without the yield, the whole loop runs inside one frame and the balloon is full at once. A while (true) loop is fine in a coroutine as long as it yields on every pass; without a yield it never gives the frame back, and the Unity editor freezes.