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.