Loop initialization, condition, update in Java - Step-by-Step Execution

Concept Flow - Loop initialization, condition, update

Initialize loop variable

↓

Check loop condition

No→Exit loop

Yes↓

Execute loop body

↓

Update loop variable

↩Back to Check loop condition

The loop starts by setting a variable, then checks a condition. If true, it runs the loop body, updates the variable, and repeats. If false, it stops.

Execution Sample

Java

for (int i = 0; i < 3; i++) {
    System.out.println(i);
}

Prints numbers 0, 1, and 2 by looping with initialization, condition check, and update.

Execution Table

Step	i (loop variable)	Condition (i < 3)	Action	Output
1	0	true	Print i=0	0
2	1	true	Print i=1	1
3	2	true	Print i=2	2
4	3	false	Exit loop

💡 i reaches 3, condition 3 < 3 is false, loop stops

Variable Tracker

Variable	Start	After 1	After 2	After 3	Final
i	0	1	2	3	3

Key Moments - 3 Insights

Why does the loop stop when i equals 3?

Because the condition i < 3 becomes false at step 4 in the execution_table, so the loop exits.

When is the loop variable i updated?

After executing the loop body at each step, before checking the condition again, as shown in the variable_tracker.

Does the loop body run when the condition is false?

No, the loop body only runs when the condition is true. At step 4, condition is false, so the loop ends without running the body.

Visual Quiz - 3 Questions

Look at the execution_table, what is the value of i at step 2?

A 2

B 0

C 1

D 3

💡 Hint

Check the 'i (loop variable)' column at step 2 in the execution_table.

At which step does the loop condition become false?

A Step 3

B Step 4

C Step 1

D Step 2

💡 Hint

Look at the 'Condition (i < 3)' column in the execution_table to find when it is false.

If the update i++ was missing, what would happen to the loop?

A It would run forever because i never changes

B It would stop immediately

C It would print only once

D It would print numbers from 0 to 3

💡 Hint

Think about the variable_tracker and how i changes each step.

Concept Snapshot

for (initialization; condition; update) {
  // loop body
}

1. Initialization runs once at start.
2. Condition checked before each loop; if false, loop ends.
3. Loop body runs only if condition is true.
4. Update runs after each loop body execution.

Full Transcript

This example shows a for loop in Java with initialization, condition, and update. First, the variable i is set to 0. Then the loop checks if i is less than 3. If yes, it prints i, then increases i by 1. This repeats until i reaches 3, when the condition becomes false and the loop stops. The execution_table tracks each step, showing i's value, condition result, action, and output. The variable_tracker shows how i changes after each iteration. Key moments clarify why the loop stops and when i updates. The visual quiz tests understanding of these steps.

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