Conditions and Decisions

Conditions and Decisions
An Introduction with Examples in Java

Prof. David Bernstein
James Madison University

Computer Science Department

bernstdh@jmu.edu

Review

Logical Operators:
- Have Boolean operands and evaluate to a Boolean
Relational Operators:
- Typically have numeric operands and evaluate to a Boolean

Motivation

An Observations:
- Some algorithms require a decision to be made before steps are performed (in other words, the steps that are performed depend on the value of one or more variables)
Operationalizing this Idea in a Program:
- Step 1: Evaluate a Boolean expression
- Step 2: Base what happens next on the evaluation

Two Conditional Statements in Java

The switch Statement
- Used to "choose" from multiple alternatives
The if Statement
- Used for almost everything else

The switch Statement

The switch statement has the following syntax :

switch (variable) { [case value: statement... [break;]]... [default: statement... [break;]] }

The switch Statement (cont.)

An Example

javaexamples/basics/SwitchExample.java (Fragment: 0)

The if Statement

The if statement has the following syntax :

if (boolean_expression) statement [else statement]

The if Statement (cont.)

The statement:
- Can be a simple statement terminating with a ;
- Can be a block statement (i.e., 0 or more simple statements enclosed in {})
White Space:
- Indenting and white space are not "relevant" syntactically
- Indenting and white space improve readability stylistically

A Railroad/Train Analogy for if Statements

Syntax

Best Practices Involving if Statements

Do Not Use "Empty" if Clauses:
- Change the condition instead (using rules of inference)
Do Not Use "Empty" else Clauses:
- Omit the clause
- Include a comment if it clarifies the situation
Remember to Have a Default:
- Either initialize to the default or have an else clause

Eliminating Empty if Clauses

An Example of an Empty if Clause:
- if (value >= 0) { } else { JMUConsole.print("-"); }
The Fix:
- if (value < 0) { JMUConsole.print("-"); }

Defaults in if Statements

Initializing to the Default:
- result = "Pass"; if (grade < 60) result = "Fail";
Including an else Clause:
- if (grade < 60) result = "Fail"; else result = "Pass";

Styles of if Statements

A Condensed Style:
- javaexamples/basics/IfExample1.java (Fragment: 0)
An Expanded Style:
- javaexamples/basics/IfExample1.java (Fragment: 1)

Nested if Statements

The statement Can Be Another if

javaexamples/basics/IfExample2.java (Fragment: 0)

Nested if Statements (cont.)

A Condensed Style for "Nesting"

javaexamples/basics/IfExample2.java (Fragment: 1)

Nested if Statements (cont.)

Multiple Nested if Statements

javaexamples/basics/IfExample3.java (Fragment: 1)

Nested if Statements (cont.)

A Condensed Style for Multiple Nested ifs

javaexamples/basics/IfExample3.java (Fragment: 0)

Nested if Statements (cont.)

A switch-Like Style

javaexamples/basics/IfExample4.java (Fragment: 0)

Common Logic Errors Involving Sequential if Statements

Is either or both of the following correct?

javaexamples/basics/IfExample5.java (Fragment: 0)

Common Logic Errors Involving Sequential if Statements (cont.)

Is either or both of the following correct?

javaexamples/basics/IfExample5.java (Fragment: 1)

There's Always More to Learn