#Lecture 07 — Dart Data Types

We have used String, int, double, and bool without asking what they really are. A data type tells Dart what kind of value a variable holds — and what you are allowed to do with it.

Contents

Prerequisites · Connection · Objectives · Lesson · Code Examples · Common Mistakes · Best Practices · Mini Challenge · Practice Questions · Answers · Takeaways

Every error message in this lecture is real. Each one was produced by running the code shown on Dart 3.12.2, so you will recognize it when you meet it yourself.

#Prerequisites

Students should already understand, from Lectures 001–006:

• main(), print(), and string interpolation with $
• declaring, initializing, and assigning variables
• var and type inference
• final and const for values that must not change

#Connection With Previous Lectures

Since Lecture 004 we have written lines like these:

String name = 'Ali';
int age = 20;
double price = 99.5;
bool isStudent = true;

We know Dart rejects age = 'twenty'. But why does 20.5 not fit in an int? Why is '20' different from 20? Is there a type for any number? To choose types for real data, we need to know what each one is for.

#Learning Objectives

By the end of this lecture you will be able to:

• Explain what a data type is and why Dart needs one for every value
• Use int for whole numbers and double for decimal numbers
• Use num when a value may be either kind of number
• Use String for text and bool for yes/no values
• Recognize that the type decides which operations are allowed
• Name Dart's collection types List, Set, and Map, and the special value null
• Choose the right type for a piece of real-world data
• Recognize the error messages Dart produces when a value has the wrong type

#1. What Is a Data Type?

A data type describes a kind of value: whole number, decimal, text, or yes/no. Every value in Dart has a type, and every variable is fixed to one.

Analogy — think of a printed form. The "Age" box expects a number, the "Name" box letters, and the "Member?" box a tick. Writing "Sara" in the Age box makes no sense. Data types are those expectations, and Dart checks them for you.

#2. The Five Basic Types at a Glance

void main() {
  int age = 20;
  double height = 5.8;
  num weight = 62;
  String name = 'Zayan';
  bool isStudent = true;
  print('$name, $age, $height, $weight, $isStudent');
}

Zayan, 20, 5.8, 62, true

#3. int — Whole Numbers

An int holds a whole number with no decimal part. It can be positive, negative, or zero.

void main() {
  int students = 30;
  int goalDifference = -4;
  print(students);
  print(goalDifference);
}

30
-4

Use int for things you count: people, items, marks.

#4. double — Decimal Numbers

A double holds a number with a decimal part, and always prints one — even .0.

void main() {
  double price = 99.99;
  double distance = 10;
  print(price);
  print(distance);
}

99.99
10.0

Use double for things you measure: prices, weights, temperatures, distances.

#5. int and double Are Different Types

Both are numbers, but Dart keeps them apart. An int variable cannot hold a decimal:

void main() {
  int age = 20.5;
  print(age);
}

Error: A value of type 'double' can't be assigned to a variable of type 'int'.
  int age = 20.5;
            ^

The other direction has a catch. A whole number written directly into a double is fine (double distance = 10;), but an int variable is not:

void main() {
  int count = 5;
  double price = count;
  print(price);
}

Error: A value of type 'int' can't be assigned to a variable of type 'double'.
  double price = count;
                 ^

Turning an int into a double (and back) is called conversion — we will study it in Lecture 010.

#6. num — Either Kind of Number

num is the general number type. Both int and double are nums, so a num variable accepts either:

void main() {
  num price = 10;
  print(price);
  price = 10.5;
  print(price);
}

10
10.5

10 prints without .0 because the value inside is still the int 10. Use num only when a value really can be either kind. Numbers get a full lecture in Lecture 009.

#7. String — Text

A String holds text inside quotes. Single and double quotes both work.

void main() {
  String name = 'Sara';
  String city = "Lahore";
  String message = "It's raining";
  print(name);
  print(city);
  print(message);
}

Sara
Lahore
It's raining

Double quotes are handy when the text contains an apostrophe. Anything in quotes is text — even digits. '20' is a String, not a number:

void main() {
  String age = 20;
  print(age);
}

Error: A value of type 'int' can't be assigned to a variable of type 'String'.
  String age = 20;
               ^

Strings have many more features; the next lecture is all about them.

#8. bool — True or False

A bool (short for Boolean) holds exactly one of two values: true or false. Give it a yes/no name such as hasPaid or isOpen.

void main() {
  bool hasPaid = true;
  bool isOpen = false;
  print('Paid: $hasPaid');
  print('Open: $isOpen');
}

Paid: true
Open: false

true and false are written without quotes. Some languages treat 1 as true; Dart does not:

void main() {
  bool isOpen = 1;
  print(isOpen);
}

Error: A value of type 'int' can't be assigned to a variable of type 'bool'.
  bool isOpen = 1;
                ^

A bool usually comes from a comparison such as "is the mark at least 50?" — comparisons arrive in Lecture 018, and decisions with if in Lecture 022.

#9. The Type Decides What You Can Do

A type decides which operations make sense. + adds two numbers, but joins two strings:

void main() {
  print(20 + 5);
  print('Ali' + ' Khan');
}

25
Ali Khan

Mixing them is rejected:

void main() {
  print(20 + ' years');
}

Error: A value of type 'String' can't be assigned to a variable of type 'num'.
  print(20 + ' years');
             ^

Adding to a number needs another number (num). To mix text and numbers, use interpolation: '$age years'.

#10. Collections and null — A First Look

Dart also has three collection types for holding many values, plus a special value meaning "nothing":

void main() {
  var marks = [80, 75, 85];
  var names = {'Ali', 'Sara'};
  var scores = {'Ali': 80, 'Sara': 92};
  print(marks);
  print(names);
  print(scores);
}

[80, 75, 85]
{Ali, Sara}
{Ali: 80, Sara: 92}

For now, just recognize the names. Lists start in Lecture 039, Sets in Lecture 047, and Maps in Lecture 049. null — and why an int cannot hold it — is the subject of Null Safety, starting in Lecture 012.

#11. Choosing the Right Type

Ask what the value is and what you will do with it:

The phone number is the classic trap: 03001234567 looks like a number, but you never add two phone numbers, and the leading 0 matters. Store it as text.

#12. A Real-World Example

void main() {
  const double taxRate = 0.05;

  String customer = 'Sara';
  String phone = '03001234567';
  int quantity = 3;
  double price = 450.0;
  bool isMember = true;

  double subtotal = price * quantity;
  double tax = subtotal * taxRate;
  double total = subtotal + tax;

  print('Customer: $customer ($phone)');
  print('Member: $isMember');
  print('Items: $quantity');
  print('Total: $total');
}

Customer: Sara (03001234567)
Member: true
Items: 3
Total: 1417.5

Every value has the type that matches what it is: text, a count, money, or a yes/no. taxRate is const because it never changes (Lecture 006).

#Code Examples

#Example 1 — Whole Numbers

void main() {
  int apples = 12;
  int floor = -2;
  print(apples);
  print(floor);
}

12
-2

#Example 2 — Decimal Numbers

void main() {
  double weight = 62.5;
  double height = 6;
  print(weight);
  print(height);
}

62.5
6.0

#Example 3 — A num Holding Both Kinds

void main() {
  num reading = 7;
  print(reading);
  reading = 7.25;
  print(reading);
}

7
7.25

#Example 4 — Text in Both Quote Styles

void main() {
  String first = 'Zayan';
  String greeting = "Zayan's notebook";
  print(first);
  print(greeting);
}

Zayan
Zayan's notebook

#Example 5 — Yes/No Values

void main() {
  bool isRaining = false;
  bool hasTicket = true;
  print('Raining: $isRaining');
  print('Ticket: $hasTicket');
}

Raining: false
Ticket: true

#Example 6 — Same +, Different Types

void main() {
  int a = 10;
  int b = 20;
  String c = '10';
  String d = '20';
  print(a + b);
  print(c + d);
}

30
1020

How it works

1. a and b are int, so + adds them: 30.
2. c and d are String, so + joins them side by side: '1020'.
3. The digits look the same; the type decides the result.

#Example 7 — Student Record

void main() {
  String student = 'Ali';
  int marks = 412;
  double percentage = 82.4;
  bool passed = true;
  print('Student: $student');
  print('Marks: $marks');
  print('Percentage: $percentage');
  print('Passed: $passed');
}

Student: Ali
Marks: 412
Percentage: 82.4
Passed: true

How it works

1. Text is a String; counted marks are an int; the percentage may have decimals, so it is a double.
2. "Passed" has only two possible answers, so it is a bool.

#Example 8 — Shopping Total

void main() {
  String product = 'Notebook';
  int quantity = 4;
  double price = 120.5;
  double total = price * quantity;
  print('$quantity x $product');
  print('Total: $total');
}

4 x Notebook
Total: 482.0

How it works

1. double times int gives a double, so total must be a double.
2. 120.5 * 4 is 482.0, printed with its decimal part.

#Example 9 — Bank Balance

void main() {
  String owner = 'Sara';
  String accountNumber = '00451239';
  double balance = 2500.0;
  bool isActive = true;
  balance = balance - 750.25;
  print('Owner: $owner');
  print('Account: $accountNumber');
  print('Active: $isActive');
  print('Balance: $balance');
}

Owner: Sara
Account: 00451239
Active: true
Balance: 1749.75

How it works

1. accountNumber is a String: it is never calculated with, and its leading zeros matter.
2. balance is a double because money can have cents, so 1749.75 fits.

#Example 10 — Temperature Reading

void main() {
  String city = 'Karachi';
  int day = 3;
  num celsius = 31;
  bool isHot = true;
  print('Day $day in $city: $celsius°C');
  celsius = 29.5;
  print('Evening: $celsius°C');
  print('Hot day: $isHot');
}

Day 3 in Karachi: 31°C
Evening: 29.5°C
Hot day: true

How it works

1. celsius starts as the whole number 31 and later becomes 29.5. A num accepts both.
2. 31 prints without .0 because the value inside the num is an int.

#Common Beginner Mistakes

1. Putting a decimal into an int

void main() {
  // int age = 20.5; // ✗
}

Error: A value of type 'double' can't be assigned to a variable of type 'int'.

Use double when a value may have a decimal part.

2. Copying an int variable into a double

void main() {
  int count = 5;
  // double price = count; // ✗
}

Error: A value of type 'int' can't be assigned to a variable of type 'double'.

A whole number written directly is fine; an int variable is not. See Lecture 010.

3. Storing a number where text is expected

void main() {
  // String age = 20; // ✗
}

Error: A value of type 'int' can't be assigned to a variable of type 'String'.

Either the variable should be an int, or the value should be text: '20'.

4. Writing true in quotes, or using 1

void main() {
  // bool isOpen = 'true'; // ✗
  // bool isOpen = 1; // ✗
}

Error: A value of type 'String' can't be assigned to a variable of type 'bool'.
Error: A value of type 'int' can't be assigned to a variable of type 'bool'.

A bool accepts only true or false, with no quotes.

5. Adding text to a number

void main() {
  // print(20 + ' years'); // ✗
}

Error: A value of type 'String' can't be assigned to a variable of type 'num'.

Use interpolation instead: print('20 years'); or print('$age years');.

6. Storing a phone number as an int

You never calculate with a phone number, and an int drops its leading 0. Store phone, ID, and account numbers as String.

#Best Practices

• Choose the type from what the value is and what you will do with it.
• Use int for counts and double for measurements and money.
• Use num only when a value genuinely can be either kind of number.
• Store digits you never calculate with — phone numbers, IDs — as String.
• Give bool variables yes/no names: isMember, hasPaid, isOpen.
• Read type errors carefully — they name both the type you gave and the type expected.

#Mini Challenge

Store a library book's details: title 'Dart Basics', number of pages 320, price 850.5, and whether it is available (true). Choose the right type for each and print them.

Your program should print exactly:

Title: Dart Basics
Pages: 320
Price: 850.5
Available: true

Try it yourself before opening the solution.

void main() {
  String title = 'Dart Basics';
  int pages = 320;
  double price = 850.5;
  bool isAvailable = true;
  print('Title: $title');
  print('Pages: $pages');
  print('Price: $price');
  print('Available: $isAvailable');
}

Title: Dart Basics
Pages: 320
Price: 850.5
Available: true

#Practice Questions

Work through these in order — they run from beginner to application to challenge. Every question gives you exact values, so you can compare your output with the answers.

Beginner

1. Create an int named chairs holding 24 and a double named width holding 1.5. Print both, each on its own line.
2. Create a String named country holding 'Pakistan' and a bool named isSunny holding true. Print both, each on its own line.
3. Create a num named score holding 40, print it, change it to 40.5, and print it again.

Application

4. Choose a type for each: an age in years, a bag's weight in kilograms, an email address, whether a light is on. Store age = 25, weight = 3.2, email = 'ali@mail.com', and isOn = false with those types and print all four on one line, separated by commas.

5. Create String x = '5'; and String y = '7'; and print x + y. Then create int p = 5; and int q = 7; and print p + q. Predict both outputs before running.

6. This program does not compile. Change the type of one variable so it prints 72.5:

   dartCopy

   void main() {
     int weight = 72.5;
     print(weight);
   }

7. Store Zayan's ID card number '4210112345678' and his age 6 using suitable types. Print ID: 4210112345678 and Age: 6.

8. Store a product name 'Pen', quantity 10, and price 15.5. Calculate the total and print Pen total: 155.0.

Challenge

9. A student has marks 78, 85, and 91 out of 300. Store the marks as int, calculate the total, then the percentage (total * 100 / 300), and store whether the student passed as bool passed = true;. Print Total: 254, Percentage: 84.66666666666667, and Passed: true.

10. This program does not compile. Fix it without changing any variable's type, so that it prints Ali is 20 years old:

    dartCopy

    void main() {
      String name = 'Ali';
      int age = 20;
      print(name + ' is ' + age + ' years old');
    }

#Practice Answers

#Question 1 — Solution

void main() {
  int chairs = 24;
  double width = 1.5;
  print(chairs);
  print(width);
}

24
1.5

#Question 2 — Solution

void main() {
  String country = 'Pakistan';
  bool isSunny = true;
  print(country);
  print(isSunny);
}

Pakistan
true

#Question 3 — Solution

void main() {
  num score = 40;
  print(score);
  score = 40.5;
  print(score);
}

40
40.5

#Question 4 — Solution

Age is counted (int), weight is measured (double), an email is text (String), and on/off is yes/no (bool).

void main() {
  int age = 25;
  double weight = 3.2;
  String email = 'ali@mail.com';
  bool isOn = false;
  print('$age, $weight, $email, $isOn');
}

25, 3.2, ali@mail.com, false

#Question 5 — Solution

void main() {
  String x = '5';
  String y = '7';
  print(x + y);
  int p = 5;
  int q = 7;
  print(p + q);
}

57
12

For strings, + joins '5' and '7' into '57'. For integers, it adds them to 12.

#Question 6 — Solution

void main() {
  double weight = 72.5;
  print(weight);
}

72.5

#Question 7 — Solution

void main() {
  String idNumber = '4210112345678';
  int age = 6;
  print('ID: $idNumber');
  print('Age: $age');
}

ID: 4210112345678
Age: 6

The ID card number is a String because it is never calculated with.

#Question 8 — Solution

void main() {
  String product = 'Pen';
  int quantity = 10;
  double price = 15.5;
  double total = price * quantity;
  print('$product total: $total');
}

Pen total: 155.0

#Question 9 — Solution

void main() {
  int math = 78;
  int science = 85;
  int english = 91;
  int total = math + science + english;
  double percentage = total * 100 / 300;
  bool passed = true;
  print('Total: $total');
  print('Percentage: $percentage');
  print('Passed: $passed');
}

Total: 254
Percentage: 84.66666666666667
Passed: true

Division with / always gives a double, so percentage must be a double. Rounding long decimals is covered in Lecture 009.

#Question 10 — Solution

void main() {
  String name = 'Ali';
  int age = 20;
  print('$name is $age years old');
}

Ali is 20 years old

The original adds an int to a String, which Dart rejects. Interpolation puts age into the text without changing any type.

#Quick Revision

1. What is a data type?
2. Which type holds whole numbers, and which holds decimals?
3. Why does double distance = 10; print 10.0?
4. What kind of value can a num variable hold?
5. Is '42' a number or text?
6. What are the only two bool values?
7. What does + do with two strings?
8. Why should a phone number be a String?

#Key Takeaways

• A data type describes what kind of value something is, and what you can do with it.
• int holds whole numbers; double holds decimal numbers and always prints a decimal part.
• int and double are different types; copying one into the other needs conversion.
• num accepts both int and double; use it only when either is possible.
• String holds text in single or double quotes — digits in quotes are text.
• bool holds only true or false, without quotes.
• The type decides the operation: + adds numbers but joins strings.
• List, Set, Map, and null exist too — each gets its own lectures later.

#What's Next?

Lecture 08 — Strings. Text is everywhere: names, messages, receipts. Now that you know String is Dart's type for text, the next lecture explores it properly — quotes and special characters, multi-line text, interpolation with expressions, and everyday string tools.