Switch Case, Fall Through and Menu-driven Programs in Java (ICSE Class 10)

Understand Switch Case, Fall Through and Menu-driven Programs

switch (expression) {
    case value1: statements; break;
    case value2: statements; break;
    default: statements;
}

Switch Case, Fall Through and Menu-driven Programs: key facts

Switch rules

• •Expression: int, char (byte, short, String)
• •Case labels: constants, all different
• •Tests equality only
• •default optional

double, float and boolean cannot be used.

Fall through

No break: control continues into the next case | break: leaves the switch

The last case may omit break.

Shared action for several labels

• •case 'a': case 'e': case 'i':
• •statements;
• •break;

Empty cases fall through to the first statements.

Terminating a program

System.exit(0); | 0 means normal end

Often used in the exit option of a menu.

How to answer Switch Case, Fall Through and Menu-driven Programs questions

1. 1Evaluate the expression and find the first case label equal to it (or default if none).
2. 2Start executing from that label.
3. 3Keep executing the statements of the following cases until you meet a break or the end of the switch.
4. 4Remember that default also executes when control falls into it from above.
5. 5Write the printed output exactly, using print or println as given.

Switch Case, Fall Through and Menu-driven Programs in real life

The idea, met in an everyday situation. Answer the question in your own words, then compare with the model answer. The one-line hook is what to remember.

Tricks to remember Switch Case, Fall Through and Menu-driven Programs

Short memory aids. Say each one aloud once, then use it on a question the same day.

• Break is the brake

  Without a break the switch keeps rolling downhill into the next case.

  Use it when: Fall-through output.

• Switch tests =, not >

  If the condition has > or <, use if-else-if.

  Use it when: Choosing a construct.

• Default is the safety net

  default catches every value that no case caught.

  Use it when: Menu programs.

Switch Case, Fall Through and Menu-driven Programs: solved examples

Ordered from board level to HOTS. Try each one before you open the solution.

int d = 3;
switch (d) {
    case 1: System.out.println("Mon"); break;
    case 2: System.out.println("Tue"); break;
    case 3: System.out.println("Wed"); break;
    default: System.out.println("Other");
}

switch (g) {
    case 'A': System.out.print("Excellent ");
    case 'B': System.out.print("Good "); break;
    case 'C': System.out.print("Fair ");
    default: System.out.print("Check");
}

switch (ch) {
    case 'a':
    case 'e':
    case 'i':
    case 'o':
    case 'u':
        System.out.println("vowel"); break;
    default:
        System.out.println("other");
}

import java.util.Scanner;
public class Menu {
    public static void main(String[] args) {
        Scanner in = new Scanner(System.in);
        System.out.println("1. Square area");
        System.out.println("2. Rectangle area");
        System.out.print("Enter choice: ");
        int ch = in.nextInt();
        switch (ch) {
            case 1:
                System.out.print("Side: ");
                double s = in.nextDouble();
                System.out.println("Area = " + s * s);
                break;
            case 2:
                System.out.print("Length and breadth: ");
                double l = in.nextDouble(), b = in.nextDouble();
                System.out.println("Area = " + l * b);
                break;
            default:
                System.out.println("Wrong choice");
        }
    }
}

int m = 68;
String g;
if (m >= 90) g = "A";
else if (m >= 75) g = "B";
else if (m >= 60) g = "C";
else g = "D";
int x = 15;
if (x > 10) System.out.print("P");
if (x > 20) System.out.print("Q");
else System.out.print("R");
System.out.println(g);

Where students lose marks in Switch Case, Fall Through and Menu-driven Programs

The slips examiners see most often, and what to write instead.

• ✗Mistake: Stopping at the matching case when there is no break

  Execution falls through to later cases.

  Wrong working

  n = 2 prints B only

  Right working

  n = 2 prints B and then C (until break)

  ✓Do this: Look for the next break each time.

  Usually costs: 1 to 2 marks

• ✗Mistake: Using a range or condition as a case label

  case labels must be constants; switch tests equality only.

  Wrong working

  case marks > 50:

  Right working

  Use if (marks > 50) ... else if ...

  ✓Do this: Choose if-else-if for ranges.

  Usually costs: 1 mark

• ✗Mistake: Using the same variable name in two cases

  All cases share one block, so a repeated declaration is a compile-time error.

  Wrong working

  case 1: double x = ...; case 2: double x = ...;

  Right working

  Use different names or declare the variable before the switch.

  ✓Do this: Declare once before the switch.

  Usually costs: 1 mark

Practice questions on Switch Case, Fall Through and Menu-driven Programs

Write each answer on paper first. Open the hint only if you are stuck, and the answer only when you have finished.

1. Q1 · 1 mark · MCQBoard level

   The expression of a switch statement can NOT be of the type:
   (a) int (b) char (c) double (d) short

   Hint

   Switch needs exact matches.

   Answer

   (c) double

2. Q2 · 1 mark · Very short answerBoard level

   What is meant by fall through in a switch statement?

   Hint

   What if break is missing?

   Answer

   When a matching case has no break, control continues into the statements of the following cases until a break or the end of the switch.

3. Q3 · 2 marks · Short answerHarder board level

   Write the output:

   int n = 6;
   switch (n % 3) {
       case 0: System.out.print("zero ");
       case 1: System.out.print("one ");
       case 2: System.out.print("two "); break;
       default: System.out.print("none");
   }

   Hint

   Evaluate n % 3 first; where does execution start and stop?

   Answer

   zero one two (6 % 3 = 0, so case 0 prints "zero ", falls through case 1 and case 2, and the break stops it)

4. Q4 · 3 marks · Short answerHarder board level

   Rewrite using switch:

   if (c == '+') r = a + b;
   else if (c == '-') r = a - b;
   else r = 0;

   Hint

   c is a char; end each case with break.

   Answer

   switch (c) {
   case '+': r = a + b; break;
   case '-': r = a - b; break;
   default: r = 0;
   }

5. Q5 · 1 mark · Assertion-reasonHOTS / competency

   Assertion (A): Missing break statements can make more than one case execute.
   Reason (R): After a matching case, control passes to the next case label without testing it when no break is met.
   (a) Both A and R are true and R is the correct explanation of A. (b) Both A and R are true but R is not the correct explanation of A. (c) A is true but R is false. (d) A is false but R is true.

   Hint

   Does the next label get tested?

   Answer

   (a) Both are true and R explains A.

Test your recall

Answer in your head or on paper first, then open the card. Pulling an answer out of memory makes it stick far better than re-reading.

1. 1.What does break do in a switch?Show answer

   Leaves the switch statement.

2. 2.When does default run?Show answer

   When no case matches (or when control falls into it).

3. 3.Can a switch test n > 5?Show answer

   No; it tests equality only.

4. 4.What does System.exit(0) do?Show answer

   Ends the program immediately; 0 means a normal end.

5. 5.Name two types allowed in a switch expression.Show answer

   int and char.

6. 6.Can two case labels have the same value?Show answer

   No; labels must be different constants.

Switch Case, Fall Through and Menu-driven Programs: frequently asked questions