#Lecture 007 — Data Types

Every value in Dart has a type: whole numbers, decimals, text, true/false, collections. We meet the built-in types and learn how to check a value's type.

Contents

Prerequisites · Objectives · Lesson · Code Examples · Common Mistakes · Practice Questions · Answers · Takeaways

#Prerequisites

Lecture 006: declaring variables with a type or var.

#Learning Objectives

• Use int, double, num, String and bool
• Recognise List and Map
• Know what dynamic and Object are, and why to avoid dynamic
• Check a type with is and runtimeType
• Convert between types

#1. Numbers: int, double and num

int students = 30;        // whole numbers
double price = 4.5;       // numbers with a decimal point
num anything = 7;         // either an int or a double
anything = 7.5;           // still fine

double d = 3;   // fine: Dart stores 3.0
// int i = 3.5; // error: a double isn't an int

#2. Text: String

String greeting = 'Hello';
String quote = "It's Day 3";   // double quotes when the text has an apostrophe

Strings get their own lecture — Lecture 009.

#3. True or False: bool

bool isOpen = true;
bool hasPaid = false;

Only true and false — Dart never treats 0 or '' as false.

#4. Collections: List and Map

List<String> fruits = ['apple', 'mango', 'banana'];
Map<String, int> ages = {'Ali': 21, 'Sara': 19};

A List is an ordered row of values; a Map pairs keys with values. Day 5 covers both.

#5. dynamic and Object

dynamic box = 5;
box = 'now text';   // allowed: dynamic switches off type checking

Object thing = 5;
thing = 'text';     // also allowed: everything is an Object

The difference: with dynamic, Dart lets you call anything on it and only fails when the program runs. With Object, Dart checks first. Prefer a real type, then Object, and dynamic only when you truly must.

#6. Checking a Type

var value = 42;
print(value is int);        // true
print(value is String);     // false
print(value.runtimeType);   // int

#7. Converting Between Types

#Code Examples

#Example 1 — A Product Record

void main() {
  String product = 'Notebook';
  int stock = 25;
  double price = 120.5;
  bool onSale = true;
  List<String> colors = ['red', 'blue'];
  Map<String, int> sold = {'Monday': 4, 'Tuesday': 7};

  print('$product: $stock in stock at $price');
  print('On sale: $onSale');
  print('Colors: $colors');
  print('Sold: $sold');

  var total = stock * price;
  print('Stock value: $total (${total.runtimeType})');
  print('Rounded: ${total.round()}');
}

Output

Notebook: 25 in stock at 120.5
On sale: true
Colors: [red, blue]
Sold: {Monday: 4, Tuesday: 7}
Stock value: 3012.5 (double)
Rounded: 3013

An int times a double gives a double.

#Common Beginner Mistakes

1. Putting a decimal in an int — int price = 4.5; is an error. Use double.

2. Numbers in quotes — '25' is a String, not a number. '25' + 1 doesn't work; use int.parse('25') + 1.

3. Reaching for dynamic — it hides mistakes until the program crashes. Use real types.

#Practice Questions

1. Which type holds 3.14? Which holds 3?
2. Can a num variable hold both? Can an int?
3. What does print(7.8.toInt()); print?
4. How do you check whether x is a String?

#Practice Answers

1. double; int.
2. Yes; no — an int can't hold a double.
3. 7.
4. x is String.

#Key Takeaways

• int, double (and num for either), String, bool, List, Map.
• dynamic turns off type checks — avoid it; Object keeps them.
• is tests a type; runtimeType names it.
• Convert with int.parse, toString, toInt, round, toDouble.