University of Washington, CSE 142

Lab 7: Arrays

Except where otherwise noted, the contents of this document are Copyright 2013 Stuart Reges and Marty Stepp.

lab document created by Marty Stepp, Stuart Reges and Whitaker Brand

Basic lab instructions

Today's lab

Goals for today:

Making arrays

An array is an object that can store many values of the same type. Making an array requires declaring how many values the array can hold, and what type of values the array will hold. The array type and capacity cannot change once the array has been made. 

// makes an integer array with 4 slots in it. Default: all values are 0.
int[] arrayName = new int[4]; 
// another way to make an array: makes an integer array with 4 slots, all values being 0
int[] arrayName = {0, 0, 0, 0};
// arrays can be of any type!
// when object arrays are made, default value of each slot is null.
String[] arrayName = new String[5];

Exercise : Array declaration syntax practice-it

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

Exercise : Quick initialization syntax practice-it

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

Accessing elements in an array

An array can be thought of as a series of cubbyholes. We can put values in and take values out of the cubbyholes by specifying which cubbyhole we want to work with. Just like with Strings, each cubby is indexed from 0 to the the length of the String/array. 

int[] a = new int[5];

// notice that unlike with Strings, we do not put a () after length
int length = a.length;

int[] a = new int[10];
  
// makes the value at index 5 equal to 8.
// the pre-existing value at index 5 is ignored and overwritten.
a[5] = 8;

// sets num equal to 8, the value at index 5 of a. Does not change the value of a[5].
int num = a[5];

ArrayIndexOutOfBoundsException

If you try to access an index smaller than 0 or greater than array.length - 1, your program will crash with an ArrayIndexOutOfBoundsException. 

int[] a = new int[4];

// changes every value in array to equal 5.
for (int i = 0; i < a.length; i++) {
   a[i] = 5;
}

// causes program to crash with ArrayIndexOutOfBoundsException
a[6] = 5;

// also causes program to crash with ArrayIndexOutOfBoundsException
int num = a[4];

Exercise a: Array code tracing practice-it

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];
index 0 1 2 3 4 5 6 7
value 3 3 0 0 5 0 0 -18

Reference semantics

An array is an object.

Whether the elements of the array are objects or not depends on what type of values the array stores.

continued on the next slide...

Reference semantics

With primitive values, we use value semantics: 
int x = 5;
int y = x;
x = 10;
// here, x = 10, y = 5
Setting y = x meant y = the value that x had at that point in time. y does not change values when x changes values.

With Objects, we use reference semantics: 
int[] a = new int[5];     // a ==> [0, 0, 0, 0, 0]
int[] b = a;              // a ==> [0, 0, 0, 0, 0] <== b
a[0] = 10;                // a ==> [10, 0, 0, 0, 0] <== b 
b[1] = 8;                 // a ==> [10, 8, 0, 0, 0] <== b
Setting b = a means b and a now reference the same array. Changes to either a or b change both a and b.

Exercise b: Array code tracing practice-it

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];
index 0 1 2 3 4 5 6 7
value 3 3 0 0 6 9 0 -18

Exercise : Array code tracing 2 practice-it

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]);
}
index 0 1 2 3 4 5
value 3 24 8 -5 6 1

Exercise : PromptNumbers practice-it

Arrays as parameter/return (declare)

public static returnType methodName(arrayType[] parameterName) {   // pass array parameter

public static returnArrayType[] methodName(parameters) {            // return array

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;
}

Arrays as parameter/return (call)

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]
Note: to print out the array we said System.out.println(Arrays.toString(arrayName)). Why's that? If we just say System.out.println(arrayName), we'd get a weird jumble of letters and numbers. To get a well formatted array, we need to ask the Arrays class to turn the array into a nice String for us!

Arrays class methods

In fact, the Arrays class has a ton of useful methods!
Method name Description
Arrays.binarySearch(array, value) returns index of value in a sorted array (< 0 if not found)
Arrays.copyOf(array, length) returns a new copy of an array
Arrays.equals(array1, array2) returns true if the two arrays contain same elements
Arrays.fill(array, value) sets every element to the given value
Arrays.sort(array) arranges the elements into sorted order
Arrays.toString(array) returns a string for the array, such as "[10, 30, -25, 17]"

Exercise : array mystery practice-it

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.

Hint: it might be helpful to draw out an array an update its values as you trace through the code! 
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];
        }
    }
}
int[] a1 = {2, 4};
mystery(a1);		 
{4, 4}
[^0-9,]+
int[] a2 = {1, 3, 6};
mystery(a2);		 
{3, 6, 6}
[^0-9,]+
int[] a3 = {7, 2, 8, 4};
mystery(a3);		 
{7, 8, 8, 4}
[^0-9,]+
int[] a4 = {5, 2, 7, 2, 4};
mystery(a4);		 
{5, 7, 7, 4, 4}
[^0-9,]+
int[] a5 = {2, 4, 6, 3, 7, 9};
mystery(a5);		 
{4, 6, 6, 7, 9, 9}
[^0-9,]+

Exercise : PromptNumbers2

Exercise - answer

    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);
    }

    public static void printForward(int[] a) {
        for (int i = 0; i < a.length; i++) {
            System.out.println(a[i]);
        }
    }

    public static void printBackward(int[] a) {
        for (int i = a.length - 1; i >= 0; i--) {
            System.out.println(a[i]);
        }
    }

Do we need to return arrays?

It depends!

Exercise : do we need to return?

For the following, do we need to return the array? Write yes or no: 
// takes in an integer array, adds 5 to each element
public static ??? addFive(int[] a) {
   for (int i = 0; i < a.length; i++) {
      a[i] = a[i] + 5;
   }
   return a? 
no

}
// takes in an integer array, returns an array with each element increased by 5.
// does not modify given array.
public static ??? addFive(int[] a) {
   int[] returnArray = new int[a.length];
   for (int i = 0; i < a.length; i++) {
      returnArray[i] = a[i] + 5;
   }
   return returnArray? 
yes

}

Exercise : Array reference mystery practice-it

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

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]++;
    }
}
index 0 1 2 3
value 3 5 0 4

Checkpoint: Congratulations!

Nice job making it this far--labs are tough! Feel free to work with the person next to you for the remaining slides. Labs are a unique opportunity (unlike homework) to collaborate directly on ideas, and practice peer programming.

These next problems get a little more challenging as we explore earlier concepts further.

We put a lot of problems in here so that you have plenty to refer back to later when working on homework. Don't feel bad if you don't finish all of them--Brett can't finish them all in a 50 minute lab, either! :)

Forest the cat says good job!

Exercise : max errors practice-it 

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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 some 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 - solution 

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// Returns the largest value in the given array.
public static int max(int[] data) {
    // every value of data could be negative, in which case setting max = 0 is incorrect
    // max = data[0] is safe as the value at the 0th index is definitely in data
    int max = data[0];
    for (int i = 1; i < data.length; i++) {
        if (data[i] > max) {
            max = data[i];
        }
    }
    return max;
}

Exercise : jGRASP Debugger

Exercise - jGRASP Debugger

continued on the next slide...

Exercise - jGRASP Debugger

Exercise : percentEven practice-it

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 : minGap practice-it

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 : swapAll practice-it

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 practice-it

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 : equals practice-it

Write a method named equals that accepts two arrays of integers as parameters and returns true if they contain exactly the same elements in the same order, and false otherwise. Note that the arrays might not be the same length; if the lengths differ, return false. Do not call Arrays.equals in your solution.

For example, if the following arrays are declared: 

int[] a1 = {10, 20, 30, 40, 50, 60};
int[] a2 = {10, 20, 30, 40, 50, 60};
int[] a3 = {20, 3, 50, 10, 68};

The call equals(a1, a2) returns true but the call equals(a1, a3) returns false.

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 the book and try to solve them!