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)

       switch (seatLocationCode)
       {
          case 100:
             vip   = true;
             price = 40.0;
             break;
                
          case 200:
             price = 30.0;
             break;
                
          case 300:
             price = 15.0;
             break;
                
          default:
             price = 0.0;
             break;
       }

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)

       if (grade < 60) result = "Fail";
       else result = "Pass";

An Expanded Style:

javaexamples/basics/IfExample1.java

(Fragment: 1)

       if (grade < 60) 
       {
          result = "Fail";
       } 
       else 
       {
          result = "Pass";
       }

Nested if Statements

The statement Can Be Another if

javaexamples/basics/IfExample2.java

(Fragment: 0)

       if (income < 30000) 
       {
          level = "Low";
       }
       else
       {
          if (income < 60000) 
          {
             level = "Middle";
          }
          else
          {
             level = "High";
          }
       }

Nested if Statements (cont.)

A Condensed Style for "Nesting"

javaexamples/basics/IfExample2.java

(Fragment: 1)

       if      (income < 30000) level = "Low";
       else if (income < 60000) level = "Middle";
       else                     level = "High";

Nested if Statements (cont.)

Multiple Nested if Statements

javaexamples/basics/IfExample3.java

(Fragment: 1)

       if (years <= 1)
       {
          title = "Freshman";
       }
       else
       {
          if  (years <= 2) 
          {
             title = "Sophomore";
          }
          else 
          {
             if  (years <= 3) 
             {
                title = "Junior";
             }
             else 
             {
                title = "Senior";
             }
          }
       }

Nested if Statements (cont.)

A Condensed Style for Multiple Nested ifs

javaexamples/basics/IfExample3.java

(Fragment: 0)

       if (years <= 1) title = "Freshman";
       else if  (years <= 2) title = "Sophomore";
       else if  (years <= 3) title = "Junior";
       else title = "Senior";

Nested if Statements (cont.)

A switch-Like Style

javaexamples/basics/IfExample4.java

(Fragment: 0)

       if        (seatLocationCode == 100)   // case 100:
       {
          vip   = true;
          price = 40.0;
       } 
       else if (seatLocationCode == 200)    // case 200:
       {
          price = 30.0;
       } 
       else if (seatLocationCode == 300)    // case 300:
       {
          price = 15.0;
       }
       else                                 // default:
       {
          price = 0.0;
       }

Common Logic Errors Involving Sequential if Statements

Is either or both of the following correct?

javaexamples/basics/IfExample5.java

(Fragment: 0)

       //    One approach (that might not work)
       if (height < 58) idealWeight = 111;
       if (height < 59) idealWeight = 113;
       if (height < 60) idealWeight = 115;

       //    Another approach (that might not work)
       if (height < 60) idealWeight = 115;
       if (height < 59) idealWeight = 113;
       if (height < 58) idealWeight = 111;

Common Logic Errors Involving Sequential if Statements (cont.)

Is either or both of the following correct?

javaexamples/basics/IfExample5.java

(Fragment: 1)

       //    One approach (that might not work)
       if (height < 58) idealWeight = 111;
       if (height < 59) idealWeight = 113;
       if (height < 60) idealWeight = 115;

       //    Still another approach (that might not work)
       if      (height < 58) idealWeight = 111;
       else if (height < 59) idealWeight = 113;
       else if (height < 60) idealWeight = 115;