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This example is for Wiring version 1.0 build 0100+. If you have a previous version, use the examples included with your software. If you see any errors or have comments, please let us know.

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);
    }