// pins 3, 5, 9, 11, and any pin with ~ beside it, are all capable of PWM
int led1Pin = 3,
    led2Pin = 5,
    led3Pin = 9,
    led4Pin = 11;
void setup() {
  // put your setup code here, to run once:
  pinMode(led1Pin, OUTPUT);
  pinMode(led2Pin, OUTPUT);
  pinMode(led3Pin, OUTPUT);
  pinMode(led4Pin, OUTPUT);
}

void loop() {
  /* put your main code here, to run repeatedly:
  * PWM --> using digital signal to control analog signal (digital -> analog)
  * map(value, fromLow, fromHigh, toLow, toHigh). 
  * This function is used to remap a value, 
  * which will return a new value whose percentage in the range of 
  * toLow-toHigh is equal to the percentage of "value" 
  * in the range of fromLow-fromHigh.
  * fromLow - fromHigh is the duty cycle --> ratio of pulse width to period. 
  * voltage of analog signal also corresponds to duty cycle
  * toLow - toHigh is the range --> (0, 255)
  */ 
  analogWrite(led1Pin, map(2, 0, 100, 0, 255));
  analogWrite(led2Pin, map(10, 0, 100, 0, 255));
  analogWrite(led3Pin, map(50, 0, 100, 0, 255));
  analogWrite(led4Pin, map(100, 0, 100, 0, 255));
}