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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 toInt() by Tom Igoe
Reads a serial input string until it sees a newline, then converts the string to a number if the characters are digits.

    String inString = "";    // string to hold input

    void setup()
    {
      // Initialize serial communications:
      Serial.begin(9600);
    }

    void loop()
    {
      // Read serial input:
      while (Serial.available() > 0)
      {
        int inChar = Serial.read();
        if (isDigit(inChar))
        {
          // convert the incoming byte to a char
          // and add it to the string:
          inString += (char)inChar;
        }
        // if you get a newline, print the string,
        // then the string's value:
        if (inChar == '\n')
        {
          Serial.print("Value:");
          Serial.println(inString.toInt());
          Serial.print("String: ");
          Serial.println(inString);
          // clear the string for new input:
          inString = "";
        }
      }
    }