Variables & Data Types — ToolScope Notes

Free PDF of this lesson — save it, print it, revise offline. The video is the same lesson on the Code Scope YouTube channel.

Introduction

An app is data at work: names, marks, prices, yes-or-no answers. Dart keeps every piece of data in a variable — a labelled box — and every box has a type that says what kind of thing may go inside. Get variables right and every later lesson (loops, functions, classes, Flutter screens) becomes easy, because all of them move data in and out of these boxes. (Roman Urdu: Variable aik label wala dabba hai — type batata hai ke dabbe mein kis qisam ki cheez jayegi.)

Key points

The four types you will use daily:
- int — whole numbers: int age = 20;
- double — decimal numbers: double height = 5.9;
- String — text, always in quotes: String name = 'Ali';
- bool — only true or false: bool isStudent = true;
You declare a variable by writing its type, a name, =, and the value: int marks = 92;. After that, use just the name — print(marks); shows the value, while print('marks'); shows the word "marks". That one difference (quotes or no quotes) decides everything.(Roman Urdu: Naam baghair quotes ke likho to dabbe ki value aati hai; quotes ke sath wohi lafz chhap jata hai.)
var lets Dart guess the type from the first value: var city = 'Sahiwal'; becomes a String forever. After that, city = 5; is an error — a box never changes its type.
final = the value is set once and never changes, but it can be decided when the program runs (for example, from user input). const = the value must be known before the program runs (like const pi = 3.14159;).
String interpolation puts values inside text: $name for a simple variable, ${maths + urdu} when a small calculation is needed inside the text. Use this instead of joining pieces with +.
Variable names use lowerCamelCase: first word small, next words capitalised — studentName, totalMarks. Names must describe the thing (marks yes, x no), because you will read this code again after a month.

Code examples

Example 1 — The four types in one program

void main() {
  String name = 'Ali';
  int age = 20;
  double height = 5.9;
  bool isStudent = true;

  print(name);
  print(age);
  print(height);
  print(isStudent);
}

What it does: Four boxes, one of each type, each printed on its own line. Notice the values wear no quotes in the code except the String — text is the only type that needs them.

What you will see: four lines — Ali, 20, 5.9, true. Try int age = 'twenty'; and Dart refuses to run: a word cannot live in a number box. That strictness is Dart protecting you, and it is called type safety.

Example 2 — Interpolation: values inside a sentence

void main() {
  String name = 'Sara';
  int maths = 92;
  int urdu = 85;

  print('My name is $name');
  print('Total marks: ${maths + urdu}');
}

What it does: $name drops the variable's value straight into the sentence. ${ ... } runs a small calculation first (92 + 85) and drops the answer in. No + joining, no commas — the sentence reads like normal English.

What you will see: two lines — My name is Sara and Total marks: 177. If you wrote 'My name is name' (no $), the console would show the word "name" — the $ is what makes it a value, not a word.

Example 3 — var, final and const together

void main() {
  var city = 'Sahiwal'; // Dart khud samjha: yeh String hai
  city = 'Multan'; // OK — ab bhi String hi hai
  print(city);

  final country = 'Pakistan'; // aik dafa set, phir change nahi
  const pi = 3.14159; // pehle se maloom value

  print(country);
  print(pi);
}

What it does: var guesses String from the first value, so a new city name is fine but a number would be an error. final locks the country after one setting. const locks pi at the value everyone already knows.

What you will see: three lines — Multan, Pakistan, 3.14159. Now try adding country = 'India'; — Dart stops you before running, because final was a promise. In app code, that promise prevents a whole family of bugs.

Quick recap

Variable = labelled box; type = what may live in the box: int, double, String, bool.
print(marks) shows the value; print('marks') shows the word. Quotes decide.
var = Dart guesses the type once; it never changes after that.
final = set once at run time. const = known before run time.
$name and ${calculation} = interpolation — values inside sentences.

Common mistakes

Putting a value in the wrong type of box — int age = '20'; looks harmless but the quotes make it a String, and Dart errors. Numbers do not wear quotes; text always does.
Changing a var to another type later — var score = 10; then score = 'ten'; is an error. var is not "any type forever"; it is "type decided on line one."
Joining sentences with + instead of interpolation — print('Name: ' + name + ', Age: ' + age); even fails when age is an int, and is hard to read when it works. Write print('Name: $name, Age: $age'); and let Dart do the joining.

Practice

Make variables for a shop item: name (String), price (int), discount percent (double), in stock (bool). Print all four with labels, using interpolation in one sentence each.
Write a program with final for your university name and const for the days in a week. Then try to change both — read Dart's two different error messages and note which keyword stopped you.