Convert a String to an Integer

The Arduino is a fantastic little piece of hardware. The little Atmega microcontroller under the hood is capable of some amazing stuff! In this tutorial, we’re going to learn how to convert a string to an Integer on the Arduino! And in the case that you are using a character array, we’ll also learn how to covert those to an integer!

Sometimes we get data input from a text box on a web page, from a database, or even over the I2C bus from a Raspberry Pi, or some other device. That data is often a number, but delivered to us as a string. When that happens, we will need to convert it to an integer in order to be able to use it for computational uses.

You cannot perform arithmetic on a string. So let’s learn how to deal with this problem!

Let’s start with converting a string to an Integer. To do this, we use .toInt() method of the String() function. The code works like this:

myInt = myString.toInt();

Used in a full sketch, we will convert a string to an integer, and then add 1 to it every second.

/*
 *  CONVERTING A STRING TO AN INTEGER: STRING.TOINT()
 *  By: TheGeekPub.com
 *  More Arduino Tutorials: https://www.thegeekpub.com/arduino-tutorials/
 */

String myString = "12345"; //create a string a populate it with "12345"
int myInt; // our integer

void setup() {
  Serial.begin(9600);  //turn on serial console
  myInt = myString.toInt();  //convert string to int
  Serial.println(myInt);  //print the integer
}

void loop() {
  //this loop will add 1 to our integer every second and show in on the screen
  delay(1000);
  myInt++;
  Serial.print("Adding +1: ");
  Serial.println(myInt);
}

On the serial console you should see the following output:

Convert a Character Array to an Integer

Sometimes instead of converting a string to an integer, we will need to convert a character array (char array) to an integer. That can be done using the atoi() function. It stands for “Array to Integer”.

The code looks like this at its basics.

myInt = atoi(myString);

Updating our previous string sketch, the character array sketch would look like this. Same fire drill. We convert the character array to an integer, and then our loop() will add +1 to the number every second.

/*
 *  CONVERTING A CHARACTER ARRAY TO AN INTEGER: ATOI()
 *  By: TheGeekPub.com
 *  More Arduino Tutorials: https://www.thegeekpub.com/arduino-tutorials/
 */

char myString[6] = "12345"; //create an array a populate it with "12345"
int myInt; // our integer

void setup() {
  Serial.begin(9600);  //turn on serial console
  myInt = atoi(myString);  //convert char array to int
  Serial.println(myInt);  //print the integer
}

void loop() {
  //this loop will add 1 to our integer every second and show in on the screen
  delay(1000);
  myInt++;
  Serial.print("Adding +1: ");
  Serial.println(myInt);
}

On the Serial Console you should see the following output: