Java Notes: Using Loops with for and for-each

What is a Loop?

A loop repeats code multiple times.

Loops are useful when you want to:

Two important Java loops are:

1. The for Loop

A for loop is used when you know how many times you want the loop to run.

Basic Syntax

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

Parts of the for Loop

  1. Initialization - This creates and starts the loop variable.
  2. Condition - The loop runs while this is true.
  3. Update - This changes the loop variable after each loop.
Example parts:
Initialization: int i = 0
Condition: i < 5
Update: i++

Example: Print numbers 1 to 5

for (int i = 1; i <= 5; i++) { System.out.println(i); }
1 2 3 4 5

2. Commands Commonly Used with a for Loop

System.out.println()

Prints output to the screen.

System.out.println("Hello");

Example with a loop

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

i++

Adds 1 to the variable.

i++;

Same as:

i = i + 1;

i--

Subtracts 1 from the variable.

i--;

Example: Count backward

for (int i = 5; i >= 1; i--) { System.out.println(i); }

+=

Adds a value other than 1.

i += 2;

Example: Count by 2s

for (int i = 0; i <= 10; i += 2) { System.out.println(i); }
0 2 4 6 8 10

3. Using a for Loop with Arrays

An array stores multiple values in one variable.

Example array

int[] nums = {10, 20, 30, 40};

Use a for loop to visit each element

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

Important command: .length

.length tells how many items are in an array.

nums.length

In this example, nums.length is 4.

4. The for-each Loop

A for-each loop is used to go through every item in an array or collection.

It is simpler than a regular for loop when you do not need the index.

Basic Syntax

for (dataType variableName : arrayName) { // code to repeat }

Example: Print each number in an array

int[] nums = {10, 20, 30, 40}; for (int num : nums) { System.out.println(num); }
10 20 30 40

Example: Print each name in a String array

String[] names = {"Joe", "Ana", "Luis", "Mia"}; for (String name : names) { System.out.println(name); }

5. Commands Commonly Used with a for-each Loop

System.out.println()

Used to print each item.

for (String name : names) { System.out.println(name); }

Adding values with a loop

You can use a for-each loop to total numbers.

int[] nums = {5, 10, 15, 20}; int sum = 0; for (int num : nums) { sum = sum + num; } System.out.println(sum);
50

6. for Loop vs for-each Loop

Use a for loop when... Use a for-each loop when...
you need the index you want every item in an array
you want to count up or down you do not need the index
you want to skip by 2s, 5s, etc. you want simpler code
you want more control you are mainly reading each value

Regular for loop

int[] scores = {90, 85, 100, 70}; for (int i = 0; i < scores.length; i++) { System.out.println(scores[i]); }

for-each loop

int[] scores = {90, 85, 100, 70}; for (int score : scores) { System.out.println(score); }

Both print the same values, but the for-each loop is shorter.

7. Common Mistakes

Mistake 1: Missing semicolons in a for loop

Wrong:

for (int i = 0 i < 5 i++)

Correct:

for (int i = 0; i < 5; i++)

Mistake 2: Going past the end of an array

Wrong:

for (int i = 0; i <= nums.length; i++)

Correct:

for (int i = 0; i < nums.length; i++)

Why? Because the last index is length - 1.

Mistake 3: Using for-each when you need the index

A for-each loop gives the value, not the position.

for (int num : nums) { System.out.println(num); }

You get the value, but not the index number.

8. Example Programs

Example A: Using a for loop

public class ForExample { public static void main(String[] args) { for (int i = 1; i <= 5; i++) { System.out.println("Number: " + i); } } }

Example B: Using a for-each loop

public class ForEachExample { public static void main(String[] args) { String[] colors = {"Red", "Blue", "Green"}; for (String color : colors) { System.out.println(color); } } }

Example C: Add numbers in an array

public class SumExample { public static void main(String[] args) { int[] nums = {2, 4, 6, 8}; int total = 0; for (int num : nums) { total = total + num; } System.out.println("Total = " + total); } }

9. Quick Summary

10. Practice Questions

  1. What are the 3 parts of a for loop?
  2. What does i++ mean?
  3. What does .length do?
  4. When should you use a for-each loop?
  5. Why is this wrong?
    for (int i = 0; i <= nums.length; i++)

11. Mini Practice

  1. Write a for loop that prints numbers 1 through 10.
  2. Write a for loop that prints even numbers from 2 to 20.
  3. Write a for-each loop that prints each value in this array:
    int[] data = {4, 8, 12, 16};
  4. Write a loop that adds all numbers in an array.
Java Classroom Handout - Using Loops with for and for-each