Read from analog then cast not showing the correct value

Question

I'm trying to read the analog value of the light sensor and print its percentage but for unknown reasons I'm getting negative values when I'm printing the value.

void setup() {
 Serial.begin(9600);
}
void loop() {
 // Let's say 1002 is the analogValue
 int analogValue = (int) analogRead(0);
 // get the percentage
 int value = 100 * analogValue / 1023;
 Serial.println("~~~~~~~~~~~~~~~~~~~~~~~~~");
 Serial.println(analogValue); // print the analog value 0...1023
 Serial.println(100 * analogValue); // just for test
 Serial.println(value); // The percentage
 Serial.println("~~~~~~~~~~~~~~~~~~~~~~~~~");
 delay(500);
}

I don't know what's going in background, but I'm getting negative values when I try to print the percentage:

~~~~~~~~~~~~~~~~~~~~~~~~~
1002
-30772
-30
~~~~~~~~~~~~~~~~~~~~~~~~~

Answer 1

Max value for int is 32,767. But 100 * 1002 = 100,200. You are overflowing the int. Use unsigned long. Or float and cast to int when printing.

• Edgar Bonet: Simplest would be to replace 100 by 100L.
• 001: Yes, but OP would have to be sure to do that on both lines - the original calculation as well as the Serial.println line (even though it's "just for test"). Either way works, though.