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.

Relevant Groundwork

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

  • MSP-EXP430G2 LaunchPad


Only LaunchPad is required for this example.

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


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Code Explanation

In the program, we take pull a character from one string and append it to the end of another string. We then replace the character in position 13 of the first string with an '='.


  String charAt() and setCharAt()

 Examples of how to get and set characters of a String

 created 27 July 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:

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

  // add blank space:

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

  // do nothing while true:

Working Video

Try it out

  • Set one string as all the letter of the alphabet, pull out the vowels, and print to serial.

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