Week 1 - Friday What did we talk about last time? Java: Types if - PowerPoint PPT Presentation

Week 1 - Friday

 What did we talk about last time?  Java:  Types  if statements

 But there's sometimes a cleaner way to express a list of possibilities  Enter: the switch statement

switch( data ) { case value1: statements 1; case value2: statements 2; … case valuen: statements n; default: default statements; }

switch( base ) { case 'A': System.out.println("Adenine"); break; case 'C': System.out.println("Cytosine"); break; case 'G': System.out.println("Guanine"); break; case 'T': System.out.println("Thymine"); break; default: System.out.println("Your base" + "doesn't belong to us"); break; // unnecessary }

Both "Three" int data = 3; and "Four" switch( data ) { are printed case 3: System.out.println("Three"); case 4: System.out.println("Four"); break; case 5: System.out.println("Five"); The } break is The default optional is optional too

1. The data that you are performing your switch on must be an int , a char , or a String 2. The value for each case must be a literal 3. Execution will jump to the case that matches 4. If no case matches, it will go to default 5. If there is no default , it will skip the whole switch block 6. Execution will continue until it hits a break

switch( base ) { case 'A': case 'a': System.out.println("Adenine"); break; case 'C': case 'c': System.out.println("Cytosine"); break; case 'G': case 'g': System.out.println("Guanine"); break; case 'T': case 't': System.out.println("Thymine"); break; default: System.out.println("Your base doesn't belong to us"); break; // unnecessary }

 Using if -statements is usually safer  if -statements are generally clearer and more flexible  switch statements are only for long lists of specific cases  Be careful about inconsistent use of break

1. while loops  Used when you don’t know how many times you are going to need to repeat 2. for loops  Used when you do know how many times you are going to repeat 3. do-while loops  Used never  Oh, okay, they are used whenever you need to be guaranteed the loop runs at least once

 Any problem that uses loops can use any kind of loop  The choice is supposed to make things easier on the programmer  Some loops are more convenient for certain kinds of problems

 The simplest loop in Java is the while loop  It looks similar to an if statement  The difference is that, when you get to the end of the while loop, it jumps back to the top  If the condition in the while loop is still true , it executes the body of the loop again

while( condition ) { statement1; A whole statement2; bunch of … statements statementn; }

 A while loop will usually have multiple statements in its body  However, it is possible to make a while loop with only a single statement while( condition ) statement;  Then, like an if -statement, the braces are optional

 Let's print the numbers from 1 to 100  We could do this with lots of cut/paste, or: int i = 1; while( i <= 100 ) { System.out.println(i); i++; }

 The while loop executes each statement one by one  When execution gets to the bottom, it jumps to the top  If the condition is still true (i.e., i <= 100 ), it repeats the loop

 for loops are great when you know how many times a loop will run  They are the most commonly used of all loops  They are perfect for any task that needs to run, say, 100 times  A for loop has 3 parts in its header: Initialization 1. 2. Condition Increment 3.

Starting Point Way to Progress for( init; condition; inc ) { statement1; statement2; … Ending statementn; Point }

 A for loop will usually have multiple statements in its body  However, it is possible to make a for loop with only a single statement for( init; condition; inc ) statement;  Then, like if -statements and while -loops, the braces are optional

 Let's print the numbers from 1 to 100 (again)  Remember how this was done with while : int i = 1; while( i <= 100 ) { System.out.println(i); i++; }

 A for loop is specifically designed for this sort of thing: for( int i = 1; i <= 100; i++ ) { System.out.println(i); }  The initialization and the increment are built-in

 They work just like while loops  The only difference is that they are guaranteed to execute at least once  Unlike a while loop, the condition isn’t checked the first time you go into the loop  Sometimes this is especially useful for getting input from the user  The syntax is a little bit different

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

 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

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

 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: int[] list; list = new int[100];  This code creates an array of 100 int s

 You can access an element of an array by indexing into it, using square brackets and a number list[9] = 142; System.out.println(list[9]);  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

 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

 When you create an int , double , char , or boolean array, the array is automatically filled with certain values Type Value int 0 double 0.0 char '\0' boolean false  For other types, including String s, each index in the array must be filled explicitly

 Explicit initialization can be done with a list: String[] days = {"Monday", "Tuesday", "Wednesday", "Thursday", "Friday", "Saturday", "Sunday"};  Or, a loop could be used to set all the values: 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 524 Addresses 524 528 532 536 540 544 548 552 556 560 12 43 -9 6 789 0 -23 23 10 6 0 1 2 3 4 5 6 7 8 9 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 int sum = 0; for( int i = 0; i < list.length; i++ ) sum += list[i];  We don’t even need to know how big the array is ahead of time

 So far, our code has been inside of the main() method  What about a big program?  The main() method is going to get really long and hard to read  Sometimes you need to do similar things several times:  Read some input and check that it falls within certain values  Draw a green hexagon at several different locations  Find the square root of several different numbers

 Methods allow you to package up some code to run over and over  Methods usually take some input (like numbers or Strings ) so that they can be customized  Methods often give back an answer (like the square root of a number)  Also called functions in some other languages

 More modular programming  Break a program into separate tasks  Each task could be assigned to a different programmer  Code reusability  Use code over and over  Even from other programs (like Math.sqrt() )  Less code duplication  Improved readability  Each method can do a few, clear tasks