Control flow (if, for, while, switch) in Flutter - Cheat Sheet & Quick Revision

Recall & Review

beginner

What does an if statement do in Flutter?

An if statement checks a condition and runs code only if that condition is true. It helps the app decide what to do next.

beginner

How does a for loop work in Flutter?

A for loop repeats a block of code a set number of times. It’s like telling the app to do something again and again.

intermediate

What is the purpose of a while loop?

A while loop keeps running code as long as a condition stays true. It’s useful when you don’t know how many times you need to repeat beforehand.

intermediate

Explain the switch statement in Flutter.

A switch statement checks a value and runs code for the matching case. It’s like choosing a path based on different options.

beginner

Why is control flow important in mobile apps?

Control flow lets apps make decisions and repeat tasks. It helps apps respond to user actions and show the right screens or data.

What will this if statement do if the condition is false?

if (isLoggedIn) {
  showHomePage();
}

A Restart the app

B Show the home page

C Show an error

D Do nothing

If the condition is false, the code inside the if block does not run, so nothing happens.

How many times will this for loop run?

for (int i = 0; i < 3; i++) {
  print(i);
}

A 3 times

B 2 times

C 1 time

D Infinite times

The loop runs while i is less than 3, so it runs 3 times with i values 0, 1, and 2.

What happens if the while loop condition is never true?

A The loop runs once

B The loop does not run at all

C The loop runs forever

D The app crashes

If the condition is false at the start, the while loop does not run any code.

Which statement best describes a switch?

A Repeats code forever

B Checks if a condition is true

C Chooses code to run based on a value

D Stops the app

A switch selects which block of code to run depending on the value it checks.

Why use control flow in a mobile app?

A To make decisions and repeat tasks

B To change the app icon

C To speed up the internet

D To store user data

Control flow helps the app decide what to do and repeat actions when needed.

Describe how an if statement controls app behavior.

Think about how the app decides to do something only sometimes.

Explain the difference between a for loop and a while loop.

One counts how many times, the other checks a condition each time.

Practice

1. Which control flow statement in Flutter is used to make decisions based on conditions?

easy

A. if

B. for

C. while

D. switch

Solution

1. Step 1: Understand the purpose of control flow statements

   if statements let the app choose between actions based on conditions.

2. Step 2: Identify the decision-making statement

   Among the options, only if directly tests a condition to decide what code runs.

3. Final Answer:

   if -> Option A

4. Quick Check:

   Decision making = if [OK]

Hint: Use if to check conditions and decide [OK]

Common Mistakes:

• Confusing for loops with decision making
• Thinking while is for choices instead of repetition
• Assuming switch is the only decision tool

2. Which of the following is the correct syntax for a for loop in Flutter to print numbers 0 to 4?

easy

A. for (int i = 1; i < 5; i++) { print(i); }

B. for i in 0 to 4 { print(i); }

C. for (int i = 0; i < 5; i++) { print(i); }

D. for (i = 0; i <= 5; i++) print(i);

Solution

1. Step 1: Check the loop initialization and condition

   for (int i = 0; i < 5; i++) { print(i); } initializes i at 0 and loops while i < 5, which prints 0 to 4.

2. Step 2: Verify syntax correctness

   for (int i = 0; i < 5; i++) { print(i); } uses correct Dart syntax with types and braces.

3. Final Answer:

   for (int i = 0; i < 5; i++) { print(i); } -> Option C

4. Quick Check:

   Correct Dart for loop syntax = for (int i = 0; i < 5; i++) { print(i); } [OK]

Hint: Remember Dart for loops use semicolons and braces [OK]

Common Mistakes:

• Using Python-style loops like 'for i in 0 to 4'
• Off-by-one errors in loop conditions
• Missing type declaration for loop variable

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

int count = 0;
while (count < 3) {
  print(count);
  count++;
}

medium

A. 1 2 3

B. No output

C. 0 1 2 3

D. 0 1 2

Solution

1. Step 1: Analyze the while loop condition and increments

   The loop starts with count = 0 and runs while count < 3, printing count each time.

2. Step 2: Trace the printed values

   It prints 0, then increments to 1, prints 1, increments to 2, prints 2, increments to 3, then stops.

3. Final Answer:

   0 1 2 -> Option D

4. Quick Check:

   Loop prints values less than 3 = 0 1 2 [OK]

Hint: Count starts at 0 and stops before 3, so prints 0,1,2 [OK]

Common Mistakes:

• Including 3 in output mistakenly
• Confusing post-increment behavior
• Thinking loop runs zero times

4. Identify the error in this Flutter Dart code snippet:

int i = 0;
while (i < 5) {
  print(i);
}

medium

A. Missing increment of i inside the loop

B. Incorrect while loop syntax

C. Variable i should be declared inside the loop

D. print() cannot be used inside loops

Solution

1. Step 1: Check loop control variables

   The variable i is initialized but never changed inside the loop.

2. Step 2: Understand loop behavior

   Without incrementing i, the condition i < 5 is always true, causing an infinite loop.

3. Final Answer:

   Missing increment of i inside the loop -> Option A

4. Quick Check:

   Loops need changing variables to stop [OK]

Hint: Always update loop variables to avoid infinite loops [OK]

Common Mistakes:

• Forgetting to increment loop counters
• Assuming loop ends automatically
• Misunderstanding while loop syntax

5. You want to display a message in Flutter based on a user's score:

int score = 75;
switch (score ~/ 10) {
  case 10:
  case 9:
    print('Excellent');
    break;
  case 8:
    print('Very Good');
    break;
  case 7:
    print('Good');
    break;
  default:
    print('Keep Trying');
}

What message will be printed?

hard

A. Excellent

B. Good

C. Very Good

D. Keep Trying

Solution

1. Step 1: Calculate the switch expression value

   score is 75, so score ~/ 10 is integer division 75/10 = 7.

2. Step 2: Match the case for value 7

   Case 7 matches and prints 'Good'.

3. Final Answer:

   Good -> Option B

4. Quick Check:

   Integer division selects case 7 = 'Good' [OK]

Hint: Use integer division to group scores in switch cases [OK]

Common Mistakes:

• Forgetting integer division operator ~/
• Assuming score matches case directly
• Missing break statements causing fall-through