String charAt() and setCharAt() by Tom Igoe

Examples of how to get and set characters of a String

void setup()
{
  Serial.begin(9600);
  Serial.println("\n\nString  charAt() and setCharAt():");
}

void loop()
{
  // make a string to report a sensor reading:
  String reportString = "SensorReading: 456";
  Serial.println(reportString);

  // the reading's most significant digit is at position 15 in the reportString:
  String mostSignificantDigit = (String) reportString.charAt(15);
  Serial.println("Most significant digit of the sensor reading is: " + mostSignificantDigit);

  // add blank space:
  Serial.println();

  // you can also set the character of a string. Change the : to a = character
  reportString.setCharAt(13, '=');
  Serial.println(reportString);

  // do nothing while true:
  while (true);
}