Working with Arrays

This lesson expands on Introduction to Arrays by showing how to use arrays to perform meaningful tasks.

Some of these techniques might help you solve the practice problems in the previous set as well, so it would be worth revisiting your solutions.

For every technique here, we'll look at:

1. The goal — What problem we are trying to solve
2. The design approach — How arrays help us solve it
3. The code — A clearly written example program

1. Accessing Elements

Goal

Retrieve specific values from inside an array (first, last, any index).

Design Approach

To access an element, use:

arrayName[index]

You must know the correct index:

• First element → index 0
• Last element → array.length - 1

This ensures your code works for any array size.

Code

int[] marks = {72, 85, 90, 66, 88};

System.out.println("First: " + marks[0]);
System.out.println("Last: " + marks[marks.length - 1]);

Example Output

First: 72
Last: 88

2. Updating Elements

Goal

Change one value inside the array without touching the others.

Design Approach

Select the index you want to modify and assign a new value.
This is just like assigning a variable—except you choose which element.

Code

int[] marks = {72, 85, 90, 66, 88};

marks[2] = 95;   // Update index 2 with a new mark

3. Traversing Arrays

Goal

Process every element in the array.

Design Approach

Use an index-based loop that runs from 0 to array.length - 1.
This ensures the loop scales for any array length.

Code

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

Example Output

72
85
90
66
88

4a. Computing Aggregate Values - Sum of All Elements

This section covers common operations over entire arrays.

Goal

Add all values in the array.

Design Approach

Use a running total and loop through every index.

Code

int sum = 0;

for (int i = 0; i < marks.length; i++) {
    sum += marks[i];
}

System.out.println("Sum = " + sum);

4b. Computing Aggregate Values - Average

Goal

Compute mean score from a list of numbers.

Design Approach

Use a double calculation:
average = sum / length

Code

double avg = (double) sum / marks.length;
System.out.println("Average = " + avg);

4c. Computing Aggregate Values - Minimum and Maximum

Goal

Find the smallest and largest elements.

Design Approach

Start with the first element, then compare each remaining one.

Code

int min = marks[0];
int max = marks[0];

for (int i = 1; i < marks.length; i++) {
    if (marks[i] < min) {
        min = marks[i];
    }
    if (marks[i] > max) {
        max = marks[i];
    }
}

5. Searching an Array (Linear Search)

Goal

Check whether a target value exists in the array.

Design Approach

Move index by index until the value is found.

Code (existence only)

boolean found = false;
int target = 90;

for (int i = 0; i < marks.length; i++) {
    if (marks[i] == target) {
        found = true;
    }
}

Code (recording the index)

int pos = -1; // means “not found”

for (int i = 0; i < marks.length; i++) {
    if (marks[i] == target) {
        pos = i;
    }
}

6. Counting Occurrences

Goal

Count how many elements satisfy a condition.

Design Approach

Use an index loop and increment a counter whenever the condition is true.

Code

int count = 0;

for (int i = 0; i < marks.length; i++) {
    if (marks[i] >= 80) {
        count++;
    }
}

7. Reversing Output (Printing Backwards)

Goal

Print array values backwards without changing the array.

Design Approach

Start the loop at length - 1, stop at 0, and move downward.

Code

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

Example Output

88
66
90
85
72

Practice Problems — Working With Arrays

Complete the following practice problems. Starter code template files can be found in the src/ folder of this repo.

Annotated solutions to these problems can be found here.

Problem 1 — Print All Values

Given:

int[] nums = {5, 12, 3, 9, 1};

Print each value on its own line using a loop.

Example Output

5
12
3
9
1

Problem 2 — Replace by Rule

Given an array of 6 integers, replace every value less than 10 with 10.
Print the updated array on one line.

Example Input

Original array: [4, 11, 2, 19, 10, 7]

Example Output

Updated: 10 11 10 19 10 10

Problem 3 — Count How Many

Ask the user for 5 temperatures, store them in an array, and count how many are above 20.

Example Input

Enter 5 temperatures:
18
22
25
17
21

Example Output

There are 3 temperatures above 20.

Problem 4 — Sum and Average

Create an array of 7 quiz scores.
Compute and print the sum and average.

Example Input

Scores: [7, 9, 10, 8, 6, 9, 7]

Example Output

Sum = 56
Average = 8.0

Problem 5 — Find the Minimum

Given:

int[] ages = {18, 22, 17, 30, 25, 20};

Print the youngest age.

Example Output

Youngest age: 17

Problem 6 — Does It Exist?

Ask the user for a number between 1 and 100.

Given:

int[] data = {12, 44, 29, 67, 44, 18};

Print “Found” or “Not found”.

Example Input

Enter a number: 44

Example Output

Found

Problem 7 — First and Last Match

Given:

double[] values = {3.5, 1.2, 6.8, 1.2, 3.5};

Check if the first and last values are the same.

Example Output

First and last match: true

Problem 8 — Index of Target

Given:

int[] points = {14, 9, 27, 31, 18};

Ask the user for a target number and print the index where it appears, or -1 if not found.

Example Input

Enter a target: 27

Example Output

Found at index: 2

Problem 9 — Print in Reverse

Ask the user for 5 numbers.
Print them in reverse order.

Example Input

Enter 5 numbers:
3
9
1
4
7

Example Output

7 4 1 9 3

Problem 10 — Above Average

Ask the user for 8 test scores.
Compute the average.
Print all scores above the average.

Example Input

Scores:
10
7
8
9
6
12
11
5

Example Output

Average = 8.5
Above average: 10 9 12 11