CS 1713/1711 Introduction to Computer Science Lecture 15

CS 1713 Lecture 15: Nested Selection and the switch Statement
October 4, 1999

Nested if statements:
- When either of the clauses of an if-else statement are themselves if statements, they are sometimes hard to read. (This is especially true of if within an if clause.)
- Rule is the else clause is matched with closest preceding unmatched if. (Indentation is not relevant.)
- Multiple-Alternative Decision: when each of the else clauses is another if except the last one. Convention is that you line up the else's with the first if.

Example: If X is between 0 and 3, increment n otherwise print
out an error message.


 The following is NOT a solution  (why not?) :
      if (0 < X)
         if (X < 3)
            n = n + 1;
      else
         System.out.println("X is not between 0 and 3");

Solution using boolean operators:
      if (0 < X && X < 3)
         n = n + 1;
      else
         System.out.println("X is not between 0 and 3");

Example: Calculate the letter grade based on exam score using multiple-alternative decision. ExamScore Letter Grade 90 and above A 80 - 89 B 70-79 C 60-69 D Below 60 F

Example: Students are asked to code the following table:

Number Of Passengers    Yearly Fee
 < 2    $100  
 2 - 10   $300  
 11 - 20   $500  
 > 20   $1000 

Analyze the solutions they produced are:

Proposed solution 1:
     Fee = 100;
     if (Passengers > 20)
        Fee = 1000;
     if (Passengers > 10)
        Fee = 500;
     if (Passengers > 2)
        Fee = 300;


Proposed solution 2:
     Fee = 100;
     if (Passengers > 2) 
        Fee = Fee + 200;
     if (Passengers > 10)
        Fee = Fee + 400;
     if (Passengers > 20)
        Fee = Fee + 900;


Proposed solution 3:
     Fee = 100;
     if (Passengers >= 2) 
        Fee = Fee + 200;
     else if (Passengers > 10)
        Fee = Fee + 400;
     else if (Passengers > 20)
        Fee = Fee + 900;


Proposed solution 4: 
     Fee = 100;
     if (Passengers >= 2)
        if (Passengers >= 10)
            if (Passengers >= 20)
                Fee = Fee + 900;
            else 
                Fee = Fee + 400;
         else
            Fee = Fee + 200;


Proposed solution 5: 
     Fee = 1000;
     if (Passengers < 20)
        Fee = 500;
     if (Passengers < 10)
        Fee = 300;
     if (Passengers < 2)
        Fee = 100;


Proposed solution 6:
     Fee = 100;
     if (2 <= Passengers && Passengers <= 10)
        Fee = 300;
     else if (10 < Passengers && Passengers <= 20)
        Fee = 500;
     else 
        Fee = 1000;

The switch statement is useful for alternatives that
can be enumerated  (e.g. ordinal types such as single characters,
integers or boolean --- not String or double.):


Example: Finds points earned for grade in calculation of GPA
   char grade;

   switch (grade) {
       case 'A' : 
               points = 4; 
               break;
       case 'B' :
               points = 3;
               break;
       case 'C' :
               points = 2;
               break;
       case 'D' :
               points = 1;
               break;
       case 'E' : case 'I': case 'W':
               points = 0;
               break;
       default:
               points = 0;
               System.out.println("Bad grade");
    }


Example: determine whether the value of letter is a vowel.
       if (letter == 'a' || letter = 'A' ||
           letter == 'e' || letter = 'E' ||
           letter == 'i' || letter = 'I' ||
           letter == 'o' || letter = 'O' ||
           letter == 'u' || letter = 'U' )
           isAVowel = true;
       else
           isAVowel = false;


Alternatively this can be done as using a switch statement:
     switch(letter) {
         case 'a': case 'e': case 'i': case 'o': case 'u': 
         case 'A': case 'E': case 'I': case 'O': case 'U': 
                      isAVowel = true;
                      break;
         default:
                      isAVowel = false;
      }


        
          

    



Objective: Gain skill with more complicated logic 

Last revision:  October 5, 1999 at 11:40 am

ExamScore	Letter Grade
90 and above	A
80 - 89	B
70-79	C
60-69	D
Below 60	F

Number Of Passengers	Yearly Fee
< 2	$100
2 - 10	$300
11 - 20	$500
> 20	$1000