Why don't I need a decrementing loop to fade OUT an Arduino LED? - for-loop

Question

I've moved past blinking an LED, and have successfully begun fading an LED using the following sketch.
void setup() {
pinMode(12, OUTPUT);
}
void loop() {
for (int i=0;i<255;i++) {
analogWrite(12,i);
delay(10);
}
}
However, I just realized that there's no for(int i=254;i=0;i--) to fade OUT. Why don't I need that? Is there something in the loop() function that automatically returns to the beginning state?

Answer 1

Why don't I need that?
Why don't you?
Of course you do. That program makes the LED get brighter slowly, then it turns off instantly, and starts over. That is a sawtooth profile, and just lame IMO.
Everyone wants their LED to fade smoothly bright and then dim. This is much cooler:
void setup() {
pinMode(13, OUTPUT);
}
boolean fadein = true;
int bright = 0;
void loop() {
// adjust brightness based on current direction
if(fadein) {
bright += 1;
}
else {
bright -= 1;
}
// apply current light level
analogWrite(13,bright);
// when get to full bright, turn around
if(255 == bright) {
fadein = false;
}
// when get to full off, turn around
if(0 == bright) {
fadein = true;
}
delay(10);
// The delay is just a placeholder
// here is where your program could do other useful things
// in addition to the cool LED fade in fade out
return;
}

Answer 2

Your setup() and loop() are part of a larger program (let's call it a dispatcher for this discussion). That program calls setup() once at the start of a run. It then executes loop(), if loop() exits then loop() gets called again by the dispatcher.
So, the for loop runs once and returns to the dispatcher,loop gets called again and the for loop runs again. The for loop appears to be running forever, because the amount of time spent in the dispatcher is minimal compared to the length of time spent in the for loop.
Finally, just for your information, the LED appears to be dimmed because the analogWrite() creates a square wave signal on the pin in a format called Pulse Width Modulation (PWM). The brightness of the LED depends on the PWM's duty cycle, which is the second parameter to analogWrite().
Try a program where the value for the PWM duty cycle is stepped up and down thru a range of values. Find a delay that causes the LED to go from off to completely on within 5 or 10 seconds (or for whatever interval you want!)
Signed, A Former Graduate TA