The Conditional Operator and the 'switch' Statement

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Course:	CS101: Introduction to Computer Science I
Book:	The Conditional Operator and the 'switch' Statement

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Date:	Wednesday, May 14, 2025, 2:26 AM

Description

Read this chapter, which discusses the switch and '?' operators to write conditional statements. As you read this tutorial, you will learn that sometimes it is better to use a 'switch' statement when there are multiple choices to choose from. Under such conditions, an if/else structure can become very long, obscure, and difficult to comprehend. Once you have read the tutorial, you will understand the similarity of the logic used for the 'if/else' statement, '?', and 'switch' statements and how they can be used alternately.

1. More Ways to Make Decisions
2. If-Else Absolute Value
3. Maximum and Minimum
4. More Complicated Expressions
5. Many-way Branches
6. Rules for switch Statements
7. Example
8. Flowchart of a break Statement
9. Using break
10. Corrected Program
11. Upper and Lower Case
12. if Statement Equivalent
13. Range Testing
14. switch with Strings
15. Example with Strings
16. End of Chapter

1. More Ways to Make Decisions

Nested if and else statements can be used to choose between any number of options. But, using nested statements is sometimes awkward. This chapter discusses two other statements that choose between several options.

Neither of these two statements does anything that can't be done with ifs and elses. But they are sometimes useful, and you will need to know them to understand programs written by other programmers.

Some programmers avoid using these statements. Neither statement is used in the AP Computer Science test. This entire chapter may be skipped. These statements are not used in the chapters that follow.

Chapter Topics:

The conditional operator ?
The switch statement

Question 1:
What is the absolute value of -9?
What is the absolute value of +9?

Source: Bradley Kjell, http://programmedlessons.org/Java9/chap34/ch34_01.html
This work is licensed under a Creative Commons Attribution-NonCommercial 4.0 License.

2. If-Else Absolute Value

Answer:
What is the absolute value of -9?+9
What is the absolute value of +9?+9

If-Else Absolute Value

An if statement can be used to compute absolute values:

if ( value < 0 )
  abs = -value;
else
  abs = value;

This is awkward for such a simple idea. The following does the same thing in one statement:

abs = (value < 0 ) ? -value : value ;

This statement uses a conditional operator. The right side of the = is a conditional expression. The expression is evaluated to produce a value, which is then assigned to the variable, abs.

The conditional operator is used in an expression. It asks for a value to be computed but does not by itself change any variable. In the example, the variable value is not changed. Usually, an assignment operator is used to make a change.

double value = -34.569;
double abs;

abs = (value < 0 )   ?   -value : value ;
      -------------       ------
      1. condition         2.  this is evaluated,
         is true               to +34.569
                      
----
3.  The +34.569 is assigned to abs

The conditional operator is used like this:

true-or-false-condition ? value-if-true : value-if-false

Here is how it works:

The true-or-false-condition evaluates to true or false.
That value selects one choice:
- If the true-or-false-condition is true, then evaluate the expression between ? and :
- If the true-or-false-condition is false, then evaluate the expression between : and the end .
The result of evaluation is the value given to the entire conditional expression.

Question 2:
Given
int a = 7, b = 21;
What is the value of:
a > b ? a : b 
(Even though it looks funny, the entire expression stands for a single value.)

3. Maximum and Minimum

Answer:
The expression is evaluated to 21.

Maximum and Minimum

int a = 7, b = 21;
a > b ? a : b

The expression evaluates to the maximum of two values:

The condition a > b is false, so
The part after the colon (:) is evaluated, to 21.
That value is given to the entire conditional expression.

(Usually such an expression is part of a longer statement that does something with the value.)

Here is a program fragment that prints the minimum of two variables.

int a = 7, b = 21;
System.out.println( "The min is: " + (a ______ b ? a : b ) );

Other than the blank, the fragment is correct. The value of the conditional expression is used with the concatenation operator + in the println() statement.

Question 3:
Fill in the blank so that the fragment works correctly.

4. More Complicated Expressions

Answer:
int a = 7, b = 21;
System.out.println( "The min is: " + (a < b ? a : b ) );

More Complicated Expressions

Here is a slightly more interesting example:

A program computes the average grade of each student in a class. Students whose average grade is below 60 are given a bonus of 5 points. All other students are given a bonus of just 3 points.

Here is the program fragment that does this:

... average grade is computed here ...
average += (average < 60 ) ?  5 : 3;

The subexpressions that make up a conditional expression can be as complicated as you want. Now say that grades below 60 are to be increased by 10 percent and that other grades are to be increased by 5 percent.

Here is the program fragment that does this:

... average grade is computed here ...
average += 
  (average < 60 ) ? average*0.10 : average*0.05;

This is probably about as complicated as you want to get with the conditional operator. If you need to do something more complicated than this, do it with if-else statements.

Question 4:
Write an assignment statement that adds one to the integer number if it is odd, but adds nothing if it is even.
Click here for a

5. Many-way Branches

Answer:
number += (number % 2 == 1 ) ? 1 : 0 ;

Many-way Branches

Often a program needs to make a choice among several options based on the value of a single expression. For example, a clothing store might offer a discount that depends on the quality of the goods:

Class 'A' goods are not discounted at all.
Class 'B' goods are discounted 10%.
Class 'C' goods are discounted 20%.
anything else is discounted 30%.

The program fragment does that. A choice is made between four options based on the value in code.

The switch statement looks at the cases to find a match for the value in code.

It then executes the statements between the matching case and the following break. All other cases are skipped. If there is no match, the default case is chosen.

The default case is optional. If present, it must be the last case.

Warning: the complete rules for switch statements are complicated. Read on to get the details.

double discount;        
char code = 'B' ;
switch ( code )
{
  case 'A':
    discount = 0.0;
    break;
  case 'B':
    discount = 0.1;
    break;
  case 'C':
    discount = 0.2;
    break;
  default:
    discount = 0.3;
}

Question 5:
If code is 'C', what is the discount?

6. Rules for switch Statements

Answer:
0.2

Rules for `switch` Statements

Here is what a switch statement looks like:

switch ( expression )
{
  case label1:
    statementList1 
    break;

  case label2:
    statementList2 
    break;

  case label3:
    statementList3 
    break;

  . . . other cases like the above

  default:
     defaultStatementList 
}

Here is how it works:

Only one case is selected per execution of the switch statement.
The value of expression determines which case is selected.
expression must evaluate to byte, short, char, or int primitive data, a String, or a few other types not discussed further here.
The type of expression and the type of the labels must agree.
Each label must be an integer literal (like 0, 23, or 'A'), a String literal (like "green"), but not an expression or variable.
There can be any number of statements in a statementList.
The statementList is usually followed with break;
Each time the switch statement is executed, the following happens:
1. The expression is evaluated.
2. The labels after each case are inspected one by one, starting with the first.
3. The first label that matches has its statementList execute.
4. The statements execute until a break statement is encountered.
5. Now the entire switch statement is complete (no further statements are executed.)
If no case label matches the value of expression, then the default case is picked, and its statements execute.
If no case label matches the value of expression, and there is no default case, no statements execute.

Question 6:
Could the type of the expression be a float or a double?

7. Example

Answer:
No

Rules for `switch` Statements

We will get to those other rules shortly. Here is the previous example, with more explanation:


double discount;

char   code = 'B' ;   // Usually the value for code would come from data
                      // or from user interaction

switch ( code )
{
  case 'A':
    discount = 0.0;
    break;

  case 'B':
    discount = 0.1;
    break;

  case 'C':
    discount = 0.2;
    break;

  default:
    discount = 0.3;
}

System.out.println( "discount is: " + discount );

The expression is evaluated.
- In this example, the expression is the variable, code, which evaluates to the character 'B'.
The case labels are inspected starting with the first.
The first one that matches is case 'B'
The statementList after case 'B' starts executing.
- In this example, there is just one statement.
- The statement assigns 0.1 to discount.
The break statement is encountered which ends the statementList.
The statement after the entire switch statement is executed.
- In this example, that is the println() statement

Question 7:
If code is 'W' what is discount?

8. Flowchart of a break Statement

Answer:
0.3

Flowchart of a `break` Statement

Here is the example program fragment (again) and a flowchart that shows how it works. The box in the flowchart "evaluate code" means to get the current value of code. In a larger program this would usually be different every time.

Annotation 2020-03-09 210049

Question 8:
(Trick Question:) What would be the discount if code were 'a' ?

9. Using break

Answer:
0.3
Notice that the code is a lower case 'a', not upper case.

Using `break`

There is one label per case statement, and there must be an exact match with the expression in the switch for the case to be chosen.

The break at the end of each case is optional. If it is omitted, then after the statements in the selected case have executed, execution will continue with the following case. This is not usually what you want.

The fragment will print:

Color is yellow green blue violet unknown

This is probably not what the program was intended to do.

Examine the following fragment:

class Switcher
{
  public static void main ( String[] args )
  {
    char   color = 'Y' ;    
    String message = "Color is";
    
    switch ( color )
    {
    
      case 'R':
        message = message + " red" ;
                        
      case 'O':
        message = message + " orange" ;
                        
      case 'Y':
        message = message + " yellow" ;
                        
      case 'G':
        message = message + " green" ;
                        
      case 'B':
        message = message + " blue" ;
                        
      case 'V':
        message = message + " violet" ;
                        
      default:
        message = message + " unknown" ;
            
    }

  System.out.println ( message ) ;
  }
}

Question 9:
Mentally fix the program fragment so that it prints just one color name.

10. Corrected Program

Answer:
The correct program fragment is given below.

Corrected Program

Here is the corrected program. It would be OK if you put a break after the statement in the default case, but it is not needed.

class Switcher
{
  public static void main ( String[] args )
  {
    char   color = 'Y' ;    
    String message = "Color is";
    
    switch ( color )
    {
    
      case 'R':
        message = message + " red" ;
        break;
                        
      case 'O':
        message = message + " orange" ;
        break;
                        
      case 'Y':
        message = message + " yellow" ;
        break;
                        
      case 'G':
        message = message + " green" ;
        break;
                        
      case 'B':
        message = message + " blue" ;
        break;
                        
      case 'V':
        message = message + " violet" ;
        break;
                        
      default:
        message = message + " unknown" ;
            
    }

  System.out.println ( message ) ;
  }
}

Often what you really want is for several characters to select a single case. This can be done using several case statements, followed by just one list of statements.

For example, here both 'y' and 'Y' select the same statement:

      case 'y':                        
      case 'Y':
        message = message + " yellow" ;
        break;

Question 10:
Mentally insert extra case statements into the program so that upper and lower case 
characters work for each color. (Or, even better: copy the program to an editor, fix it, and run it.)

11. Upper and Lower Case

Answer:
The completed program is given below.

Upper and Lower Case

The program has also been improved by accepting input from the user. The program uses the charAt( int index ) method of the String class. This method returns a single character from a string.

The first character is at index 0, the next is at index 1, and so on. (Remember: a String is an object, even if it contains only one character. The charAt() method must be used here to get a char that can be used in the switch.)


import java.util.Scanner;
class Switcher
{
  public static void main ( String[] args ) 
  {
    String lineIn;
    char   color  ;    
    String message = "Color is";
    
    Scanner scan = new Scanner( System.in );

    System.out.print("Enter a color letter: ");
    lineIn = scan.nextLine();
    color = lineIn.charAt( 0 );  // get the first character

    switch ( color )
    {
    
      case 'r':
      case 'R':
        message = message + " red" ;
        break;
               
      case 'o':                  
      case 'O':
        message = message + " orange" ;
        break;
               
      case 'y':                  
      case 'Y':
        message = message + " yellow" ;
        break;
               
      case 'g':                  
      case 'G':
        message = message + " green" ;
        break;
               
      case 'b':  
      case 'B':
        message = message + " blue" ;
        break;
               
      case 'v':  
      case 'V':
        message = message + " violet" ;
        break;
                        
      default:
        message = message + " unknown" ;
            
    }

  System.out.println ( message ) ;
  }
}

Question 11:
Could the above program be written using if statements?

12. if Statement Equivalent

Answer:
Of course. Any switch statement can be done with nested ifs .

`if` Statement Equivalent

If you need to select among several options based on complicated requirements, use nested if statements, or if else if statements (which are really the same thing.) Here is the previous program, rewritten with equivalent if else if statements.


import java.util.Scanner;

class Switcher
{
  public static void main ( String[] args ) throws IOException
  {
    String lineIn;
    char   color  ;    
    String message = "Color is";
    
    Scanner scan = new Scanner( System.in );

    System.out.println("Enter a color letter:");
    lineIn = scan.nextLine();
    color = lineIn.charAt( 0 );  // get the first character

    if      ( color=='r' || color=='R' )    
      message = message + " red" ;

    else if ( color=='o' || color=='O' )               
      message = message + " orange" ;
               
    else if ( color=='y' || color=='Y' )               
      message = message + " yellow" ;
               
    else if ( color=='g' || color=='G' )               
      message = message + " green" ;
               
    else if ( color=='b' || color=='B' )               
      message = message + " blue" ;

    else if ( color=='v' || color=='V' )               
      message = message + " violet" ;

    else 
      message = message + " unknown" ;
            
    System.out.println ( message ) ;
  }
}

Question 12:
Is the program correct? Is == being used correctly?

13. Range Testing

Answer:
There is nothing wrong with the program. Since color is a primitive data type, an expression such as
color == 'V'
compares the contents of color (a character) to the character literal 'V'. However, the following is wrong:
color == "V"
This is asking to compare the primitive character value in color with a reference to the String object "V".

Range Testing

Say that you have the following problem: Assign a Beaufort number to bNumber based on wind speed in meters per second.

wind speed	Beaufort number
0 - 0.3	0
0.3 - 1.6	1
1.6 - 3.4	2
3.4 - 5.5	3
5.5 - 8.0	4
8.0 - 10.8	5
10.8 - 13.9	6
13.9 - 17.2	7
17.2 - 20.8	8
20.8 - 24.5	9
24.5 - 28.5	10
28.5 - 32.7	11
>=32.7	12

Assume that the upper number in each range is not included.

Lots of cases! You would hope that the switch statement would work for this. But it does not. Floating point values don't work with switch statements. You need to use if and else if.

if      ( speed <  0.3 ) bNumber = 0;
else if ( speed <  1.6 ) bNumber = 1;
else if ( speed <  3.4 ) bNumber = 2;
else if ( speed <  5.5 ) bNumber = 3;
else if ( speed <  8.0 ) bNumber = 4;
else if ( speed < 10.8 ) bNumber = 5;
else if ( speed < 13.9 ) bNumber = 6;
else if ( speed < 17.2 ) bNumber = 7;
else if ( speed < 20.8 ) bNumber = 8;
else if ( speed < 24.5 ) bNumber = 9;
else if ( speed < 28.5 ) bNumber = 10;
else if ( speed < 32.7 ) bNumber = 11;
else                     bNumber = 12;

Question 13:
Could integer ranges be used with a switch statement? Say that value is expected to be in 
the range 1..10 and you want three categories: 1 to 3, 4 to 7, 8 to 10.

14. switch with Strings

Answer:
This could be done with multiple case: labels, but the result is awkward.
switch ( value )
{
case 1: case 2: case3:
  do-something;
  break;
  
case 4: case 5: case6: case 7:
  do-something;
  break;

case 8: case 9: case 10:
  do-something;
  break;
}

`switch` with `String`s

Recall the syntax of the switch statement:

switch ( expression )
{
  case label1:
    statementList1 
    break;

  case label2:
    statementList2 
    break;

  case label3:
    statementList3 
    break;

  . . . other cases like the above

  default:
     defaultStatementList 
}

Starting with Java 7.0 the expression can be a String reference and the case labels can be String literals.

Matching of the expression with the case labels is done as if by String.equals().

Question 14:
Is "BTW".equals( " BTW ") true or false?

15. Example with Strings

Answer:
"BTW".equals( " BTW ") is false
Blank spaces matter in string comparison.

Example with Strings

Here is a program that asks the user for an acronym and then prints out its meaning:


import java.util.Scanner;
public class StringSwitcher
{
  public static void main ( String[] args ) 
  {
    String phrase;
    char   color  ;    
    String message = "Phrase is: ";
    
    Scanner scan = new Scanner( System.in );

    System.out.print("Enter Acronym: ");
    phrase = scan.nextLine().trim().toUpperCase();
 
    switch ( phrase )
    {
      case "LOL":
        message = message + "Laugh Out Loud" ;
        break;
               
      case "BFF":
        message = message + "Best Friends Forever" ;
        break;
               
      case "SO":
        message = message + "Significant Other" ;
        break;
              
      case "THS":                  
      case "THKS":                  
      case "TX":                  
        message = message + "Thanks" ;
        break;
                           
      default:
        message = message + "unknown" ;
            
    }

    System.out.println ( message ) ;
  }
}

Users might put spaces before or after the acronym. The function String.trim() removes white space from both sides of a string.

Another problem is that users might enter upper or lower case. The function String.toUpperCase() creates a string with all upper case.

Question 15:
Could the strings in the case statements use punctuation?

16. End of Chapter

Answer:
Yes. Any string literal can be used.

End of Chapter

This is the end of the chapter. A strong case can be made for reviewing the following subjects.

conditional operator The conditional operator.
switch statement Syntax of the switch statement.
switch statement, example Example of the switch statement.
switch statement, flowchart Flowchart of the switch statement.
break statement Role of the break statment.
switch statement, labels Using several labels per group of statements.

The Conditional Operator and the 'switch' Statement

Description

Table of contents

1. More Ways to Make Decisions

Question 1:

2. If-Else Absolute Value

Answer:

If-Else Absolute Value

If-Else Absolute Value

Question 2:

3. Maximum and Minimum

Answer:

Maximum and Minimum

Maximum and Minimum

Question 3:

4. More Complicated Expressions

Answer:

More Complicated Expressions

More Complicated Expressions

Question 4:

5. Many-way Branches

Answer:

number += (number % 2 == 1 ) ? 1 : 0 ;

Many-way Branches

Many-way Branches

double discount; char code = 'B' ; switch ( code ){ case 'A': discount = 0.0; break; case 'B': discount = 0.1; break; case 'C': discount = 0.2; break; default: discount = 0.3;}

Question 5:

6. Rules for switch Statements

Answer:

0.2

Rules for switch Statements

Rules for switch Statements

switch ( expression ) { case label1: statementList1 break; case label2: statementList2 break; case label3: statementList3 break; . . . other cases like the above default: defaultStatementList }

Question 6:

7. Example

Answer:

No

Rules for switch Statements

Rules for switch Statements

Question 7:

8. Flowchart of a break Statement

Answer:

0.3

Flowchart of a break Statement

Flowchart of a break Statement

Question 8:

9. Using break

Answer:

Using break

Using break

Question 9:

10. Corrected Program

Answer:

Corrected Program

Corrected Program

Question 10:

11. Upper and Lower Case

Answer:

Upper and Lower Case

Upper and Lower Case

Question 11:

12. if Statement Equivalent

Answer:

if Statement Equivalent

if Statement Equivalent

Question 12:

13. Range Testing

Answer:

Range Testing

Range Testing

Question 13:

14. switch with Strings

Answer:

switch with Strings

switch with Strings

Question 14:

15. Example with Strings

Answer:

Example with Strings

Example with Strings

double discount;
char code = 'B' ; switch ( code )
{
case 'A':
discount = 0.0;
break; case 'B':
discount = 0.1;
break; case 'C':
discount = 0.2;
break; default:
discount = 0.3;
}

Rules for `switch` Statements

Rules for `switch` Statements

Rules for `switch` Statements

Rules for `switch` Statements

Flowchart of a `break` Statement

Flowchart of a `break` Statement

Using `break`

Using `break`

`if` Statement Equivalent

`if` Statement Equivalent

`switch` with `String`s

`switch` with `String`s