Lecture 3

Decisions (Conditionals)

One of the essential features of computer programs is their ability to make decisions. Like a train that changes tracks depending on how the switches are set, a program can take different actions depending on inputs and other circumstances.

In this chapter, you will learn how to program simple and complex decisions. You will apply what you learn to the task of checking user input.

radius < 0

"Incorrect input" area = radius * radius * PI "Area is" + area

Alternative Paths of Execution

true

false

Comparison Operators

conditional

do something

true

false

do next thing

if (conditional) do something;do next thing;

One-Way if Statements

if (conditional) do something;do next thing;

conditional

do something

true

false

do next thing

do other thing

Two-Way if Statements

conditional1

do something

true

false

do next thing

do other thing

conditional2

true

false

yet another thing

if (condition 1) do something;else if (condition 2) do other thing;else yet another thing;do next thing;

if-else if-else

Example Program

import java.util.Scanner;public class LetterGradeDemo{ public static void main(String[] args) { Scanner input = new Scanner(System.in); System.out.print("Enter final average (0-100)... "); double avg = input.nextDouble(); char grade; if (avg >= 90.0) { grade = 'A'; } else if (avg >= 80.0) { grade = 'B'; } else if (avg >= 70.0) { grade = 'C'; } else if (avg >= 60.0) { grade = 'D'; } else { grade = 'F'; } if(grade == 'A') System.out.println("An average of " + avg + " is an " + grade); else System.out.println("An average of " + avg + " is a " + grade); }}

Boolean Operators

Example Program

case 0: case 0 stuff break

case 1:

case 2:

default:

case 1 stuff

case 2 stuff

default stuff

break

break

Switch Statements

Switch Statement Rules

(and String types starting with JDK 7)

Alternative Form for Conditional Statements

For both version of the conditional constructs below, if x is greater than 0 then y is assigned the value 1, otherwise y is assign the value -1.

This alternative version of the two-way conditional statement will not be used in this course. It is provided here for purposes of comparison and the understanding of java programs written by others.

Formatting Console Outputspecifying display data types

Formatting Console Outputspecifying width and precision

Operator Precedence

Confirmation Dialogs

Summary