Week 6 - Friday

 What did we talk about last time?  Loop examples

do { statement1; statement2; … statementn; } while( condition );

 I said that the do-while loop is rarely used, but certain kinds of input are well suited to a do-while  For example, what about a program that gives a menu of options 1. Add two numbers 2. Subtract two numbers 3. Multiply two numbers 4. Quit

 Loops are great  But, without a way to talk about a list of variables, we can’t get the full potential out of a loop  Enter: the array

 An array is a homogeneous, static data structure  Homogeneous means that everything in the array is the same type: int, double, String, etc.  Static (in this case) means that the size of the array is fixed when you create it

 The args variable passed into the main() method is an array of type String  This array has a set number of String s (maybe zero) that you can use as input to your program  Now, we are giving you the ability to create and manipulate your own arrays

 To declare an array of a specified type with a given name :  Example with a list of type int :  Just like any variable declaration, but with [] type[] name; int[] list;

 When you declare an array, you are only creating a variable that can hold an array  At first, it holds nothing, also know as null  To use it, you have to create an array, supplying a specific size:  This code creates an array of 100 int s int[] list; list = new int[100]; int[] list; list = new int[100];

 You can access an element of an array by indexing into it, using square brackets and a number  Once you have indexed into an array, that variable behaves exactly like any other variable of that type  You can read values from it and store values into it  Indexing starts at 0 and stops at 1 less than the length list[9] = 142; System.out.println(list[9]); list[9] = 142; System.out.println(list[9]);

 When you instantiate an array, you specify the length  Sometimes (like in the case of args ) you are given an array of unknown length  You can use its length member to find out int[] list = new int[42]; int size = list.length; System.out.println("List has " + size + " elements"); //prints 42 int[] list = new int[42]; int size = list.length; System.out.println("List has " + size + " elements"); //prints 42

 When you create an int, double, char, or boolean array, the array is automatically filled with certain values  For other types, including String s, each index in the array must be filled explicitly TypeValue int0 double0.0 char'\0' booleanfalse

 Explicit initialization can be done with a list:  Or, a loop could be used to set all the values: String[] days = {"Monday", "Tuesday", "Wednesday", "Thursday", "Friday", "Saturday", "Sunday"}; String[] numbers = new String[100]; for(int i = 0; i < numbers.length; i++) numbers[i] = "" + (i + 1); String[] numbers = new String[100]; for(int i = 0; i < numbers.length; i++) numbers[i] = "" + (i + 1);

 An array takes up the size of each element times the length of the array  Each array starts at some point in computer memory  The index used for the array is actually an offset from that starting point  That’s why the first element is at index 0

 We can imagine that we have an array of type int of length 10  Java decides what address in memory is going to be used, but let’s say it starts at Addresses Indexes

 Arrays are a fixed size list of a single kind of data  A for loop is ideal for iterating over every item and performing some operation  for loops and arrays will crop up again and again  Of course, a while loop can be used with an array, but it rarely is

 Imagine that we have an array of int s called list  Let’s use a for loop to sum up those int s  Super easy, just like before  We don’t even need to know how big the array is ahead of time int sum = 0; for( int i = 0; i < list.length; i++ ) sum += list[i]; int sum = 0; for( int i = 0; i < list.length; i++ ) sum += list[i];

 More on arrays  StdDraw

 Keep reading Chapter 6 of the textbook  Keep working on Project 2