String Character Functions

The String functions charAt() and setCharAt() are used to get or set the value of a character at a given position in a String.

At their simplest, these functions help you search and replace a given character. For example, the following replaces the colon in a given String with an equals sign:

 String reportString = "SensorReading: 456";
 int colonPosition = reportString.indexOf(':');
 reportString.setCharAt(colonPosition, '='); 

Here's an example that checks to see if the first letter of the second word is 'B':

 String reportString = "Franklin, Benjamin";
 int spacePosition = reportString.indexOf(' ');
 if (reportString.charAt(spacePosition + 1) == 'B') {
    Serial.println("You might have found the Benjamins.")

Caution: If you try to get the charAt or try to setCharAt() a value that's longer than the String's length, you'll get unexpected results. If you're not sure, check to see that the position you want to set or get is less than the string's length using the length() function.

Hardware Required

  • Arduino or Genuino Board


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 charAt() and setCharAt()

  Examples of how to get and set characters of a String

  created 27 Jul 2010
  modified 2 Apr 2012
  by Tom Igoe

  This example code is in the public domain.

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

  Serial.println("\n\nString charAt() and setCharAt():");

void loop() {
  // make a String to report a sensor reading:
  String reportString = "SensorReading: 456";

  // the reading's most significant digit is at position 15 in the reportString:
  char mostSignificantDigit = reportString.charAt(15);

  String message = "Most significant digit of the sensor reading is: ";
  Serial.println(message + mostSignificantDigit);

  // add blank space:

  // you can also set the character of a String. Change the : to a = character
  reportString.setCharAt(13, '=');

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

See Also

Last revision 2015/08/11 by SM