#Lecture 006 — Declaring & Initializing Variables

A variable is a named box that holds a value. We create boxes (declare), put a first value in (initialize), change the value, and learn the naming rules and keywords.

Contents

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

#Prerequisites

Lectures 004–005: main, print and comments.

#Learning Objectives

• Tell declaring a variable from initializing it
• Declare with a type or with var
• Change a variable's value
• Put variables inside strings with $
• Follow Dart's naming rules and avoid keywords

#1. Declare and Initialize

String city = 'Lahore';

Read it as: type String, name city, =, value 'Lahore'.

• Declaring creates the box: String city.
• Initializing puts the first value in: = 'Lahore'.

You can do both at once (above) or in two steps:

int age;       // declared
age = 21;      // initialized
print(age);    // 21

Dart won't let you read a variable before it has a value — that is caught before the program runs.

#2. var — Let Dart Work Out the Type

var city = 'Lahore';   // Dart sees a String
var students = 30;     // Dart sees an int

The type is still fixed — var only saves you typing it:

var students = 30;
students = 31;        // fine: still an int
// students = 'many'; // error: a String can't go in an int variable

#3. Changing a Value

int score = 10;
score = 15;
score = score + 5;   // now 20

Write the type only the first time. After that, just the name.

#4. Variables in Text

var name = 'Bilal';
var age = 19;
print('$name is $age years old.');
print('Next year: ${age + 1}');

$name puts a variable in a string; ${...} puts a whole expression in. Lecture 009 covers this in depth.

#5. Naming Rules

• Letters, digits, _ and $ only; can't start with a digit — 2nd ✗, second ✓.
• No spaces — total price ✗, totalPrice ✓.
• Case matters — age and Age are different variables.
• Style: lowerCamelCase — totalPrice, isLoggedIn.

#6. Keywords

Keywords are words Dart has reserved for itself, so they can't be variable names:

// var class = 'A';     // error: class is a keyword
var className = 'A';    // fine

#Code Examples

#Example 1 — A Student Card

void main() {
  String name = 'Zainab';
  int rollNumber;
  rollNumber = 42;          // initialized later
  var course = 'Dart Basic';
  var day = 3;

  print('Name: $name');
  print('Roll number: $rollNumber');
  print('$course — Day $day');

  day = day + 1;            // a new day
  print('Tomorrow is Day $day');
}

Output

Name: Zainab
Roll number: 42
Dart Basic — Day 3
Tomorrow is Day 4

#Common Beginner Mistakes

1. Using a variable before giving it a value — int x; print(x); gives "The non-nullable local variable 'x' must be assigned before it can be used."

2. Re-declaring — writing int score = 15; a second time is an error. Write score = 15;.

3. Expecting var to accept any type later — it locks in the first type it sees.

4. Using a keyword as a name — var if = 1; doesn't compile.

#Practice Questions

1. What is the difference between declaring and initializing?
2. What type does var total = 9.99; have?
3. Which of these names are valid: myAge, 2cars, first_name, class?
4. Print "Ali has 3 books" using two variables.

#Practice Answers

1. Declaring creates the variable; initializing gives it its first value.
2. double.
3. myAge and first_name (though firstName is Dart style). 2cars starts with a digit; class is a keyword.
4. var name = 'Ali'; var books = 3; print('$name has $books books');

#Key Takeaways

• Declare = create the box; initialize = give it its first value.
• var infers the type once; it can't change type later.
• $name and ${expression} put values into strings.
• Names: lowerCamelCase, no leading digit, no keywords.