Loop initialization, condition, update in Java - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to initialize the loop variable correctly.

Java

for ([1]; i < 5; i++) {
    System.out.println(i);
}

Drag options to blanks, or click blank then click option'

Aint = 0

Bi = 0

Cint i

Dint i = 0

Attempts:

3 left

💡 Hint

Common Mistakes

Forgetting to declare the variable type.

Using assignment without declaration.

The loop variable must be declared and initialized as int i = 0 inside the for loop.

2fill in blank

medium

Complete the code to write the correct loop condition.

Java

for (int i = 0; [1]; i++) {
    System.out.println(i);
}

Drag options to blanks, or click blank then click option'

Ai < 5

Bi == 5

Ci > 5

Di <= 5

Attempts:

3 left

💡 Hint

Common Mistakes

Using <= 5 runs the loop one extra time.

Using == 5 does not run the loop at all.

The loop should run while i is less than 5, so the condition is i < 5.

3fill in blank

hard

Fix the error in the loop update expression.

Java

for (int i = 0; i < 5; [1]) {
    System.out.println(i);
}

Drag options to blanks, or click blank then click option'

Ai++

Bi = i + 1

Ci--

Di += 2

Attempts:

3 left

💡 Hint

Common Mistakes

Using decrement i-- causes an infinite loop.

Using i += 2 skips numbers.

The update expression i++ increases i by 1 each loop, which is the standard increment.

4fill in blank

hard

Fill both blanks to create a loop that counts down from 10 to 1.

Java

for (int i = [1]; i [2] 0; i--) {
    System.out.println(i);
}

Drag options to blanks, or click blank then click option'

A10

B>

C>=

D0

Attempts:

3 left

💡 Hint

Common Mistakes

Starting at 0 instead of 10.

Using i < 0 as condition causes no iterations.

The loop starts at 10 and runs while i > 0, printing numbers from 10 down to 1.

5fill in blank

hard

Fill all three blanks to create a loop that prints even numbers from 2 to 10.

Java

for (int [1] = [2]; [1] [3] 10; [1] += 2) {
    System.out.println([1]);
}

Drag options to blanks, or click blank then click option'

Ai

B2

C<=

Dj

Attempts:

3 left

💡 Hint

Common Mistakes

Using wrong variable names.

Starting at 0 or 1 instead of 2.

Using < instead of <= causes missing 10.

The loop variable is i, starting at 2, running while i <= 10, and increasing by 2 each time to print even numbers.

Practice

1. Which part of a for loop in Java sets the starting value of the loop variable?

easy

A. Initialization

B. Condition

C. Update

D. Body

Solution

1. Step 1: Understand the parts of a for loop

   A for loop has three parts: initialization, condition, and update.

2. Step 2: Identify the starting value part

   The initialization part sets the starting value of the loop variable before the loop runs.

3. Final Answer:

   Initialization -> Option A

4. Quick Check:

   Starting value = Initialization [OK]

Hint: Initialization always sets the loop's start value [OK]

Common Mistakes:

• Confusing condition with initialization
• Thinking update sets the start value
• Mixing loop body with initialization

2. Which of the following is the correct syntax for a for loop in Java that counts from 0 to 4?

easy

A. for(int i = 0; i < 4; i++) {}

B. for(int i = 0; i <= 4; i++) {}

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

D. for(int i = 0; i != 5; i--) {}

Solution

1. Step 1: Check initialization and condition

   for(int i = 0; i <= 4; i++) {} starts at 0 and runs while i is less than or equal to 4, which covers 0 to 4.

2. Step 2: Check update direction

   for(int i = 0; i <= 4; i++) {} increments i by 1 each time, moving forward correctly.

3. Final Answer:

   for(int i = 0; i <= 4; i++) {} -> Option B

4. Quick Check:

   0 to 4 inclusive = for(int i = 0; i <= 4; i++) {} [OK]

Hint: Use i <= max to include last number [OK]

Common Mistakes:

• Using i < 4 excludes 4
• Starting at 1 misses 0
• Decrementing i causes infinite loop

3. What is the output of this Java code?

for(int i = 1; i < 5; i += 2) {
    System.out.print(i + " ");
}

medium

A. 1 2 3 4

B. 1 3 5

C. 2 4

D. 1 3

Solution

1. Step 1: Trace loop values

   i starts at 1, prints 1, then i increases by 2 to 3, prints 3, then i increases by 2 to 5.

2. Step 2: Check loop condition

   Loop runs while i < 5. When i is 5, condition fails, so loop stops.

3. Final Answer:

   1 3 -> Option D

4. Quick Check:

   Prints 1 and 3 only [OK]

Hint: Add update to i and check condition carefully [OK]

Common Mistakes:

• Assuming loop runs when i equals 5
• Forgetting i increments by 2
• Printing values after loop ends

4. Identify the error in this Java loop:

for(int i = 0; i > 5; i++) {
    System.out.println(i);
}

medium

A. Condition causes loop to never run

B. Update should decrement i

C. Initialization is incorrect

D. Missing loop body braces

Solution

1. Step 1: Analyze the loop condition

   The condition is i > 5, but i starts at 0, so 0 > 5 is false immediately.

2. Step 2: Understand loop execution

   Since condition is false at start, loop body never runs.

3. Final Answer:

   Condition causes loop to never run -> Option A

4. Quick Check:

   False condition stops loop immediately [OK]

Hint: Check if condition is true at start to run loop [OK]

Common Mistakes:

• Thinking loop runs at least once
• Confusing > with < in condition
• Assuming update fixes condition

5. You want to print all even numbers from 2 to 20 using a for loop in Java. Which loop correctly does this?

hard

A. for(int i = 2; i < 20; i++) { if(i % 2 == 0) System.out.print(i + " "); }

B. for(int i = 1; i < 20; i++) { if(i % 2 == 0) System.out.print(i + " "); }

C. for(int i = 2; i <= 20; i += 2) { System.out.print(i + " "); }

D. for(int i = 0; i <= 20; i += 2) { System.out.print(i + " "); }

Solution

1. Step 1: Check initialization and update for even numbers

   for(int i = 2; i <= 20; i += 2) { System.out.print(i + " "); } starts at 2 and increments by 2, printing even numbers directly up to 20.

2. Step 2: Verify condition includes 20

   Condition i <= 20 ensures 20 is included in output.

3. Final Answer:

   for(int i = 2; i <= 20; i += 2) { System.out.print(i + " "); } -> Option C

4. Quick Check:

   Start at 2, step 2, up to 20 [OK]

Hint: Start at first even, add 2 each time [OK]

Common Mistakes:

• Starting at 0 prints unwanted 0
• Using < 20 excludes 20
• Forgetting to increment by 2