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Activity 5.2 — Array Traversal with for Loops

Introduction

Activity 5.2

Array Traversal with for Loops

Warm-Up Question

If an array has 100 elements…
Do you really want to type arr[0], arr[1], … arr[99]?
Traversal means visiting every element using a loop.

Key Vocabulary (Again)

Index: position (starts at 0)
Element: value stored at a position
Length: number of elements (arr.length)

The One Loop Pattern You Must Know

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// Standard array traversal pattern
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int[] nums = {4, 7, 2, 9};
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for (int i = 0; i < nums.length; i++)
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{
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  System.out.println("Index " + i + " => " + nums[i]);
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}
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// i is the index (position)
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// nums[i] is the element (value at that position)

i starts at 0 (first index)
loop continues while i < arr.length
arr[i] means “element at index i”

Index vs Element

i is the index
arr[i] is the element
Index = where, Element = what

Off-by-One (Most Common Bug)

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// Common mistake: off-by-one (DO NOT DO THIS)
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int[] nums = {4, 7, 2, 9};
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// When i == nums.length, nums[i] is out of bounds
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for (int i = 0; i <= nums.length; i++)
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{
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  System.out.println(nums[i]);
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}
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// Correct condition is: i < nums.length

Valid indexes go from 0 to length - 1
i < length stops the loop before it breaks

Tracing Example (AP Style)

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// Tracing practice: What prints?
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int[] a = {10, 20, 30};
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for (int i = 0; i < a.length; i++)
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{
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  System.out.println(a[i] + i);
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}
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// i=0 prints 10+0
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// i=1 prints 20+1
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// i=2 prints 30+2

Before you run code, you should be able to predict output.
Write the values of i and what prints each time.

Printing Arrays (Reminder)

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// Using import to print arrays nicely
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import java.util.Arrays;
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int[] nums = {4, 7, 2, 9};
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System.out.println(Arrays.toString(nums));
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// Without Arrays.toString, printing an array isn't readable.

import lets us use helper tools from Java libraries.
Arrays.toString(...) converts an array into readable text.
Today: you'll mostly print elements in a loop — that's even more readable.

Today's Skills

Traverse an array using a for loop
Compute a sum and average
Count elements that match a condition
Find a minimum or maximum value

'F' → Fullscreen

Objectives

Use a for loop to visit every element in an array.
Explain why the correct loop condition is i < arr.length.
Write traversal code that computes a sum/average or count.
Use traversal to find a minimum and maximum value.

Activity Tasks

Create a new project named Activity5_2_ArrayTraversal.
Complete each task in order.
Use only index-based for loops today (no enhanced for loop yet).

Task 1: Print Every Element

Traverse an array and print each element.
Make sure your loop condition uses i < nums.length.

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public class Program
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{
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  public static void main(String[] args)
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  {
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    // Task 1 — Print Every Element (Traversal Basics)
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    int[] nums = {4, 7, 2, 9, 5};
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    // Use a for loop to print each element on its own line.
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    // Example output line: "Value: 4"
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    for (int i = 0; i < nums.length; i++)
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    {
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      System.out.println("Value: " + nums[i]);
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    }
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  }
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}

Task 2: Sum + Average

Use a loop to add every score into sum.
Compute the average as a double.

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public class Program
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{
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  public static void main(String[] args)
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  {
9 collapsed lines
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    // Task 1 — Print Every Element (Traversal Basics)
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    int[] nums = {4, 7, 2, 9, 5};
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    // Use a for loop to print each element on its own line.
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    // Example output line: "Value: 4"
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    for (int i = 0; i < nums.length; i++)
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    {
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      System.out.println("Value: " + nums[i]);
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    }
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    // Task 2 — Sum + Average
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    int[] scores = {84, 91, 76, 88, 95};
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    int sum = 0;
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    // Add every element into sum
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    for (int i = 0; i < scores.length; i++)
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    {
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      sum += scores[i];
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    }
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    double avg = (double) sum / scores.length;
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    System.out.println("Sum: " + sum);
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    System.out.println("Average: " + avg);
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  }
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}

Task 3: Count Values Meeting a Condition

Count how many items match a rule using an if inside the loop.
This pattern shows up constantly on AP-style questions.

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public class Program
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{
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  public static void main(String[] args)
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  {
26 collapsed lines
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    // Task 1 — Print Every Element (Traversal Basics)
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    int[] nums = {4, 7, 2, 9, 5};
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    // Use a for loop to print each element on its own line.
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    // Example output line: "Value: 4"
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    for (int i = 0; i < nums.length; i++)
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    {
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      System.out.println("Value: " + nums[i]);
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    }
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    // Task 2 — Sum + Average
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    int[] scores = {84, 91, 76, 88, 95};
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    int sum = 0;
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    // Add every element into sum
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    for (int i = 0; i < scores.length; i++)
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    {
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      sum += scores[i];
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    }
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    double avg = (double) sum / scores.length;
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    System.out.println("Sum: " + sum);
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    System.out.println("Average: " + avg);
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    // Task 3 — Count Values Meeting a Condition
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    int[] temps = {72, 68, 75, 70, 69, 76, 71};
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    // Count how many temps are 72 or higher
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    int count = 0;
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    for (int i = 0; i < temps.length; i++)
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    {
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      if (temps[i] >= 72)
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      {
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        count++;
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      }
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    }
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    System.out.println("Temps >= 72: " + count);
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  }
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}

Task 4: Find Min and Max

Initialize min and max to the first element.
Traverse from index 1 to the end, updating as needed.

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public class Program
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{
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  public static void main(String[] args)
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  {
43 collapsed lines
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    // Task 1 — Print Every Element (Traversal Basics)
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    int[] nums = {4, 7, 2, 9, 5};
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    // Use a for loop to print each element on its own line.
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    // Example output line: "Value: 4"
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    for (int i = 0; i < nums.length; i++)
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    {
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      System.out.println("Value: " + nums[i]);
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    }
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    // Task 2 — Sum + Average
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    int[] scores = {84, 91, 76, 88, 95};
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    int sum = 0;
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    // Add every element into sum
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    for (int i = 0; i < scores.length; i++)
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    {
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      sum += scores[i];
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    }
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    double avg = (double) sum / scores.length;
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    System.out.println("Sum: " + sum);
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    System.out.println("Average: " + avg);
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    // Task 3 — Count Values Meeting a Condition
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    int[] temps = {72, 68, 75, 70, 69, 76, 71};
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    // Count how many temps are 72 or higher
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    int count = 0;
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    for (int i = 0; i < temps.length; i++)
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    {
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      if (temps[i] >= 72)
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      {
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        count++;
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      }
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    }
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    System.out.println("Temps >= 72: " + count);
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    // Task 4 — Find Min and Max (Classic AP Pattern)
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    int[] nums = {4, 7, 2, 9, 5};
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    // Start min and max at the first element
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    int min = nums[0];
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    int max = nums[0];
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    for (int i = 1; i < nums.length; i++) {
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      if (nums[i] < min)
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      {
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        min = nums[i];
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      }
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      if (nums[i] > max)
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      {
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        max = nums[i];
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      }
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    }
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    System.out.println("Min: " + min);
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    System.out.println("Max: " + max);
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  }
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}

Sample Output

Your program output should something similar to the sample output below.

Value: 4
Value: 7
Value: 2
Value: 9
Value: 5
Sum: 434
Average: 86.8
Temps >= 72: 3
Min: 2
Max: 9

Reflection Questions

You may write your reflection answers as comments at the bottom of your code.

In one sentence: what does it mean to traverse an array?
Why is i < arr.length correct, but i <= arr.length is not?
What is the difference between i and arr[i]?

Submission

Submit your activity and reflection answers to the appropriate dropbox.