Dart double

Summary: in this tutorial, you’ll learn about the Dart double type that represents double-precision floating-point numbers.

Introduction to Dart double type

Dart uses double type to represent double-precision floating-point numbers. For example, the following declares a variable with the double type:

void main() {
  double price = 9.99;
  print(price);
}Code language: Dart (dart)

Output:

9.99Code language: Dart (dart)

Comparing doubles

Since computers can only represent floating-point numbers approximately, comparing them may lead to an expected result. Consider the following example:

void main() {
  double x = 0.3;
  double y = 0.1 + 0.1 + 0.1;
  bool isEqual = x == y;

  print("$x == $y -> $isEqual ");
}Code language: Dart (dart)

In this example, you may expect x to be equal to y but the result is different:

0.3 == 0.30000000000000004 -> falseCode language: Dart (dart)

The reason is that the 0.1 + 0.1 + 0.1 is not 0.3 but 0.30000000000000004. This is because internally Dart only can represent the number 0.1 approximately.

It’s important to note that this problem is not Dart’s specific. It illustrates how the computers represent floating-point numbers internally.

Converting a string into a double

To convert a string to a double, you use the double.parse() method. For example:

void main() {
  String priceStr = "1.55";
  double price = double.parse(priceStr);

  print(price);
}Code language: Dart (dart)

In this example, we convert the string "1.55" to a double. The double.parse() will throw an exception if the string cannot be converted into a double:

void main() {
  String priceStr = "1.55x";
  double price = double.parse(priceStr);

  print(price);
}Code language: Dart (dart)

Error:

Unhandled exception:
FormatException: Invalid double
1.55xCode language: Dart (dart)

In this example, the double.parse() couldn’t convert the string “1.55x” to a double. Therefore, it raised an exception.

Convert an integer into a double

To convert an integer into a double, you use the int.toDouble() like the following example:

void main() {
  int qty = 10;
  double totalQty = qty.toDouble();

  print(totalQty);
}Code language: Dart (dart)

How it works.

First, declare the qty variable as an integer and initialize its value to 10.

int qty = 10;Code language: Dart (dart)

Second, convert the qty into a double and assign its value to totalQty variable:

double totalQty = qty.toDouble();Code language: Dart (dart)

Third, display the totalQty variable:

print(totalQty);Code language: Dart (dart)

Summary

• Use double type to represent double-precision floating-point numbers.
• Use double.parse() to convert a string into a double.
• Use int.toDouble() to convert an integer into a double.