Iterative Constructs in Java ICSE Class 10 Computer Applications

What Iterative Constructs in Java is about

Weightage: CISCE publishes no chapter weightage. Section A (40 marks) is compulsory: 1-mark MCQs and 2-mark short answers including output and error questions; in Section B you answer any four of six 15-mark programming questions written in Java.Source: CISCE ICSE Computer Applications (661) specimen paper structure for 2027

Key concepts

for, while and do-while

Every loop has four parts: initialisation, condition (test), body and update.

for loop (entry controlled; best when the number of passes is known):
for (int i = 1; i <= 5; i++)
System.out.print(i + " ");
Output: 1 2 3 4 5

while loop (entry controlled; best when passes depend on a condition, such as digits of a number):
int n = 1234, s = 0;
while (n > 0)
{
s = s * 10 + n % 10;
n = n / 10;
}
After the loop s = 4321 (the reverse).

do-while loop (exit controlled; body runs at least once; note the semicolon after while):
int k = 10;
do
{
System.out.println(k);
k++;
} while (k < 5);
Output: 10 (once), although the condition is false at the start.

Worked example

(2 marks) Distinguish between entry-controlled and exit-controlled loops.

Try it first, then show the solutionSolution

1. Entry controlled (for, while): condition tested before the body; may run zero times - 1 mark.
2. Exit controlled (do-while): condition tested after the body; runs at least once - 1 mark.

Answer: In an entry-controlled loop (for, while) the condition is checked before the body, so the body may not run at all; in an exit-controlled loop (do-while) the condition is checked after the body, so it runs at least once.

Counting iterations and output

To trace, make a table of the loop variable, the condition and what is printed.

Without braces only the next statement is the loop body:
for (x = 10; x > 20; x++)
System.out.println(x);
System.out.println(x * 2);
The condition 10 > 20 is false at once: the loop runs 0 times; only the second println runs, printing 20.

A semicolon right after for(...) makes an empty body:
for (p = 4; p > 0; p -= 2);
System.out.print(p);
System.out.println(p);
The loop changes p: 4, 2, 0 and stops; then 0 is printed twice on one line: 00.

Worked example

(2 marks) Give the output and the number of iterations:
int i;
for (i = 1; i <= 20; i += 4)
System.out.print(i + " ");

Try it first, then show the solutionSolution

1. i takes 1, 5, 9, 13, 17; next 21 fails - 1 mark.
2. 5 iterations; output on one line - 1 mark.

Answer: 1 5 9 13 17 (5 iterations)

break and continue

break ends the loop at once; control goes to the statement after the loop.
continue skips the rest of the current pass and goes to the next pass (in a for loop, the update runs next).

for (int i = 1; i <= 10; i++)
{
if (i % 3 == 0) continue;
if (i == 8) break;
System.out.print(i + " ");
}
Trace: 1 printed, 2 printed, 3 skipped, 4, 5 printed, 6 skipped, 7 printed, 8 breaks.
Output: 1 2 4 5 7

Worked example

(2 marks) Give the output:
int s = 0;
for (int k = 1; k <= 6; k++)
{
if (k == 4) continue;
s += k;
}
System.out.println(s);

Try it first, then show the solutionSolution

1. k = 4 is skipped; s = 1 + 2 + 3 + 5 + 6 - 1 mark.
2. s = 17 - 1 mark.

Answer: 17

Loop conversion

To convert, keep the same initialisation, condition and update, and move them into the new form.

for → while:
for (i = 1; i <= 5; i++) sum += i;
becomes
i = 1;
while (i <= 5)
{
sum += i;
i++;
}

for → do-while (exit controlled):
int a, b;
for (a = 10, b = 1; a >= 1; a -= 2) { b += a; b++; }
becomes
a = 10; b = 1;
do
{
b += a;
b++;
a -= 2;
} while (a >= 1);
Both give b = 36 (b: 12, 21, 28, 33, 36 for a = 10, 8, 6, 4, 2).

do-while → for: a do-while always runs once, so the converted for gives the same result only if the condition is true at the start. Multiple variables go in the header separated by commas: for (x = 10, y = 20; x <= 20; x++, y++);

Worked example

(2 marks) Rewrite using for and give the output:
x = 10; y = 20;
do
{
x++;
y++;
} while (x <= 20);
System.out.println(x * y);

Try it first, then show the solutionSolution

1. for (x = 10, y = 20; x <= 20; x++, y++); (empty body; the condition is true at the start, so results match) - 1 mark.
2. Loop ends at x = 21, y = 31; output 21 × 31 = 651 - 1 mark.

Answer: for (x = 10, y = 20; x <= 20; x++, y++);
System.out.println(x * y);
Output: 651

Finite, infinite and delay loops; digit extraction

A finite loop ends after a fixed number of passes. An infinite loop never ends because its condition never becomes false: for (int i = 2; i != 0; i -= 3) (i = 2, -1, -4, ... never 0); while (true) { } without break; for ( ; ; ).

A delay loop has an empty body and only wastes time: for (long t = 1; t <= 100000; t++);

Digit extraction pattern (used in most number programs):
int copy = n; // keep the original
while (n > 0)
{
int d = n % 10; // last digit
... use d ...
n = n / 10; // remove last digit
}
With it you can find the sum, product, count, largest or smallest digit, and the reverse (rev = rev * 10 + d).

Worked example

(1 mark) Which is an infinite loop?
(a) for (int i = 1; i <= 10; i++)
(b) for (int i = 2; i != 0; i -= 3)
(c) for (int i = 5; i <= 5; i++)
(d) for (int i = 1; i >= 1; i--)

Try it first, then show the solutionSolution

1. (b): i = 2, -1, -4, ... skips 0, so i != 0 is always true.

Answer: (b) for (int i = 2; i != 0; i -= 3)

Iterative Constructs in Java in real life

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

Topics in this chapter

• Loop Tracing, break, continue and Loop Conversion

Iterative Constructs in Java: syntax and key facts

for syntax

for (init; condition; update) { body }

Commas for multiple variables.

while / do-while

while (condition) { body } do { body } while (condition);

do-while ends with a semicolon.

Digit loop

while (n > 0) { d = n % 10; … ; n = n / 10; }

Save a copy of n first; reverse: rev = rev * 10 + d.

Jumps

break: leave loop; continue: next pass

How the board asks Iterative Constructs in Java

The question patterns to expect, and the method that works for each.

Tricks to remember Iterative Constructs in Java

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

• ICBU

  Initialise, Check, Body, Update: the order of every for and while.

  Use it when: Tracing and converting loops.

• Do-while: do it, then doubt it

  Body first, condition later; always once.

  Use it when: Exit-controlled questions.

• Mod takes, divide drops

  n % 10 takes the last digit; n / 10 drops it.

  Use it when: Digit programs.

Iterative Constructs in Java: board-level solved questions

Ordered from board level to HOTS. Cover the solution, write your own, then compare step by step.

Case-study practice (Section B style)

Read the situation, then answer every part. In the board paper this is a 10-mark question split into 2 + 3 + 5 marks.

Where the marks are

How a typical answer is split into step marks. Show every step: a right final answer alone rarely earns full marks.

Where students lose marks in Iterative Constructs in Java

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

• ✗Mistake: Ignoring the semicolon after for

  The 2025 examiners noted candidates ignored the ; at the end of the for statement.

  ✓Do this: A ; after for(...) means an empty body; the next statement runs once, after the loop.

  Usually costs: 1 mark.

• ✗Mistake: Treating both statements as the loop body

  In 2024 candidates printed two lines per pass, missing that without braces only the next statement is in the loop.

  ✓Do this: Without { }, the loop controls one statement only.

  Usually costs: 1 to 2 marks.

• ✗Mistake: Missing the semicolon after while in do-while, or the update

  The 2025 examiners reported these errors in loop conversion.

  ✓Do this: do { …; update; } while (condition);

  Usually costs: 1 mark.

• ✗Mistake: Not saving the original number

  In the 2024 EvenPal program candidates compared the reverse with the destroyed n.

  ✓Do this: int copy = n; before the digit loop.

  Usually costs: 2 to 3 marks of 15.

• ✗Mistake: Wrong initial max/min or sum inside the loop

  In 2025 candidates initialised max and min wrongly and found the sum inside the loop.

  ✓Do this: max = 0, min = 9 for digits; compute the sum after the loop.

  Usually costs: 2 to 4 marks of 15.

Practice questions on Iterative Constructs in Java

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 loop that always executes at least once is:
   (a) for
   (b) while
   (c) do-while
   (d) none

   Hint

   Exit controlled.

   Answer

   (c) do-while

2. Q2 · 1 mark · MCQBoard level

   Which statement skips the remaining body and starts the next iteration?
   (a) break
   (b) continue
   (c) exit
   (d) return

   Hint

   Go on.

   Answer

   (b) continue

3. Q3 · 2 marks · Very short answerHarder board level

   Give the output:
   int i = 1;
   while (i < 30)
   {
   i = i * 3;
   System.out.print(i + " ");
   }

   Hint

   Multiply then print.

   Answer

   3 9 27 81 (i = 27 < 30 runs once more, printing 81)

4. Q4 · 2 marks · Very short answerBoard level

   Convert to a while loop: for (int k = 5; k >= 1; k--) System.out.print(k);

   Hint

   Move init and update.

   Answer

   int k = 5;
   while (k >= 1)
   {
   System.out.print(k);
   k--;
   }
   Output: 54321

5. Q5 · 2 marks · Short answerHOTS / competency

   Give the output:
   for (int i = 1; i <= 10; i++)
   {
   if (i % 3 == 0) continue;
   if (i == 8) break;
   System.out.print(i + " ");
   }

   Hint

   Skip multiples of 3; stop at 8.

   Answer

   1 2 4 5 7

6. Q6 · 2 marks · Short answerHOTS / competency

   How many times does this loop run, and what is the final value of m? int m = 100; do { m = m / 3; } while (m > 5);

   Hint

   Integer division.

   Answer

   m: 33, 11, 3; it runs 3 times (after 3, 3 > 5 is false). Final m = 3.

7. Q7 · 15 marks · Long answerHarder board level

   Write a program to print the sum of the series 1 + 1/2 + 1/3 + ... + 1/n for a given n.

   Hint

   Use 1.0 / i.

   Answer

   import java.util.Scanner;
   class Series
   {
   public static void main(String args[])
   {
   Scanner sc = new Scanner(System.in);
   System.out.print("n: ");
   int n = sc.nextInt();
   double s = 0;
   for (int i = 1; i <= n; i++)
   s = s + 1.0 / i;
   System.out.println("Sum = " + s);
   }
   }
   Trace n = 4: 1 + 0.5 = 1.5; + 0.333... = 1.8333...; + 0.25 = 2.0833...; printed as Sum = 2.083333333333333. (1 / i would give 0 for i > 1.)

8. Q8 · 15 marks · Long answerHOTS / competency

   Write a program to accept a number and check whether it is a Spy number (sum of digits equals product of digits). Example: 1124 → sum 8, product 8.

   Hint

   Product starts at 1.

   Answer

   import java.util.Scanner;
   class Spy
   {
   public static void main(String args[])
   {
   Scanner sc = new Scanner(System.in);
   System.out.print("Number: ");
   int n = sc.nextInt();
   int sum = 0, prod = 1;
   while (n > 0)
   {
   int d = n % 10;
   sum += d;
   prod *= d;
   n /= 10;
   }
   if (sum == prod)
   System.out.println("Spy number");
   else
   System.out.println("Not a Spy number");
   }
   }
   Traced: 1124 → sum 8, product 8 → Spy number; 123 → sum 6, product 6 → Spy number; 124 → 7 and 8 → Not a Spy number.

9. Q9 · 2 marks · Very short answerBoard level

   What is a delay loop? Write one.

   Hint

   Empty body.

   Answer

   A loop with an empty body used only to pass time, e.g. for (long t = 1; t <= 500000; t++);

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.Which loops are entry controlled?Show answer

   for and while.

2. 2.What does n % 10 give?Show answer

   The last digit of n.

3. 3.What does break do in a loop?Show answer

   Ends the loop immediately.

4. 4.Write an infinite for loop.Show answer

   for ( ; ; )

5. 5.Initial value of a product accumulator?Show answer

   1

Quick revision checklist

The night before the exam: can you do each of these without looking?

• ☐for/while: test first (may run 0 times); do-while: test last (runs ≥ 1).
• ☐No braces → one statement in the loop; ; after for → empty body.
• ☐break exits; continue skips to next pass.
• ☐Convert by moving init, condition, update.
• ☐Digits: d = n % 10; n /= 10; save a copy first.

Iterative Constructs in Java: frequently asked questions