Data types (int, double, String, bool) in Flutter - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is an int in Flutter?

An int is a data type that stores whole numbers without decimals, like 1, 42, or -7.

beginner

How is a double different from an int?

A double stores numbers with decimals, like 3.14 or -0.5, while an int stores whole numbers only.

beginner

What kind of data does a String hold?

A String holds text, like words or sentences, for example: "Hello, Flutter!".

beginner

What values can a bool have?

A bool can only be true or false. It is used for yes/no or on/off decisions.

beginner

Why is it important to use the correct data type in Flutter?

Using the correct data type helps Flutter understand what kind of data you have, so it can store and use it properly without errors.

Which data type would you use to store the number 10?

A int

B double

C String

D bool

The number 10 is a whole number, so it should be stored as an int.

What data type is best for storing the text "Flutter Rocks"?

A int

B double

C bool

D String

Text like "Flutter Rocks" is stored as a String.

Which data type can only be true or false?

A int

B bool

C String

D double

The bool type holds only true or false values.

Which data type would you use for the number 3.1415?

A String

B int

C double

D bool

3.1415 has decimals, so it should be stored as a double.

What happens if you try to store text in an int variable?

A You get an error

B It works fine

C Flutter converts it automatically

D It stores only the first letter

Storing text in an int variable causes an error because the types don't match.

Explain the difference between int and double data types in Flutter.

Think about numbers with and without decimals.

Describe when you would use a bool data type in a Flutter app.

Think about simple yes/no questions in your app.

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