Serial.print()

Projects Programming
8

I tried the following example but could not see the output on serial monitor:
(just pasting the relevant lines)

 void setup()
 {
   Serial.begin(9600);
   attachInterrupt(0, rpm_fun, RISING);

  }

 void loop()
 {
   if (half_revolutions >= 20) { 
     rpm = 30*1000/(millis() - timeold)*half_revolutions;
     timeold = millis();
     half_revolutions = 0;
     Serial.println(rpm,DEC);
   }
 }

 void rpm_fun()
 {
   half_revolutions++;
   //Each rotation, this interrupt function is run twice
 }