sizeof

The sizeof operator returns the number of bytes in a variable type, or the number of bytes occupied by an array.

sizeof(variable)

variable: any variable type or array (e.g. int, float, byte)

The sizeof operator is useful for dealing with arrays (such as strings) where it is convenient to be able to change the size of the array without breaking other parts of the program.

This program prints out a text string one character at a time. Try changing the text phrase.

char myStr[] = "this is a test";
int i;
 
void setup(){
  Serial.begin(9600);
}
 
void loop() { 
  for (i = 0; i < sizeof(myStr) - 1; i++){
    Serial.print(i, DEC);
    Serial.print(" = ");
    Serial.write(myStr[i]);
    Serial.println();
  }
  delay(5000); // slow down the program
}

Note that sizeof returns the total number of bytes. So for larger variable types such as ints, the for loop would look something like this. Note also that a properly formatted string ends with the NULL symbol, which has ASCII value 0.

for (i = 0; i < (sizeof(myInts)/sizeof(int)) - 1; i++) {
  // do something with myInts[i]
}

Source: arduino.cc