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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 length() by Tom Igoe
Examples of how to use length() in a String. Open the Serial Monitor and start sending characters to see the results.

    String txtMsg = "";                         // a string for incoming text
    int lastStringLength = txtMsg.length();     // previous length of the String

    void setup()
    {
      // open the serial port:
      Serial.begin(9600);
    }

    void loop()
    {
      // add any incoming characters to the String:
      while (Serial.available() > 0)
      {
        char inChar = Serial.read();
        txtMsg += inChar;
      }

      // print the message and a notice if it's changed:
      if (txtMsg.length() != lastStringLength)
      {
        Serial.println(txtMsg);
        Serial.println(txtMsg.length());
        // if the String's longer than 140 characters, complain:
        if (txtMsg.length() < 140)
        {
          Serial.println("That's a perfectly acceptable text message");
        }
        else
        {
          Serial.println("That's too long for a text message.");
        }
        // note the length for next time through the loop:
        lastStringLength = txtMsg.length();
      }
    }