Strange issue trying fade on arduino on multiple LEDs - c++

Question

I am trying to test out Fade on multiple LEDs on an Arduino Uno. Here's the code i have written
int led[2] = {9,10}; // the pin that the LED is attached to
int brightness[2] = {0,0}; // how bright the LED is
int fadeAmount[2] = {5,15}; // how many points to fade the LED by
long previousMillis[2] = {0,0}; // will store last time LED was updated
long interval[2] = {30, 50};
// the setup routine runs once when you press reset:
void setup() {
// declare pin 9 to be an output:
pinMode(led[0], OUTPUT);
pinMode(led[1], OUTPUT);
Serial.begin(9600);
}
// the loop routine runs over and over again forever:
void loop() {
// set the brightness of pins:
for (int counter = 0; counter < 2; counter++) {
unsigned long currentMillis = millis();
analogWrite(led[counter], brightness[counter]);
Serial.print("LED ");
Serial.print(led[counter]);
Serial.print(": Brightness ");
Serial.println(brightness[counter]);
if (currentMillis - previousMillis[counter] > interval[counter]) {
// change the brightness for next time through the loop:
brightness[counter] = brightness[counter] + fadeAmount[counter];
// reverse the direction of the fading at the ends of the fade:
if (brightness[counter] == 0 || brightness[counter] == 255) {
fadeAmount[counter] = -fadeAmount[counter] ;
}
}
}
}
Here's the weird thing. If i comment out the Serial stuff (print and begin), the fades don't work. They just "flicker" a little.
Any idea what is wrong?

Answer 1

I figured it out. It looks like I forgot a line of code.
previousMillis[counter] = currentMillis;
I should have put that in on the first if statement in the code. I guess the act of observing by using the serial print slowed things down enough that the code seemed to work.
Boy, do i feel foolish.