Java Loops, break, continue and Scope of Variables (ISC Class 11 Computer Science)

Understand Loops in Java: for, while, do-while, break, continue, Nested Loops and Scope

int n = 4271, s = 0;
while (n > 0) { s += n % 10; n /= 10; }   // s = 14

int i = 10;
do { System.out.print(i + " "); i++; } while (i < 5);   // prints 10

for (int i = 1; i <= 10; i++) {
    if (i % 3 == 0) continue;     // skip multiples of 3
    if (i > 7) break;             // stop the loop
    System.out.print(i + " ");
}                                 // 1 2 4 5 7

int x = 10;
{ int y = 5; x = x + y; }
System.out.println(x);      // 15
// System.out.println(y);   // error: y not in scope

Loops in Java: for, while, do-while, break, continue, Nested Loops and Scope: key facts

Loop parts

• •for (initialise; test; update)
• •while (test)
• •do { } while (test);

Three loops.

Number of iterations

for i = a to b step 1: b - a + 1 passes | do-while: at least 1

If b >= a.

Scope

Local	its block
Loop variable	the loop
Instance	the object

Visibility.

How to answer Loops in Java: for, while, do-while, break, continue, Nested Loops and Scope questions

1. 1List the variables with starting values.
2. 2Test the condition before each pass (after, for do-while).
3. 3Apply break and continue exactly where they occur.
4. 4Write the output in the order printed.

Loops in Java: for, while, do-while, break, continue, Nested Loops and 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 Loops in Java: for, while, do-while, break, continue, Nested Loops and Scope

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

• Entry vs exit

  for and while test first; do-while tests last.

  Use it when: Loops.

• Continue skips, break stops

  continue goes to the next pass; break leaves the loop.

  Use it when: Jumps.

• Brace is the wall

  A local variable dies at its closing brace.

  Use it when: Scope.

Loops in Java: for, while, do-while, break, continue, Nested Loops and Scope: solved examples

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

Where students lose marks in Loops in Java: for, while, do-while, break, continue, Nested Loops and Scope

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

• ✗Mistake: Counting a skipped iteration as printed

  Ignoring continue.

  Wrong working

  1 2 3 4 5 6 7

  Right working

  1 2 4 5 7

  ✓Do this: Apply continue first.

  Usually costs: 2 marks.

• ✗Mistake: Assuming do-while does not run when the condition is false

  Treating it like while.

  Wrong working

  No output

  Right working

  One pass

  ✓Do this: Body first, then test.

  Usually costs: 1-2 marks.

• ✗Mistake: Using a block variable outside the block

  Scope misunderstanding.

  Wrong working

  Using y after the block

  Right working

  Declare y before the block

  ✓Do this: Declare in the outer scope.

  Usually costs: 1-2 marks.

Practice questions on Loops in Java: for, while, do-while, break, continue, Nested Loops and 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) none

   Hint

   Exit-controlled.

   Answer

   (c) do-while

2. Q2 · 2 marks · Short answerBoard level

   Write the output: int i = 1; while (i <= 20) { i = i * 2; System.out.print(i + " "); } (2 marks)

   Hint

   Doubling.

   Answer

   2 4 8 16 32 (when i is 16 the test 16 <= 20 is true, so 32 is printed and the loop then stops)

3. Q3 · 4 marks · Short answerHarder board level

   Write a program fragment using a loop to print the first 6 terms of the series 2, 5, 10, 17, ... (n squared + 1), and the sum of the terms. (4 marks)

   Hint

   1 + 1, 4 + 1, 9 + 1.

   Answer

   int s = 0; for (int n = 1; n <= 6; n++) { int t = n * n + 1; s += t; System.out.print(t + " "); } System.out.println("Sum = " + s); Terms: 2 5 10 17 26 37; sum = 97.

4. Q4 · 4 marks · Short answerHarder board level

   Give the output of: for (int i = 1; i <= 3; i++) { for (int j = 1; j <= 3; j++) { if (j == i) continue; System.out.print(i * 10 + j + " "); } } and the number of values printed. (4 marks)

   Hint

   Skip j == i.

   Answer

   i = 1: 12 13; i = 2: 21 23; i = 3: 31 32. Output: 12 13 21 23 31 32 (6 values).

5. Q5 · 3 marks · Short answerHOTS / competency

   A programmer writes: int count = 0; for (int i = 0; i < 10; i++) { int count = i; } and gets a compile-time error. Explain, correct it, and find count after the corrected loop that adds i to count. (3 marks)

   Hint

   Redeclaration.

   Answer

   count is already declared in the enclosing scope, so declaring it again in the inner block is an error. Use count += i; with no new declaration: count = 0 + 1 + ... + 9 = 45.

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.Entry-controlled loops?Show answer

   for and while.

2. 2.do-while runs at least?Show answer

   Once.

3. 3.continue does?Show answer

   Skips to the next iteration.

4. 4.Scope of a local variable?Show answer

   Its block.

5. 5.Nested loop inner runs?Show answer

   For every outer pass.

Loops in Java: for, while, do-while, break, continue, Nested Loops and Scope: frequently asked questions