Arduino/build/shared/examples/08.Strings/StringLength/StringLength.ino

/*
  String length()

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

 created 1 Aug 2010
 by Tom Igoe

 http://www.arduino.cc/en/Tutorial/StringLengthTrim

 This example code is in the public domain.
 */

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

void setup() {
  // Open serial communications and wait for port to open:
  Serial.begin(9600);
  while (!Serial) {
    ; // wait for serial port to connect. Needed for Leonardo only
  }

  // send an intro:
  Serial.println("\n\nString  length():");
  Serial.println();
}

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();
  }
}