Building Java Programs

Lab 7: Arrays

Except where otherwise noted, the contents of this document are Copyright 2010 Stuart Reges and Marty Stepp.

lab document created by Whitaker Brand and Marty Stepp

Today's lab

Goals for today:

• use arrays for storing lists of data
• practice using for loops to traverse and process array data
• pass arrays as parameters and returns from methods
• explore the idea of reference parameters
• Where you see this icon, you can click it to check the problem in Practice-It!

Exercise : Array declaration syntax

Which of the following choices is the correct syntax for declaring/initializing an array of integers?

1. []int a = [10]int;
2. int a[10];
3. int[10] a = new int[10];
4. int a[10] = new int[10];
5. int[] a = new int[10];

Exercise : Quick initialization syntax

Which of the following choices is the correct syntax for quickly declaring/initializing an array of integers to store a particular list of values?

1. int[] a = {17, -3, 42, 5, 9, 28};
2. int a {17, -3, 42, 5, 9, 28};
3. int[] a = new int[6] {17, -3, 42, 5, 9, 28};
4. int[6] a = {17, -3, 42, 5, 9, 28};
5. int[] a = new {17, -3, 42, 5, 9, 28} [6];

Exercise : Array code tracing

Fill in the array with the values that would be stored after the code executes:

int[] data = new int[8];
data[0] = 3;
data[7] = -18;
data[4] = 5;
data[1] = data[0];

int x = data[4];
data[4] = 6;
data[x] = data[0] * data[1];

Exercise : Array code tracing 2

Fill in the array with the values that would be stored after the code executes:

int[] list = {2, 18, 6, -4, 5, 1};
for (int i = 0; i < list.length; i++) {
    list[i] = list[i] + (list[i] / list[0]);
}

Recall: Arrays class methods

Exercise : PromptNumbers

• Download the following file PromptNumbers.java and to your machine and open it with jGrasp.
• The program is supposed to prompt the user to enter several integers, store them into an array, then print those integers back out in forwards and backwards order. Finish the program so that it runs properly. Make your program flexible enough that it will work no matter how many integers the user wants to type.

Recall: Arrays as parameter/return (declare)

• Arrays can be passed as parameters and returned from methods.
• This method takes an array of doubles, and returns a new array of rounded ints:

public static int[] roundAll(double[] realNums) {
    int[] roundedNums = new int[realNums.length];
    for (int i = 0; i < realNums.length; i++) {
        roundedNums[i] = (int) Math.round(realNums[i]);
    }
    return roundedNums;
}

Recall: Arrays as parameter/return (call)

• Below is an example usage of the roundAll method from the previous slide:

import java.util.*; // to use Arrays

public class MyProgram {
    public static void main(String[] args) {
        double[] realNumbers = {5.5, 7.31, 8.09, -3.234234, 2.0, 0.0};
        int[] roundedNumbers = roundAll(realNumbers);
        System.out.println(Arrays.toString(roundedNumbers));
    }
    ...
}

// Output: [5, 7, 8, -3, 2, 0]

Exercise : array mystery

Suppose that each array at right were passed as a parameter to the mystery method below. Fill in the boxes with the array contents after each method call.

public static void mystery(int[] a) {
    for (int i = 0; i < a.length - 1; i++) {
        if (a[i] < a[i + 1]) {
            a[i] = a[i + 1];
        }
    }
}

{4, 4}

[^0-9,]+

{3, 6, 6}

[^0-9,]+

{7, 8, 8, 4}

[^0-9,]+

{5, 7, 7, 4, 4}

[^0-9,]+

{4, 6, 6, 7, 9, 9}

[^0-9,]+

Exercise : PromptNumbers2

• Modify your PromptNumbers2.java program from a previous exercise.
• Add a method that accepts an array as a parameter and prints the elements of that array in forward order.
• Then add a second method that accepts an array as a parameter and prints the elements of that array in backward order.

Exercise - answer

• PromptNumbers2.java
  (complete solution)

public class PromptNumbers2 {
    public static void main(String[] args) {
        int count = console.nextInt();
        int[] nums = new int[count];
        ...
        System.out.println("Your numbers in forward order:");
        printForward(nums);

        System.out.println("Your numbers in backward order:");
        printBackward(nums);
    }

    // Prints the elements of the given array in forward order.
    public static void printForward(int[] a) {
        for (int i = 0; i < a.length; i++) {
            System.out.println(a[i]);
        }
    }
    // Prints the elements of the given array in backward order.
    public static void printBackward(int[] a) {
        for (int i = a.length - 1; i >= 0; i--) {
            System.out.println(a[i]);
        }
    }
}

Exercise : minGap

Write a method named minGap that accepts an integer array as a parameter and returns the minimum 'gap' between adjacent values in the array. The gap between two adjacent values in a array is defined as the second value minus the first value. For example, suppose a variable called array is an array of integers that stores the following sequence of values:

int[] array = {1, 3, 6, 7, 12};

The first gap is 2 (3 - 1), the second gap is 3 (6 - 3), the third gap is 1 (7 - 6) and the fourth gap is 5 (12 - 7). Thus, the call of minGap(array) should return 1 because that is the smallest gap in the array. If you are passed an array with fewer than 2 elements, you should return 0.

Click on the check-mark above to try out your solution in Practice-it!

Exercise : percentEven

Write a method named percentEven that accepts an array of integers as a parameter and returns the percentage of even numbers in the array as a real number. For example, if a variable named nums refers to an array of the elements {6, 2, 9, 11, 3}, then the call of percentEven(nums) should return 40.0. If the array contains no even elements or no elements at all, return 0.0.

Click on the check-mark above to try out your solution in Practice-it!

Exercise : Array reference mystery

What values are stored in the array at the comment in main? Note that the incrementAll returns void, but does take an int[] parameter.

public class ArrayReference {
    public static void main(String[] args) {
        int[] nums = {2, 4, -1, 3};
        incrementAll(nums);

        // HERE!
    }

    public static void incrementAll(int[] data) {
        for (int i = 0; i < data.length; i++) {
            data[i]++;
        }
    }
}

Exercise : swapAll

Write a method named swapAll that accepts two arrays of integers as parameters and swaps their entire contents. You may assume that the arrays passed are not null and are the same length.

For example, if the following arrays are passed:

int[] a1 = {11, 42, -5, 27, 0, 89};
int[] a2 = {10, 20, 30, 40, 50, 60};
swapAll(a1, a2);

After the call, the arrays should store the following elements:

a1: {10, 20, 30, 40, 50, 60}
a2: {11, 42, -5, 27, 0, 89}

Exercise : stretch

Write a method named stretch that accepts an array of integers as a parameter and returns a new array twice as large as the original, replacing every integer from the original array with a pair of integers, each half the original. If a number in the original array is odd, then the first number in the new pair should be one higher than the second so that the sum equals the original number. For example, if a variable named list refers to an array storing the values {18, 7, 4, 24, 11}, the call of stretch(list) should return a new array containing {9, 9, 4, 3, 2, 2, 12, 12, 6, 5}. (The number 18 is stretched into the pair 9, 9, the number 7 is stretched into 4, 3, the number 4 is stretched into 2, 2, the number 24 is stretched into 12, 12 and the number 11 is stretched into 6, 5.)

Click on the check-mark above to try out your solution in Practice-it!

Exercise : max errors

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// Returns the largest value in the given array.
public static int max(int data[10]) {
    int max = 0;
    for (int i = 0; i < data[].length(); i++) {
        if (array[i] > max) {
            max = array[i];
        }
    }
    return max[];
}

The above attempted solution to Practice-It problem "max" has 7 problems. Open Practice-It from the link above, copy/paste this code into it, and fix the errors. Complete the code so that it passes the test cases.

Exercise - answer

1. line 2: should not write [10] after parameter name; should write [] (without length) after int
2. line 3: starting max at 0 won't work if the array is all negative. Should initialize max variable to be data[0] and start for loop at index 1
3. line 4: should not write [] after data here
4. line 4: should not write () after length here
5. line 5: array should be data
6. line 6: array should be data
7. line 9: should not write [] after max here

Exercise - solution

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// Returns the largest value in the given array.
public static int max(int[] data) {
    int max = data[0];
    for (int i = 1; i < data.length; i++) {
        if (data[i] > max) {
            max = data[i];
        }
    }
    return max;
}

If you finish them all...

If you finish all the exercises, try out our Practice-It web tool. It lets you solve Java problems from our Building Java Programs textbook.

You can view an exercise, type a solution, and submit it to see if you have solved it correctly.

Choose some problems from Chapter 7 and try to solve them!