Week 5 - Wednesday

 What did we talk about last time?  Exam 1!  And before that?  Review!  And before that?  if and switch statements

 All the little pieces of Java syntax are confusing  I have tried hard to tell you exactly what you need to know (and no more)  Java seems like it has an unlimited amount of stuff to learn, you can already do a lot  They designed Java so that you have just enough tools, but they don't like to have redundant functionality  The key is to think about how to put together the pieces  It's like a puzzle

 Focus on the values you have and how to turn them into the values you want  Let's look at this example of incorrect code and see how to fix it: char letter = in.nextChar(); if( letter >= 'a' && <= 'z' ) System.out.println("Lowercase letter!"); char letter = in.nextChar(); if( letter >= 'a' && <= 'z' ) System.out.println("Lowercase letter!");

 Scanner can't read a char value in directly  We have to read a String String word = in.next(); //we can do this! char letter = word.charAt(0); //and get a char from the String if( letter >= 'a' && <= 'z' ) System.out.println("Lowercase letter!"); String word = in.next(); //we can do this! char letter = word.charAt(0); //and get a char from the String if( letter >= 'a' && <= 'z' ) System.out.println("Lowercase letter!"); char letter = in.nextChar(); if( letter >= 'a' && <= 'z' ) System.out.println("Lowercase letter!"); char letter = in.nextChar(); if( letter >= 'a' && <= 'z' ) System.out.println("Lowercase letter!");

 The && operator only works between two boolean values  <= 'z' isn't a boolean, it's only part of a comparison String word = in.next(); //we can do this! char letter = word.charAt(0); //and get a char from the String if( letter >= 'a' && letter <= 'z' ) //fixed! System.out.println("Lowercase letter!"); String word = in.next(); //we can do this! char letter = word.charAt(0); //and get a char from the String if( letter >= 'a' && letter <= 'z' ) //fixed! System.out.println("Lowercase letter!"); String word = in.next(); //we can do this! char letter = word.charAt(0); //and get a char from the String if( letter >= 'a' && <= 'z' ) System.out.println("Lowercase letter!"); String word = in.next(); //we can do this! char letter = word.charAt(0); //and get a char from the String if( letter >= 'a' && <= 'z' ) System.out.println("Lowercase letter!");

 You're only allowed to do certain operations:  Try to think of everything in terms of these operations  Methods are useful too (for manipulating String values or doing advanced math), but there are too many to list here First Operand Operator Second Operand Result int+, -, *, /, %int double+, -, *, /, %double number==, !=,, >=numberboolean &&, ||boolean String+ any value String

 Java works on a set of arbitrary rules  Some regular people invented them  They are internally consistent, but they are not based on reality  They don't make any deep sense  Don't waste time trying to find meaning in the rules!  Just use the rules as a set of tools you can use to solve your problem  Liker MacGuyver

 "Those who cannot remember the past are condemned to repeat it."  George Santayana  What is our past?  Basic data types  Mathematical and String operations  Input and output  Conditional execution

 To these tools, we add repetition, the ability to do something over and over again  With repetition in the mix, we can solve practically any problem that can be solved with a computer  Repetition leverages the most famous ability of the computer: speed

 The main way that repetition works in Java is through loops  A loop is a block of code that will be executed repeatedly some number of times (perhaps even zero)  As the statements are executed, the variables change  It isn’t just repeating the same thing over and over (unless a mistake was made)

 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; statement2; … statementn; } A whole bunch of statements

 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  Then, like an if -statement, the braces are optional while( condition ) statement;

 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++; } 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

//line A while( condition ) { //line B statement1; statement2; //line C … statementn; } //line D LineCondition An B C D LineCondition AUnknown B C D LineCondition AUnknown BTrue C D LineCondition AUnknown BTrue CUnknown D LineCondition AUnknown BTrue CUnknown DFalse

 We can also use while loops to help us deal with input  What if we wanted to sum all of the numbers that a person entered?  How would we know when to stop?  One solution is to keep adding numbers until the person enters a negative number  This is called using a sentinel value

 Solution: int i = 0; int sum = 0; Scanner in = new Scanner( System.in ); while( i >= 0 ) { sum += i; System.out.print("Enter number: "); i = in.nextInt(); } System.out.println("Sum: " + sum); int i = 0; int sum = 0; Scanner in = new Scanner( System.in ); while( i >= 0 ) { sum += i; System.out.print("Enter number: "); i = in.nextInt(); } System.out.println("Sum: " + sum);

 We could also find the average: int i = 0; double sum = 0; int count = 0; Scanner in = new Scanner( System.in ); while( i >= 0 ) { sum += i; count++; System.out.print("Enter number: "); i = in.nextInt(); } count--; //fixes extra count for sentinel System.out.println("Average: " + (sum / count)); int i = 0; double sum = 0; int count = 0; Scanner in = new Scanner( System.in ); while( i >= 0 ) { sum += i; count++; System.out.print("Enter number: "); i = in.nextInt(); } count--; //fixes extra count for sentinel System.out.println("Average: " + (sum / count));

 What if we wanted to find the biggest input?  Somehow we would have to check each input and see if it were larger than the current largest  Solution: use an if -statement inside of a while loop  Let’s look at Eclipse

 Let’s say that you wanted to write a program to guess a number that a person had come up  The number is between 1 and 100  Every time the computer guesses a number, the person enters:  H if the number is too high  L if the number is too low  F if the number was found

1. Start with the minimum and maximum of the range 2. Find the midpoint 3. Ask the user if the midpoint is correct 4. If the answer is too high, go to Step 1 using the minimum and the midpoint as the new range 5. If the answer is too low, go to Step 1 using the midpoint and the maximum as the new range 6. If the midpoint is correct, you’re done!

 Just as with if -statements, it’s possible to nest loops  A repetitive task can be done inside of another repetitive task  Be careful! You can make the computer do a lot of work

 Triangular numbers are 1, 3, 6, 10, …  1 = 1  3 =  6 =   Let’s write a program that expresses the n th triangular number by printing 1 on the first line, 1 and 2 on the second line, 1, 2, and 3 on the third line, and so on

 Loops can go on forever if you aren’t careful int n = 40; int i = 1; while( i <= 40 ) { System.out.println(i); //supposed to print all the numbers //less than 40, but i never increases } int n = 40; int i = 1; while( i <= 40 ) { System.out.println(i); //supposed to print all the numbers //less than 40, but i never increases }

 Overflow and underflow will make some badly written loops eventually terminate int n = 40; int i = 1; while( i <= 40 ) { System.out.println(i); i--; //whoops, should have been i++ } int n = 40; int i = 1; while( i <= 40 ) { System.out.println(i); i--; //whoops, should have been i++ }

 Being off by one is a very common loop error int n = 40; int i = 1; while( i < 40 ) //won’t //reach 40 { System.out.println(i); i++; } int n = 40; int i = 1; while( i < 40 ) //won’t //reach 40 { System.out.println(i); i++; }

 If the condition isn’t true to begin with, the loop will just be skipped int n = 40; int i = 1; while( i >= 40 ) //oops, should be <= { System.out.println(i); i++; } int n = 40; int i = 1; while( i >= 40 ) //oops, should be <= { System.out.println(i); i++; }

 A misplaced semicolon can cause an empty loop body to be executed (often infinitely) int n = 40; int i = 1; while( i <= 40 ); //semicolon is wrong { System.out.println(i); i++; } int n = 40; int i = 1; while( i <= 40 ); //semicolon is wrong { System.out.println(i); i++; }

 The condition of the while loop is not followed by a semicolon  Be careful about starting and ending conditions  When in doubt, use braces  The print statement must be inside the loop in order to get printed multiple times  There’s no magic formula; you have to think it through

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

 for loops  Lab 5

 Read Chapter 5  Start Project 2