Dart Lesson 7 of 102 2 min read

Numbers in Dart: int, double and num

Work with int, double and num in Dart: arithmetic, integer division, rounding, parsing text into numbers and the dart:math library.

Dart has two number types. int holds whole numbers and double holds numbers with a fractional part. Both are kinds of num.

Arithmetic #

void main() {
  var a = 17;
  var b = 5;

  print(a + b);  // addition
  print(a - b);  // subtraction
  print(a * b);  // multiplication
  print(a / b);  // division, always a double
  print(a ~/ b); // integer division
  print(a % b);  // remainder
}

22
12
85
3.4
3
2

The / operator always returns a double, even for 10 / 2, which gives 5.0. Use ~/ when you want a whole number.

int and double do not mix silently #

void main() {
  int whole = 7;
  double asDouble = whole.toDouble();
  int back = 7.9.toInt(); // cuts off the fraction

  print(asDouble);
  print(back);

  double price = 10; // an int literal is allowed where a double is expected
  print(price);
}

7.0
7
10.0

Rounding #

void main() {
  var n = 3.14159;

  print(n.round());            // nearest whole number
  print(n.floor());            // round down
  print(n.ceil());             // round up
  print(n.toStringAsFixed(2)); // text with 2 decimals
}

3
3
4
3.14

Useful properties and methods #

void main() {
  print(8.isEven);
  print(8.isOdd);
  print((-12).abs());
  print(15.clamp(0, 10));
  print(2.5.isNegative);
}

true
false
12
10
false

Turning text into numbers #

parse throws an error if the text is not a number. tryParse returns null instead, which is safer for anything typed by a user.

void main() {
  print(int.parse('42') + 1);
  print(double.parse('3.5') * 2);
  print(int.tryParse('forty'));
}

43
7.0
null

The dart:math library #

import 'dart:math';

void main() {
  print(sqrt(81));
  print(pow(2, 10));
  print(max(4, 9));
  print(pi.toStringAsFixed(4));

  var dice = Random().nextInt(6) + 1; // 1 to 6
  print(dice >= 1 && dice <= 6);
}

9.0
1024
9
3.1416
true

Things to know #

• Floating-point maths is not exact. 0.1 + 0.2 prints 0.30000000000000004. Compare doubles with a small tolerance, and store money as whole cents.
• On phones, desktops and servers an int is a 64-bit value. In a browser, numbers follow JavaScript rules, so very large integers lose precision.
• You can write big numbers with underscores for readability: 1_000_000.
• DartPad runs your code as JavaScript, where a double with no fraction prints without the .0. There 10 / 2 shows 5, while dart run on your computer shows 5.0. The value is the same.

Try it yourself #

Write a program that converts 500 minutes into hours and remaining minutes using ~/ and %. It should print 8 hours 20 minutes.