Lesson 4.4: Traversing Arrays

Unit 4 · Lesson 4.4 · Arrays

Lesson 4.4: Traversing Arrays

🕑 40–50 min · 10 Practice Questions · New Syntax · Code Tracing

What You'll Learn

4.4.A: Traverse the elements in a 1D array.
Write standard for loops to traverse arrays from front to back, back to front, and partial ranges.
Write enhanced for-each loops and explain when to use each loop type.
Identify the off-by-one error pattern and how it causes ArrayIndexOutOfBoundsException.
Read and modify array elements during traversal.

Key Vocabulary

Term	Definition
traversal	Visiting every element in a collection exactly once, typically using a loop.
standard for loop	A loop with an index variable that gives direct control over which elements are visited and in what order.
enhanced for loop (for-each)	A loop that automatically visits every element from front to back without an explicit index variable. Cannot modify elements or traverse in reverse.
off-by-one error	A logic error where a loop iterates one too many or one too few times, often caused by using `<=` instead of `<` or starting at 1 instead of 0.

Standard For Loop Traversal (4.4.A)

The most common pattern for traversing an array uses a standard for loop with an index variable i that starts at 0 and runs while i < array.length:

int[] scores = {92, 74, 88, 58, 95};

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

This prints every element from index 0 to index 4. The loop runs exactly scores.length times — once per element.

📌 Why `i < array.length`, not `i <= array.length`

When i equals array.length, that index doesn't exist. The last valid index is always array.length - 1. Using <= is the classic off-by-one error — the loop runs one extra time and throws ArrayIndexOutOfBoundsException.

Modifying Elements During Traversal

Because the standard for loop gives you the index, you can read and write elements in the same loop:

// Double every value in the array
for (int i = 0; i < scores.length; i++) {
    scores[i] = scores[i] * 2;
}
// scores is now {184, 148, 176, 116, 190}

Traversing in Reverse

Start at the last index and decrement:

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

This prints elements from index 4 down to index 0 — last to first.

Partial Traversal

You don't have to visit every element. Adjust the start or end condition:

// Print only the first 3 elements
for (int i = 0; i < 3; i++) {
    System.out.println(scores[i]);
}

// Print only elements at even indices (0, 2, 4...)
for (int i = 0; i < scores.length; i += 2) {
    System.out.println(scores[i]);
}

✅ Example: Finding the Maximum

int[] scores = {92, 74, 88, 58, 95};
int max = scores[0];  // assume first is largest

for (int i = 1; i < scores.length; i++) {
    if (scores[i] > max) {
        max = scores[i];
    }
}
System.out.println(max);  // 95

Start max at scores[0], then loop from index 1 onward. Starting the loop at 1 avoids comparing the first element to itself unnecessarily.

Enhanced For Loop (For-Each)

Java's enhanced for loop removes the need for an index variable when you only need to read every element front to back:

for (int score : scores) {
    System.out.println(score);
}

Read it as: "for each int named score in scores, do..." The loop automatically visits every element from first to last.

For-Each vs. Standard For — Know the Difference

Situation	Use This Loop
Read every element, front to back	for-each ✓
Modify (write to) elements	standard for ✓
Traverse in reverse	standard for ✓
Access the index value itself	standard for ✓
Partial traversal (skip elements)	standard for ✓
Compare adjacent elements	standard for ✓

⚠️ AP Exam Trap: For-Each Cannot Modify the Array

for (int score : scores) {
    score = score * 2;  // DOES NOT modify the array
}

score is a copy of each element, not a reference to the slot. Assigning to it has no effect on the array. This is one of the most common errors tested on the AP exam. To modify elements, use a standard for loop with scores[i] = ....

The Off-By-One Error

This error trips up every programmer at some point. It comes in two flavors:

// Error 1: Using <= instead of <
for (int i = 0; i <= scores.length; i++) {  // throws exception on last iteration
    System.out.println(scores[i]);
}

// Error 2: Starting at 1 instead of 0
for (int i = 1; i < scores.length; i++) {  // skips scores[0]
    System.out.println(scores[i]);
}

On the AP exam, off-by-one errors are tested both as bugs to identify and as correct-or-incorrect code questions.

Traversing Arrays of Objects

The same loop patterns work for any element type. For an array of objects, access attributes using getter methods:

Student[] roster = { new Student("Alice", 92),
                     new Student("Ben",   74),
                     new Student("Carlos", 58) };

// Print every student's name
for (Student s : roster) {
    System.out.println(s.getName());
}

// Find the highest score
int max = roster[0].getScore();
for (int i = 1; i < roster.length; i++) {
    if (roster[i].getScore() > max) {
        max = roster[i].getScore();
    }
}

Summary

Standard for loop: for (int i = 0; i < arr.length; i++) — use when you need the index, must modify elements, traverse in reverse, or do a partial traversal.
Enhanced for loop: for (Type x : arr) — use when reading every element front to back. Cleaner but cannot modify the array.
Off-by-one: using <= causes an out-of-bounds exception; starting at 1 skips the first element.
For-each copies the element value — assigning to the loop variable does not change the array.

▶ Java Code Editor

Try It Yourself

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Practice Questions

MCQ 1

What is printed by the following code?

int[] arr = {1, 2, 3, 4, 5};
int total = 0;
for (int i = 0; i < arr.length; i++) {
    total += arr[i];
}
System.out.println(total);

Predict before reading the options.

A 10

B 15

C 14

D 5

✓ B. The loop visits all 5 elements (indices 0–4). 1+2+3+4+5 = 15. A (10) would be missing 5. C (14) would be missing 1. D (5) is the last element only.

MCQ 2

Which loop correctly prints every element of arr in reverse order?

Predict before reading the options.

for (int i = arr.length; i > 0; i--) {
    System.out.println(arr[i]);
}

for (int i = arr.length; i >= 0; i--) {
    System.out.println(arr[i]);
}

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

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

✓ C. Start at arr.length - 1 (last valid index), stop when i >= 0 (includes index 0). A and B start at arr.length which is out of bounds. D uses i > 0 which stops before printing arr[0] — misses the first element.

MCQ 3

What is the result of this code?

int[] vals = {10, 20, 30, 40};
for (int v : vals) {
    v = v + 5;
}
System.out.println(vals[0]);

Predict before reading the options.

A 10

B 15

C 5

D 20

✓ A. The for-each loop copies each element into v. Assigning to v changes only the copy — the array is untouched. vals[0] remains 10. This is the most commonly tested for-each trap on the AP exam.

MCQ 4

Which of the following correctly doubles every element in an array nums?

for (int n : nums) {
    n *= 2;
}

for (int i = 1; i < nums.length; i++) {
    nums[i] *= 2;
}

for (int i = 0; i <= nums.length; i++) {
    nums[i] *= 2;
}

for (int i = 0; i < nums.length; i++) {
    nums[i] *= 2;
}

✓ D. A uses for-each — won't modify the array. B starts at index 1 — skips nums[0]. C uses <= — throws ArrayIndexOutOfBoundsException on the last iteration. D is correct: starts at 0, uses <, modifies via index.

MCQ 5

What is printed?

int[] arr = {5, 3, 8, 1, 9, 2};
int count = 0;
for (int x : arr) {
    if (x > 4) count++;
}
System.out.println(count);

Predict before reading the options.

A 2

B 4

C 3

D 6

✓ C. Elements greater than 4: 5 ✓, 3 ✗, 8 ✓, 1 ✗, 9 ✓, 2 ✗. Three elements qualify. This is a classic filter-and-count traversal — a pattern that appears on FRQ #3 every year.

Tier 3 · AP Mastery

Mastery: Traversing Arrays

MCQ 6

What is printed by the following code?

int[] arr = {2, 4, 6, 8, 10};
for (int i = 0; i < arr.length - 1; i++) {
    arr[i] = arr[i + 1];
}
System.out.println(arr[0] + " " + arr[3]);

Trace carefully — elements shift as the loop runs.

A 2 8

B 4 10

C 4 8

D 2 10

✓ B. The loop shifts every element one position left (each element gets the value of its right neighbor). Trace: i=0: arr[0]=arr[1]=4 · i=1: arr[1]=arr[2]=6 · i=2: arr[2]=arr[3]=8 · i=3: arr[3]=arr[4]=10. Final array: {4,6,8,10,10}. So arr[0]=4 and arr[3]=10.

MCQ 7

A method receives an array of integers and should return the index of the first element greater than 50. Which implementation is correct?

Predict before reading the options.

for (int x : arr) {
    if (x > 50)
        return x;
}
return -1;

for (int i = 0; i < arr.length; i++) {
    if (arr[i] > 50)
        return arr[i];
}
return -1;

for (int i = 1; i <= arr.length; i++) {
    if (arr[i] > 50)
        return i;
}
return -1;

for (int i = 0; i < arr.length; i++) {
    if (arr[i] > 50)
        return i;
}
return -1;

✓ D. The method must return the index, not the element value. A uses for-each — no index available. B returns the element value arr[i] instead of the index i. C starts at index 1 (skips index 0) and uses <= (out-of-bounds). D starts at 0, uses <, and returns i — correct.

MCQ 8

What is printed?

int[] arr = {1, 2, 3, 4, 5};
int result = 0;
for (int i = 0; i < arr.length - 1; i++) {
    if (arr[i] % 2 == 0) result += arr[i];
}
System.out.println(result);

Note the loop bound carefully before tracing.

A 6

B 2

C 4

D 10

✓ A. The bound is arr.length - 1 = 4, so the loop visits indices 0–3 only (skips index 4, which holds 5). Even values in that range: arr[1]=2 ✓, arr[3]=4 ✓. Sum = 6. D (10) would be correct if the loop included index 4, but 5 is odd so it wouldn't matter — the trap is whether students notice the loop skips the last element.

MCQ 9

A Product class has a method getPrice() that returns a double. Given Product[] inventory, which code correctly computes the total price of all products?

Predict before reading the options.

double total = 0;
for (int i = 1; i < inventory.length; i++) {
    total += inventory.getPrice();
}

double total = 0;
for (int i = 0; i < inventory.length; i++) {
    total += inventory[i];
}

double total = 0;
for (Product p : inventory) {
    total += p.getPrice();
}
return total;

double total = 0;
for (Product p : inventory) {
    total += inventory.getPrice();
}

✓ C. A calls getPrice() on the array itself (not an element) and skips index 0. B adds the object reference, not its price. D calls inventory.getPrice() — same error as A. C correctly uses for-each to visit each Product object and calls getPrice() on it.

MCQ 10

What is printed?

int[] arr = {3, 1, 4, 1, 5, 9};
int prev = arr[0];
int count = 0;
for (int i = 1; i < arr.length; i++) {
    if (arr[i] < prev) count++;
    prev = arr[i];
}
System.out.println(count);

Trace step by step — this compares adjacent elements.

A 1

B 2

C 3

D 4

✓ B. The loop counts how many times an element is less than the one before it (a "drop"). Trace: prev=3 · i=1: arr[1]=1 < 3 ✓ count=1, prev=1 · i=2: arr[2]=4 < 1? No, prev=4 · i=3: arr[3]=1 < 4 ✓ count=2, prev=1 · i=4: arr[4]=5 < 1? No, prev=5 · i=5: arr[5]=9 < 5? No. Final count=2. Note the loop starts at i=1 — this is the correct pattern for comparing adjacent elements.

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Lesson 4.4: Traversing Arrays

What You'll Learn

Key Vocabulary

Standard For Loop Traversal (4.4.A)

📌 Why i < array.length, not i <= array.length

Modifying Elements During Traversal

Traversing in Reverse

Partial Traversal

✅ Example: Finding the Maximum

Enhanced For Loop (For-Each)

For-Each vs. Standard For — Know the Difference

⚠️ AP Exam Trap: For-Each Cannot Modify the Array

The Off-By-One Error

Traversing Arrays of Objects

Summary

Practice Questions

Mastery: Traversing Arrays

Get in Touch

Message Sent!

📌 Why `i < array.length`, not `i <= array.length`