Week 4 - Monday What did we talk about last time? if statements - PowerPoint PPT Presentation

Week 4 - Monday

 What did we talk about last time?  if statements  else statements  Nested selection statements

The if part Any boolean expression if( condition ){ statements; } Executable statements

if( condition ) { statements1; } else { statements2; } Two different outcomes

if( condition1 ){ statement1; if( condition2 ) { if( condition3 ) statement2; … } }

 Sometimes you probably break the speed limit  But, there's one speed limit you can never break  The speed of light c is about 3 x 10 8 m/s  Given a variable named speed of type double , what's an if -statement that will print an error message if speed is larger than c ?

 Recall the 4 quadrants of the Cartesian coordinate system  Let's update our code to say if the point falls on the x axis, the y axis, or the origin y 2 (0,0) 1 -x x 3 4 -y

 Now you are controlling the flow of execution in your program  There is a wider range of mistakes you can make when giving instructions  Huge chunks of code can be executed or skipped by mistake  Here are a few things to watch out for

 Remember that an if -statement is not an executable statement  It does not end with a semicolon if( balance < 0 ); // empty statement { // this block always runs System.out.println("You owe a fee!"); balance -= 15; }

 In some languages, indentation actually matters  Java ignores whitespace if( enemies > 2 ) System.out.println("Run away!"); else defense = true; System.out.println("Fight!");  "Fight!" prints no matter what

 It’s easy to make logical errors when writing conditions  If an airline allows two or fewer bags on the plane, someone might code that as: if( bags < 2 ) { // only allows 1 or 0 boarding = true; }  But this is too restrictive. It should be: if( bags <= 2 ) { boarding = true; }

 Sometimes it's easy to get a condition backwards  Try not to assume you wrote the condition correctly if( number % 3 == 0 ) { System.out.println("Not divisible by 3!"); } else { System.out.println("Divisible by 3!"); }  Always double check

 Finish switch statements  More examples

 Keep reading Chapter 4 of the textbook  Start working on Project 2  Exam 1 next Monday  Review on Friday