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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.

Appending to Strings using the += operator and concat() by Tom Igoe
Examples of how to append different data types to strings

    String stringOne, stringTwo;

    void setup()
    {
      Serial.begin(9600);
      stringOne = String("Sensor ");
      stringTwo = String("value");
      Serial.println("\n\nAppending to a string:");
    }

    void loop()
    {
      Serial.println(stringOne);  // prints  "Sensor "

      // adding a string to a string:
      stringOne += stringTwo;
      Serial.println(stringOne);  // prints "Sensor value"

      // adding a constant string to a string:
      stringOne += " for input ";
      Serial.println(stringOne);  // prints "Sensor value for input"

      // adding a constant character to a string:
      stringOne += 'A';
      Serial.println(stringOne);   // prints "Sensor value for input A"

      // adding a constant integer to a string:
      stringOne += 0;
      Serial.println(stringOne);   // prints "Sensor value for input A0"

      // adding a constant string to a string:
      stringOne += ": ";
      Serial.println(stringOne);  // prints "Sensor value for input"

      // adding a variable integer to a string:
      stringOne += analogRead(A0);
      Serial.println(stringOne);   // prints "Sensor value for input A0: 456" or whatever analogRead(A0) is

      Serial.println("\n\nchanging the Strings' values");
      stringOne = "A long integer: ";
      stringTwo = "The millis(): ";

      // adding a constant long integer to a string:
      stringOne += 123456789;
      Serial.println(stringOne);   // prints "A long integer: 123456789"

      // using concat() to add a long variable to a string:
      stringTwo.concat(millis());
      Serial.println(stringTwo); // prints "The millis(): 43534" or whatever the value of the millis() is

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