The toInt() function allows you to convert a String to an integer number.

In this example, the board reads a serial input string until it sees a newline, then converts the string to a number if the characters are digits. Once you've uploaded the code to your board, open the Arduino IDE serial monitor, enter some numbers, and press send. The board will repeat these numbers back to you. Observe what happens when a non-numeric character is sent.

There is no circuit for this example, though your board must be connected to your computer via USB and the serial monitor window of the Arduino Software (IDE) should be open.

image developed using Fritzing. For more circuit examples, see the Fritzing project page

/*
  String to Integer conversion
 
 Reads a serial input string until it sees a newline, then converts
 the string to a number if the characters are digits.
 
 The circuit:
 No external components needed.
 
 created 29 Nov 2010
 by Tom Igoe
 
 This example code is in the public domain.
 */
 
String inString = "";    // string to hold input
 
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 toInt():");
  Serial.println();
}
 
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 = "";
    }
  }
}