1 Fall 2007ACS-1903 Chapter 3 Decision Structures The if Statement The if-else Statement The if-else-if Statement Nested if Statements Logical Operators Comparing String Objects The Conditional Operator The switch Statement Variable Declaration and Scope DecimalFormat Class printf

2 Fall 2007ACS-1903 if statements Suppose we need to calculate reward points for a retailer. Assume a customer receives 1 reward point for each 25 dollars spent, but if the customer spends more than 300 dollars the customer receives an extra 50 points. Examples: Dollars spent Reward Points

3 Fall 2007ACS-1903 pseudocode If sale is less than or equal to 300 Then reward points are dollar value / 25 Otherwise reward points are 50 plus (dollar value / 25)

4 Fall 2007ACS-1903 Flowchart Sale > 300 Reward points = 50 + (Dollars / 25) Reward points = Dollars / 25 true false

5 Fall 2007ACS-1903 if statement As a UML activity diagram [sale > 300] [sale<=300] rewardPoints = 50 + dollars / 25rewardPoints = dollars / 25

6 Fall 2007ACS-1903 if statement if ( sale <= 300 ) rewardPoints = dollars / 25; else rewardPoints = 50 + dollars / 25; Now … using BlueJ

7 Fall 2007ACS-1903 if statements The if statement contains a section of code that executes if the value of a boolean expression is true. Optionally, there is a section of code that executes if the value of the boolean expression is false. General syntax if (boolean expression) java statement; [ else java statement; ] Note that “[“ and “]” are ways we indicate an optional component in syntax If there is more than one statement you want for a “then” clause or an “else” clause, you can create a compound statement using curly braces {}

8 Fall 2007ACS-1903 if statements Note that an If-else is a java statement and so we allow for If’s to be nested Suppose we give reward points out as before but with an additional bonus of 100 points if the sale is for more than $500. Pseudocode: If sale <= 300 then reward = dollars /25 otherwise if sale <= 500 then reward = dollars/ otherwise reward = dollars/

9 Fall 2007ACS-1903 Flowchart Sale > 300 Reward points = 50 + (Dollars / 25) Reward points = Dollars / 25 true false Reward points = (Dollars / 25) Sale > 500 true false

10 Fall 2007ACS-1903 Java Code if ( saleAmount <= 300 ) rewardPoints = dollarsSpent / 25; else if (saleAmount <=500 ) rewardPoints = 50 + dollarsSpent / 25; else rewardPoints = dollarsSpent/25;

11 Fall 2007ACS-1903 Relational Operators In most cases, the boolean expression, used by the if statement, uses relational operators. Relational OperatorMeaning > is greater than < is less than >= is greater than or equal to <= is less than or equal to == is equal to != is not equal to

12 Fall 2007ACS-1903 Boolean Expressions A boolean expression is any variable or calculation that results in a true or false condition. ExpressionMeaning x > y Is x greater than y? x < y Is x less than y? x >= y Is x greater than or equal to y? x <= y Is x less than or equal to y. x == y Is x equal to y? x != y Is x not equal to y?

13 Fall 2007ACS-1903 Boolean Expressions if (x > y) System.out.println(“X is greater than Y”); if(x == y) System.out.println(“X is equal to Y”); if(x != y){ System.out.println(“X is not equal to Y”); x = y; System.out.println(“However, now it is.”); } Example: AverageScore.javaAverageScore.java Note the use of “{“ and “}” There are 3 statements executed when x!=y is true

14 Fall 2007ACS-1903 Comparing Strings if (gender.equals("male")) { System.out.println("Hello Mr." + name); } else { System.out.println("Hello Ms." + name); } System.out.println("Your gross pay is $" + grossPay); To compare two strings in Java you typically use equals() or equalsIgnoreCase() See documentation for class String Now … using BlueJ

15 Fall 2007ACS-1903 Programming Style Rules of thumb: The conditionally executed statement should be on the line after the if condition. The conditionally executed statement should be indented one level from the if condition. It is suggested by some to always use curly braces, but when an if statement does not have the curly braces, it is ended by the first semicolon encountered after the if condition. if(expression) statement; No semicolon here. Semicolon ends statement here.