switch-case, break, default, Fall Through and Menu-Driven Programs in Java (ICSE Class 9)

Understand switch-case, break, default, Fall Through and Menu-Driven Programs

switch-case, break, default, Fall Through and Menu-Driven Programs: key facts

switch form

• •switch (expression)
• •case constant: statements; break;
• •default: statements

Usually an int or a char.

Fall through

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

Trace downwards from the match.

Grouped cases

case 4: case 6: case 9: one block for all | one break at the end

Saves repeated code.

Exit

System.exit(0) ends the program immediately | used for the Exit option of a menu

Ends the whole program.

How to answer switch-case, break, default, Fall Through and Menu-Driven Programs questions

1. 1Evaluate the switch expression.
2. 2Find the case whose constant equals it; if none, go to default.
3. 3Execute statements downwards from that label.
4. 4Stop at the first break or the closing brace.
5. 5Write the printed items in order, joined if print() is used.

switch-case, break, default, 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, break, default, Fall Through and Menu-Driven Programs

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

• No break, no stop

  Without break execution falls into the next case.

  Use it when: Fall through.

• Default is the safety net

  default catches every unmatched value.

  Use it when: Invalid input.

• Stack the labels

  Put cases one under the other to share a block.

  Use it when: Vowel and month-type questions.

switch-case, break, default, 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.print("A");
case 2: System.out.print("B"); break;
case 3: System.out.print("C");
case 4: System.out.print("D"); break;
default: System.out.print("E");
}

switch (g)
{
case 'A': case 'B':
System.out.println("Good");
break;
case 'C':
System.out.println("Average");
break;
default:
System.out.println("Poor");
}

int a = 12, b = 5;
double res = 0;
switch (ch)
{
case 1: res = a + b; break;
case 2: res = a - b; break;
case 3: res = a * b; break;
case 4: res = (double) a / b; break;
default: System.out.println("Invalid choice");
}
System.out.println("Result = " + res);

int days;
switch (m)
{
case 2: days = 28; break;
case 4: case 6: case 9: case 11: days = 30; break;
default: days = 31;
}

Where students lose marks in switch-case, break, default, Fall Through and Menu-Driven Programs

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

• ✗Mistake: Forgetting break

  Each case is treated as separate.

  Wrong working

  case 1 prints A and case 2 prints B for the value 1: output A only

  Right working

  without break the output is AB

  ✓Do this: Trace downwards until a break.

  Usually costs: 2 marks

• ✗Mistake: Writing a range in a case

  Ladder syntax is copied.

  Wrong working

  case >= 90:

  Right working

  Use an if-else-if ladder for ranges

  ✓Do this: Case labels are constants.

  Usually costs: 1 mark

• ✗Mistake: Missing default in a menu

  Invalid input is not handled.

  Wrong working

  No default, nothing printed for 9

  Right working

  default prints Invalid choice

  ✓Do this: Always add default to menus.

  Usually costs: 1 mark

Practice questions on switch-case, break, default, 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

   Which keyword ends the execution of a case and leaves the switch?
   (a) exit (b) stop (c) break (d) default

   Hint

   It is also used in loops.

   Answer

   (c) break

2. Q2 · 2 marks · Very short answerBoard level

   State the purpose of default in a switch and say whether it is compulsory.

   Hint

   Think of unmatched values.

   Answer

   default runs when no case label matches the expression. It is optional.

3. Q3 · 2 marks · Short answerHarder board level

   Find the output:

   int ch = 2;
   switch (ch)
   {
   case 1: System.out.print(1);
   case 2: System.out.print(2);
   case 3: System.out.print(3);
   default: System.out.print(4);
   }

   Hint

   Look for break statements.

   Answer

   234 (case 2 matches; there is no break, so 3 and then default 4 also run)

4. Q4 · 2 marks · Short answerHarder board level

   Explain why a switch cannot replace the ladder that checks marks >= 90, marks >= 75 and so on, and name the statement to use instead.

   Hint

   What does a case label hold?

   Answer

   A case label holds a single constant that is compared for equality, so ranges and conditions joined with logical operators cannot be written. An if-else-if ladder is used instead.

5. Q5 · 3 marks · Short answerHOTS / competency

   Write a switch statement for a char variable c that prints "Stop" for 'R', "Get ready" for 'Y', "Go" for 'G' and "Invalid colour" for anything else.

   Hint

   Use println and break.

   Answer

   switch (c)
   {
   case 'R': System.out.println("Stop"); break;
   case 'Y': System.out.println("Get ready"); break;
   case 'G': System.out.println("Go"); break;
   default: System.out.println("Invalid colour");
   }

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 happens if break is missing?Show answer

   Execution falls through into the next case.

2. 2.Which case runs if no label matches?Show answer

   default

3. 3.Can a case label be a range?Show answer

   No, only a constant.

4. 4.Which statement ends a program at once?Show answer

   System.exit(0)

5. 5.Which two types are most often used as a switch expression in school programs?Show answer

   int and char.

6. 6.What follows a case label?Show answer

   A colon.

switch-case, break, default, Fall Through and Menu-Driven Programs: frequently asked questions