Conditional and Looping Statements, Blocks and Scope (ISC Class 12 Computer Science)

Understand Conditional and Looping Statements, Blocks and Variable Scope

Conditional and Looping Statements, Blocks and Variable Scope: key facts

Loop choice

for	known number of repeats
while	test first, may run zero times
do-while	runs at least once
	Exit-controlled means do-while

Choose by when the test is made.

break and continue

• •break leaves the nearest loop or switch
• •continue skips to the next iteration
• •Both affect only the innermost loop

Use a flag to leave an outer loop.

switch rules

• •Case labels are constants
• •No break means fall-through
• •default runs when no case matches
• •Allowed types: int, char, String, enum and similar

Execution continues from the matched label.

Scope of a variable

• •Block variable: from its declaration to the closing brace
• •Loop variable declared in for: only inside the loop
• •Local variables get no default value

Using a variable outside its block is a compile error.

How to answer Conditional and Looping Statements, Blocks and Variable Scope questions

1. 1Make a trace table with a column for each variable that changes.
2. 2For a for loop, run init once, then repeat test, body, update.
3. 3For a switch, find the matching label and carry on downwards until a break or the end.
4. 4In nested loops, finish the inner loop completely for each outer value.
5. 5Apply break and continue to the innermost loop only.
6. 6Write the output with the exact spacing and line breaks (print versus println).

Conditional and Looping Statements, Blocks and Variable Scope 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 Conditional and Looping Statements, Blocks and Variable Scope

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

• Entry and exit control

  for and while test at the entry; do-while tests at the exit.

  Use it when: Entry- versus exit-controlled loop questions.

• break is the exit sign

  break leaves the loop or switch; continue only skips the current round.

  Use it when: Distinguishing the two.

• Inner loop restarts

  Every outer round, the inner loop starts fresh from its init.

  Use it when: Nested loop tracing.

• Scope is the braces

  A name dies at the closing brace of the block where it was born.

  Use it when: Scope questions.

Conditional and Looping Statements, Blocks and Variable Scope: solved examples

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

int n = 2, s = 0;
switch (n) {
    case 1: s += 1;
    case 2: s += 2;
    case 3: s += 3; break;
    case 4: s += 4;
    default: s += 10;
}
System.out.println(s);

int count = 0;
for (int i = 1; i <= 5; i++) {
    if (i == 2) continue;
    for (int j = 1; j <= i; j++) {
        if (j == 3) break;
        count++;
    }
}
System.out.println(count);

int x = 10;
do {
    System.out.print(x + " ");
    x -= 3;
} while (x > 4);
System.out.println(x);

int total = 0;
for (int i = 1; i <= 4; i++) {
    int sq = i * i;
    if (sq % 2 == 0)
        total += sq;
}
System.out.println(total);
System.out.println(i);

Where students lose marks in Conditional and Looping Statements, Blocks and Variable Scope

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

• ✗Mistake: Forgetting fall-through

  Without break, execution runs on into the next case.

  Wrong working

  Only case 2 runs when n = 2

  Right working

  case 2 and then case 3 run until the break

  ✓Do this: Trace downwards from the matching label.

  Usually costs: 1 mark

• ✗Mistake: Applying break to the outer loop

  break stops only the innermost loop or switch.

  Wrong working

  break in the inner loop ends the whole program section

  Right working

  Only the inner loop ends; the outer loop continues

  ✓Do this: Use a flag or label to leave both loops.

  Usually costs: 1 mark

• ✗Mistake: Using a loop variable after the loop

  A variable declared in the for header does not exist outside it.

  Wrong working

  for (int i = 0; ...) {...} System.out.println(i);

  Right working

  int i; for (i = 0; ...) {...} System.out.println(i);

  ✓Do this: Declare the variable before the loop.

  Usually costs: 1 mark

Practice questions on Conditional and Looping Statements, Blocks and Variable Scope

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 loop is guaranteed to execute its body at least once?
   (a) for (b) while (c) do-while (d) enhanced for

   Hint

   The test comes after the body.

   Answer

   (c) do-while

2. Q2 · 2 marks · Short answerBoard level

   Differentiate between break and continue with a small example of each.

   Hint

   One leaves the loop, the other skips a round.

   Answer

   break ends the nearest loop or switch immediately: for (i = 1; i <= 5; i++) { if (i == 3) break; } stops at 3. continue skips the rest of the current iteration and goes on to the next: with if (i == 3) continue; the loop runs for i = 1, 2, 4, 5 with only 3 skipped.

3. Q3 · 4 marks · Short answerHarder board level

   Write a program segment that reads an integer n and prints the sum of its digits. Trace it for n = 4729.

   Hint

   Use % 10 and / 10 in a loop.

   Answer

   int sum = 0;
   int t = n;
   while (t > 0) {
       sum += t % 10;
       t /= 10;
   }
   System.out.println(sum);

   For 4729: digits 9, 2, 7, 4 are added in turn: 9, 11, 18, 22. t goes 4729, 472, 47, 4, 0. Output: 22

4. Q4 · 3 marks · Short answerHarder board level

   Write the output:

   for (int i = 1; i <= 3; i++) {
       for (int j = i; j <= 3; j++)
           System.out.print(j);
       System.out.println();
   }

   Hint

   The inner loop starts from i.

   Answer

   i = 1 prints 1, 2, 3 and a line break; i = 2 prints 2, 3; i = 3 prints 3. Output:
   123
   23
   3

5. Q5 · 3 marks · Case-basedHOTS / competency

   Write the output of the code below and rewrite it so that only the text for grade B is printed.

   char g = 'B';
   String r = "";
   switch (g) {
       case 'A': r += "Excellent ";
       case 'B': r += "Good ";
       case 'C': r += "Fair "; break;
       default: r += "Unknown";
   }
   System.out.println(r);

   Hint

   Where does execution continue after the match?

   Answer

   g = B matches case B: r = "Good ". There is no break, so case C also runs: r = "Good Fair ", then break. Output: Good Fair (with a trailing space). Fix: add break; after r += "Good "; in case B (and after case A) so the output is Good.

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.Name the exit-controlled loop in Java.Show answer

   do-while.

2. 2.What does default do in a switch?Show answer

   Runs when no case label matches.

3. 3.What does continue do?Show answer

   Skips the rest of the current iteration of the innermost loop.

4. 4.What is the ternary operator?Show answer

   cond ? valueIfTrue : valueIfFalse.

5. 5.What is the scope of a variable declared in a for header?Show answer

   Only the for loop.

6. 6.What happens when a case has no break?Show answer

   Execution falls through into the next case.

Conditional and Looping Statements, Blocks and Variable Scope: frequently asked questions