String Addition Operator

You can add Strings together in a variety of ways. This is called concatenation and it results in the original String being longer by the length of the String or character array with which you concatenate it. The + operator allows you to combine a String with another String, with a constant character array, an ASCII representation of a constant or variable number, or a constant character.

 // adding a constant integer to a string:
  stringThree =  stringOne + 123;

  // adding a constant long interger to a string:
  stringThree = stringOne + 123456789;

  // adding a constant character to a string:
  stringThree =  stringOne + 'A';

  // adding a constant string to a string:
  stringThree =  stringOne +  "abc";

  // adding two Strings together:
  stringThree = stringOne + stringTwo;

You can also use the + operator to add the results of a function to a String, if the function returns one of the allowed data types mentioned above. For example,

  stringThree = stringOne + millis();

This is allowable since the millis() function returns a long integer, which can be added to a String. You could also do this:

  stringThree = stringOne + analogRead(A0);

because analogRead() returns an integer. String concatenation can be very useful when you need to display a combination of values and the descriptions of those values into one String to display via serial communication, on an LCD display, over an Ethernet connection, or anywhere that Strings are useful.

Caution: You should be careful about concatenating multiple variable types on the same line, as you may get unexpected results. For example:

 int sensorValue = analogRead(A0); 
  String stringOne = "Sensor value: ";
  String stringThree = stringOne + sensorValue;
  Serial.println(stringThree);  

results in "Sensor Value: 402" or whatever the analogRead() result is, but

  int sensorValue = analogRead(A0); 
  String stringThree = "Sensor value: " + sensorValue;
  Serial.println(stringThree);  

gives unpredictable results because stringThree never got an initial value before you started concatenating different data types.

Here's another example where improper initialization will cause errors:

Serial.println("I want " + analogRead(A0) + " donuts");  

This won't compile because the compiler doesn't handle the operator precedence correctly. On the other hand, the following will compile, but it won't run as expected:

  int sensorValue = analogRead(A0); 
  String stringThree = "I want " + sensorValue;
  Serial.println(stringThree  + " donuts"); 

It doesn't run correctly for the same reason as before: stringThree never got an initial value before you started concatenating different data types.

For best results, initialize your Strings before you concatenate them.

Step 1: What You Need?

1 x Arduino Board ( Arduino UNO R3 used in this tutorial.)
1 x USB Type-B Cable 


Optional 

Arduino Uno R3 Acrylic Transparent Casing

Arduino Compatible UNO Ultimate Starter Kit / Learning Kit

Don't have components? Don't worry. Just click the component's name. 

Step 2: Build Your Circuit.

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.

Step 3: Upload The Code.

1. Select the Arduino board type: Select Tools >> Board >> Select your correct Arduino board used.

2. Find the port number by accessing device manager on Windows. See the section Port (COM&LPT) and look for an open port named "Arduino Uno (COMxx)". If you are using a different board, you will find a name accordingly. What matters is the xx in COMxx part. In my case, it's COM3. So my port number is 3.

Select the right port: Tools >> Port >> Select the port number.

3. You can find this code in the example of Arduino IDE.

Select File >> Examples >> 08. Strings >> StringAdditionOperator

Click press the "upload" button (see the button with right arrow mark).

Here's a working example of several different concatenation examples :

 

Download:

Arduino IDE

Sources:https://www.arduino.cc/en/Tutorial/StringAdditionOperator

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Author & Editor

My Robot Education Sdn. Bhd. (Robotedu.my) was founded in 2015 as the first robotics education centre in Malaysia to provide Arduino-based robotics courses for youths. Our vision is to be able to provide robotics education to every youth in Malaysia.