Making Decisions

1
Making Decisions

• B. Ramamurthy
• CS113B

2
Introduction

• Two roads divulged in a yellow wood,
• And sorry I could not travel both.
• Robert Frost
• From The Road Not Taken

3
Decision Statements (General)

• How to compare data values?
• Relational operators
• How to alter the sequence of program execution
  based on the result?
• If.. Else statements in non-event-driven
  programming
• Logical operators (, and ! ) and their use.
• How to deal with multiple choices?
• Switch statement

4
if Statement

• An if statement allows a program to choose
  whether or not to execute a following statement.
• Syntax
• if (condition)
• statement
• Semantics
• condition is a Boolean expression Something that
  evaluates to True or False.
• If condition is true then execute the statement
  is executed.

5
Relational Operators

• Operator Meaning
• Greater than ?
• Greater than or equal to?
• Equal to? (most troublesome)
• ! Not Equal to ?

6
An Example

• Condition in the if statement can be formed using
  relational operators which express comparison.
• P.93, p.96 Blue Book

7
If -else Statement

• An if-else statement allows a program to do one
  thing if a condition is true and a different
  thing if the condition is false.
• Syntax
• if ( condition )
• statement1
• else
• statement2
• Statements to be executed for if and else can be
  a single statement or multiple statements
  enclosed in .

8
Nested if Statement

• A body of an if statement could be another if
  statement. This situation is called nested-if.
• In this case, an else is matched with the most
  recent unmatched if.
• A nested-if allows a program to make decisions
  based on the results of the previous decision.

9
Dangling Else Problem

• if ( total Max)
• if (total
• System.out.println ( total equals Max but
  is less than sum)
• else
• System.out.println (total is not equal to
  Max)

10
Summary

• In many applications, choices are to be made
  depending on some conditions related to the
  problem. Selection or decision structures are
  used to model such situations.
• Java supports the implementation of selection
  through the if and switch statements. In this
  discussion we looked at various forms of if
  statements.