Week 6 - Monday

 What did we talk about last time?  while loop examples  Lab 5

 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 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: 1. Initialization 2. Condition 3. Increment

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

 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  Then, like if -statements and while -loops, the braces are optional for( init; condition; inc ) statement;

 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++; } int i = 1; while( i <= 100 ) { System.out.println(i); i++; }

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

 Ask the user to input a positive integer n  Now, write a for loop to print out the first n odd numbers  Example: If the user enters 10, print out:

 We can do something called a Monte Carlo approximation of π  We “throw” darts at a 1 x 1 square in the upper right corner of a circle with radius 1  We count the ones that fall inside the circle and divide by the total darts thrown  That fraction is an estimation of the area of one fourth of the circle  By multiplying by 4, we approximate π y x

 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 );

 Examples with for loops and do-while loops

 Keep reading Chapter 5 of the textbook  Keep working on Project 2