Loop execution flow in Java

Introduction

Loops help repeat actions many times without writing the same code again and again.

When you want to print numbers from 1 to 10.

When you need to check each item in a list one by one.

When you want to keep asking a user for input until they give a valid answer.

When you want to sum all values in a collection.

When you want to repeat a task a fixed number of times.

Syntax

Java

for (initialization; condition; update) {
    // code to repeat
}

while (condition) {
    // code to repeat
}

do {
    // code to repeat
} while (condition);

The for loop repeats code while a condition is true, updating a variable each time.

The while loop repeats code as long as the condition is true, checking before each run.

Examples

This prints numbers 1 to 5, increasing i by 1 each time.

Java

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

This does the same as the for loop but uses a while loop.

Java

int i = 1;
while (i <= 5) {
    System.out.println(i);
    i++;
}

This prints numbers 1 to 5 using a do-while loop, which runs the code at least once.

Java

int i = 1;
do {
    System.out.println(i);
    i++;
} while (i <= 5);

Sample Program

This program shows how three types of loops run and print counts from 1 to 3.

Java

public class LoopFlow {
    public static void main(String[] args) {
        System.out.println("For loop output:");
        for (int i = 1; i <= 3; i++) {
            System.out.println("Count: " + i);
        }

        System.out.println("While loop output:");
        int j = 1;
        while (j <= 3) {
            System.out.println("Count: " + j);
            j++;
        }

        System.out.println("Do-while loop output:");
        int k = 1;
        do {
            System.out.println("Count: " + k);
            k++;
        } while (k <= 3);
    }
}

Important Notes

Loops run the code inside their block repeatedly until the condition is false.

For loops are great when you know how many times to repeat.

While loops check the condition before running the code; do-while loops run the code once before checking.

Summary

Loops repeat actions to save writing the same code many times.

For, while, and do-while are common loop types with small differences in how they check conditions.

Understanding loop flow helps control how many times your code runs.

Practice

1. What does a for loop in Java primarily do?

easy

A. Executes code only once

B. Stops the program immediately

C. Repeats a block of code a specific number of times

D. Creates a new variable

Solution

1. Step 1: Understand the purpose of a for loop and compare options

   A for loop is designed to repeat a block of code multiple times based on a condition. Only "Repeats a block of code a specific number of times" correctly describes it.

2. Final Answer:

   Repeats a block of code a specific number of times -> Option C

3. Quick Check:

   for loop = repeat code [OK]

Hint: For loops repeat code a set number of times [OK]

Common Mistakes:

• Thinking for loops run only once
• Confusing loops with variable declarations
• Believing loops stop the program

2. Which of the following is the correct syntax for a basic while loop in Java?

easy

A. while (condition); { // code }

B. while condition { // code }

C. while: (condition) { // code }

D. while (condition) { // code }

Solution

1. Step 1: Recall Java while loop syntax and check options

   The correct syntax requires parentheses around the condition and braces for the code block. Only "while (condition) { // code }" matches: while (condition) { // code }.

2. Final Answer:

   while (condition) { // code } -> Option D

3. Quick Check:

   while loop syntax = parentheses + braces [OK]

Hint: While loops need parentheses and braces [OK]

Common Mistakes:

• Omitting parentheses around condition
• Adding semicolon after while condition
• Using colon instead of parentheses

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

int count = 0;
while (count < 3) {
  System.out.print(count + " ");
  count++;
}

medium

A. 0 1 2

B. 1 2 3

C. 0 1 2 3

D. 3 2 1

Solution

1. Step 1: Analyze the loop condition and trace iterations

   The loop runs while count is less than 3, starting from 0. It prints 0, then increments to 1; prints 1, increments to 2; prints 2, increments to 3; then stops because 3 is not less than 3.

2. Final Answer:

   0 1 2 -> Option A

3. Quick Check:

   Prints 0,1,2 then stops [OK]

Hint: Count starts at 0 and stops before 3 [OK]

Common Mistakes:

• Including 3 in output
• Starting count from 1
• Printing in reverse order

4. Identify the error in this loop code:

int i = 0;
while (i < 5) {
  System.out.println(i);
}

medium

A. Missing increment of i inside the loop

B. Incorrect loop condition syntax

C. Missing semicolon after while

D. Variable i should be declared inside the loop

Solution

1. Step 1: Check loop control variable and consequences

   The variable i is initialized but never changed inside the loop. Without incrementing i, the condition i < 5 is always true, causing an infinite loop.

2. Final Answer:

   Missing increment of i inside the loop -> Option A

3. Quick Check:

   Loop needs variable update to stop [OK]

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

Common Mistakes:

• Forgetting to increment loop variable
• Misplacing semicolons
• Declaring variable inside loop unnecessarily

5. Consider this code snippet:

int sum = 0;
for (int i = 1; i <= 5; i++) {
  if (i == 3) continue;
  sum += i;
}
System.out.println(sum);

What is the output and why?

hard

A. 15 because all numbers 1 to 5 are added

B. 12 because number 3 is skipped in addition

C. 10 because only numbers less than 3 are added

D. Error due to incorrect use of continue

Solution

1. Step 1: Trace the loop and effect of continue

   The loop runs from 1 to 5. When i equals 3, continue skips the addition for that iteration. Sum = 1 + 2 + 4 + 5 = 12 because 3 is skipped.

2. Final Answer:

   12 because number 3 is skipped in addition -> Option B

3. Quick Check:

   continue skips 3, sum = 12 [OK]

Hint: Continue skips current loop iteration [OK]

Common Mistakes:

• Adding 3 despite continue
• Thinking continue causes error
• Summing only numbers less than 3