Lesson 6

What comes next?

We will continue to learn the basics of Dart and start to learn more complex topics.

Step 3: Math

Inside Dart, we can also perform math with the following operators:

+ — addition
- — subtraction
* — multiplication
/ — division
% — modulo (the remainder of a division)
++ — add by 1
-- — subtract by 1

Let’s try some of these out!

An example of using addition is the following:


void main() {
  int number = 4 + 7;
  print(number);
}

The program will print the number 11 since 4 + 7 is 11.

It works the same way for all the other basic operations.

Using `++` and `--`

Let’s try the ++ and -- operators since they are a little different.


void main() {
  int number = 4;
  number++;
  number++;
  print(number);
}

Before running the program, predict what will happen.

You should have gotten 6.

Why did we get 6?

That is because the variable number started at 4 and was increased by 1 twice.

So:

4 + 1 + 1 = 6

The -- operator works the same way, except it subtracts 1 instead.

Using math with variables

What we can also do is perform these math operations with variables.

Here is an example:


void main() {
  int number1 = 4;
  int number2 = 6;
  int number3 = number1 * number2;
  print(number3);
}

The result of this program is 24.

This is because number3 is assigned the result of multiplying the values of number1 and number2.

So:

4 * 6 = 24

Step 4: Adding Different Variable Types

What do you do when you want to use the print() statement and print both an int and a String at the same time?

Let’s try it first:


void main() {
  String number1 = "four";
  int number2 = 6;
  print(number1 + number2);
}

You may use the example code above, or make your own code.

Did you get an error?

We can’t combine Strings and ints directly like we would with the same data types.

Well, there are 2 main ways to do this.

Method 1: `.toString()`

Let’s look at the error message:

A value of type ‘int’ can’t be assigned to a variable of type ‘String’.

It is telling us that the int value cannot be used where a String is expected.

Can we change the int into a String?

Yes!

Luckily, Dart has a very simple feature we can use to convert your variable into a String. The function is called .toString().

Let’s try to use that.

Here is an example of how we would use the function:


void main() {
  String number1 = "four";
  int number2 = 6;
  print(number1 + number2.toString());
}

Now our code runs successfully!

number2 was originally an int, but .toString() converted it into a String.

Method 2: String Interpolation

Let’s look at the second way now.

Instead of using a new function, in this method we will use something called string interpolation.

String interpolation uses a $ sign to insert a variable directly into a String.

Did that not make much sense?

Let’s look at how it is used to get a better understanding:


void main() {
  String number1 = "four";
  int number2 = 6;
  print("$number1$number2");
}

Try running this code.

Notice how we get the same result as before?

That is because $ works similarly to the .toString() function by allowing us to use the value of a variable as part of a String.

However, there is an important difference: string interpolation must be inside quotation marks.

It also works with all variable types, including String variables.

You can see this with the variable number1.

Even though it is already a String, there is still a $ sign in front of it.

What happens if you don’t use the $?

Let’s try it:


void main() {
  String number1 = "four";
  int number2 = 6;
  print("number1$number2");
}

We get:

number16

Why is that?

That is because without the $ sign, the compiler thinks number1 is just regular text.

So it combines the text "number1" with the value 6, resulting in: