Data types (int, double, String, bool) in Flutter

Introduction

Data types help us tell the app what kind of information we want to store. This makes the app work correctly and avoid mistakes.

When you want to store whole numbers like age or count.

When you need numbers with decimals like price or temperature.

When you want to store words or sentences like names or messages.

When you want to store true or false values like a switch or a yes/no answer.

Syntax

Flutter

int myNumber = 10;
double myDecimal = 3.14;
String myText = 'Hello';
bool isTrue = false;

int is for whole numbers without decimals.

double is for numbers with decimals.

String is for text inside single or double quotes.

bool is for true or false values.

Examples

This stores the age as a whole number.

Flutter

int age = 25;

This stores a price with decimals.

Flutter

double price = 19.99;

This stores a short text message.

Flutter

String greeting = 'Hi there!';

This stores a true/false value to check login status.

Flutter

bool isLoggedIn = true;

Sample App

This Flutter app shows four texts on the screen, each displaying a different data type value.

Flutter

import 'package:flutter/material.dart';

void main() {
  runApp(const MyApp());
}

class MyApp extends StatelessWidget {
  const MyApp({super.key});

  @override
  Widget build(BuildContext context) {
    int myInt = 42;
    double myDouble = 3.14;
    String myString = 'Flutter is fun!';
    bool myBool = true;

    return MaterialApp(
      home: Scaffold(
        appBar: AppBar(title: const Text('Data Types Example')),
        body: Center(
          child: Column(
            mainAxisAlignment: MainAxisAlignment.center,
            children: [
              Text('int: $myInt'),
              Text('double: $myDouble'),
              Text('String: $myString'),
              Text('bool: $myBool'),
            ],
          ),
        ),
      ),
    );
  }
}

Important Notes

Use int for whole numbers only, no decimals allowed.

double can store decimals but uses more memory than int.

String values must be inside quotes.

bool only accepts true or false.

Summary

int stores whole numbers.

double stores decimal numbers.

String stores text.

bool stores true or false.

Practice

1. Which data type in Flutter is used to store true or false values?

easy

A. bool

B. int

C. String

D. double

Solution

1. Step 1: Understand the bool data type

   The bool data type stores only two values: true or false.

2. Step 2: Compare with other data types

   int stores whole numbers, double stores decimals, and String stores text, so they do not fit true/false values.

3. Final Answer:

   bool -> Option A

4. Quick Check:

   bool = true or false [OK]

Hint: Remember: bool means true or false only [OK]

Common Mistakes:

• Confusing bool with int
• Thinking String can store true/false
• Using double for true/false

2. Which of the following is the correct way to declare a double variable named price with value 9.99 in Flutter?

easy

A. double price = 9,99;

B. double price = 9.99;

C. int price = 9.99;

D. String price = 9.99;

Solution

1. Step 1: Check the syntax for double declaration

   In Flutter, double values use a dot (.) as decimal separator, so 9.99 is correct.

2. Step 2: Verify variable type and value match

   double price = 9.99; is correct. double price = 9,99; uses comma which is invalid. int price = 9.99; uses int for decimal value. String price = 9.99; uses String but value is numeric.

3. Final Answer:

   double price = 9.99; -> Option B

4. Quick Check:

   double uses dot for decimals [OK]

Hint: Use dot (.) for decimals in double, not comma [OK]

Common Mistakes:

• Using comma instead of dot
• Assigning decimal to int
• Assigning number to String without quotes

3. What will be the output of this Flutter code?

int a = 5;
double b = 2.0;
print(a + b);

medium

A. 7.0

B. 7.00

C. 7

D. Error

Solution

1. Step 1: Understand type promotion in addition

   Adding int and double results in a double value in Flutter.

2. Step 2: Calculate the sum

   5 + 2.0 = 7.0 as a double, so print outputs 7.0.

3. Final Answer:

   7.0 -> Option A

4. Quick Check:

   int + double = double [OK]

Hint: int + double results in double output [OK]

Common Mistakes:

• Expecting int output
• Thinking print shows 7.00
• Assuming syntax error

4. Identify the error in this Flutter code snippet:

bool isActive = "true";

medium

A. Variable name is invalid

B. Missing semicolon

C. bool cannot store text in quotes

D. No error, code is correct

Solution

1. Step 1: Check bool assignment rules

   bool variables must be assigned true or false without quotes.

2. Step 2: Analyze the code

   "true" is a String, not a bool, so this causes a type error.

3. Final Answer:

   bool cannot store text in quotes -> Option C

4. Quick Check:

   bool = true or false without quotes [OK]

Hint: Assign bool without quotes, quotes mean String [OK]

Common Mistakes:

• Using quotes around true/false
• Ignoring type mismatch errors
• Assuming variable name causes error

5. You want to store a user's age, height in meters, name, and whether they are a student. Which Flutter data types should you use for age, height, name, and isStudent respectively?

hard

A. bool, String, double, int

B. double, int, bool, String

C. String, double, int, bool

D. int, double, String, bool

Solution

1. Step 1: Match each variable to its data type

   Age is a whole number -> int; height is decimal -> double; name is text -> String; isStudent is true/false -> bool.

2. Step 2: Verify option matches all types correctly

   int, double, String, bool matches int, double, String, bool in order for age, height, name, isStudent.

3. Final Answer:

   int, double, String, bool -> Option D

4. Quick Check:

   Whole number, decimal, text, true/false [OK]

Hint: Match data type to real-world meaning: number, decimal, text, true/false [OK]

Common Mistakes:

• Mixing int and double
• Using bool for text
• Confusing order of types