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

• Mouse over highlighted words if you're not sure what they mean!
• Talk to your classmates for help.
• You may want to bring your textbook to future labs to look up syntax and examples.
• Stuck? Confused? Have a question? Ask a TA for help, or look at the book or past lecture slides.
• Complete as much of the lab as you can within the allotted time. You don't need to keep working on these exercises after you leave.
• Feel free to complete problems in any order.
• Make sure you've signed in on the sign-in sheet before you leave!

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

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];

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];

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.

• If the array is storing primitive type values (like ints, doubles, or chars), then the elements are not objects.
• If the array stores objects (like Scanners or PrintStreams), the array elements are objects.

Reference semantics

int x = 5;
int y = x;
x = 10;
// here, x = 10, y = 5

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

Exercise b: 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];

Arrays as parameter/return (declare)

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

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

• 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;
}

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]

Arrays class methods

Do we need to return arrays?

It depends!

• If we are modifying an array passed in as a parameter, we do not need to return the array. If we create int[] a in methodX() and then pass a into methodY(), any changes made to the parameter array in methodY() will also be reflected in methodX().
• If methodX() calls methodY(), and then we make int[] a in methodY(), a must be returned to methodX() for methodX() to be able to use it.

Exercise : do we need to return?

// 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
}

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!

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!