Loops in Java: Tracing, break, continue and Converting Between Loops (ICSE Class 10)

Understand Loop Tracing, break, continue and Loop Conversion

for (init; condition; update) { body }

init;
while (condition) { body; update; }

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

Loop Tracing, break, continue and Loop Conversion: key facts

Loop types

for	entry controlled
while	entry controlled
do-while	exit controlled, runs at least once

do-while ends with a semicolon.

Iterations of a counting for loop

• •List the values a
• •a+s
• •a+2s
• •and so on
• •Count those that satisfy the condition

for (i = 2; i <= 10; i += 3) runs for 2, 5, 8: 3 times.

Jump statements

break: leave the loop | continue: skip to the next pass

In while loops update before continue.

Digit extraction

• •d = n % 10
• •n = n / 10
• •repeat while n > 0
• •rev = rev x 10 + d

Uses int division and remainder.

How to answer Loop Tracing, break, continue and Loop Conversion questions

1. 1Make a table with a column for every variable, including the loop variable.
2. 2Check the condition before each pass (for and while) or after it (do-while).
3. 3Execute the body statement by statement; at continue jump to the update/condition; at break stop and go past the loop.
4. 4Apply the update, then test again; stop when the condition is false.
5. 5Write the output after the loop with the final values of the variables.

Loop Tracing, break, continue and Loop Conversion 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 Loop Tracing, break, continue and Loop Conversion

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

• Entry vs exit

  for and while check at the gate (entry); do-while checks as you leave (exit).

  Use it when: Classification questions.

• Break = break up, continue = carry on

  break ends the loop; continue only ends the current pass.

  Use it when: Jump statement outputs.

• Mod takes, divide drops

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

  Use it when: Digit programs.

Loop Tracing, break, continue and Loop Conversion: solved examples

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

for (int i = 2; i <= 10; i += 3)
    System.out.print(i + " ");

int i = 0, s = 0;
while (i < 10) {
    i++;
    if (i % 3 == 0)
        continue;
    if (i > 7)
        break;
    s += i;
}
System.out.println(s + " " + i);

do {
    System.out.print(n + " ");
    n += 5;
} while (n < 10);
System.out.println(n);

while (n < 10) {
    System.out.print(n + " ");
    n += 5;
}
System.out.println(n);

int n = 2475, s = 0, c = 0, rev = 0;
while (n > 0) {
    int d = n % 10;
    s += d;
    c++;
    rev = rev * 10 + d;
    n /= 10;
}
System.out.println(s + " " + c + " " + rev);

Where students lose marks in Loop Tracing, break, continue and Loop Conversion

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

• ✗Mistake: Forgetting the semicolon after while in do-while

  The do-while statement ends with a semicolon after its condition.

  Wrong working

  do { ... } while (i < 5)

  Right working

  do { ... } while (i < 5);

  ✓Do this: Add the semicolon.

  Usually costs: 1 mark

• ✗Mistake: Putting a semicolon right after the for header

  The loop body becomes empty and the next block runs once.

  Wrong working

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

  Right working

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

  ✓Do this: No semicolon after for (...) when it has a body.

  Usually costs: 1 mark

• ✗Mistake: Placing continue before the update in a while loop

  The update is skipped, so the loop repeats the same value for ever.

  Wrong working

  while (i < 5) { if (i == 2) continue; i++; }

  Right working

  while (i < 5) { i++; if (i == 2) continue; }

  ✓Do this: Update first or use for.

  Usually costs: 1 to 2 marks

Practice questions on Loop Tracing, break, continue and Loop Conversion

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 of these is an exit controlled loop?
   (a) for (b) while (c) do-while (d) none of these

   Hint

   The test comes after the body.

   Answer

   (c) do-while

2. Q2 · 1 mark · Very short answerBoard level

   How many times does the body run?
   for (int i = 1; i <= 10; i += 2) { ... }

   Hint

   List the values of i.

   Answer

   5 times (i = 1, 3, 5, 7, 9).

3. Q3 · 2 marks · Short answerHarder board level

   Write the output of each loop:

   int s = 0;
   for (int i = 1; i <= 4; i++) { if (i == 3) continue; s += i; }
   System.out.println(s);
   s = 0;
   for (int i = 1; i <= 4; i++) { if (i == 3) break; s += i; }
   System.out.println(s);

   Hint

   continue skips one pass; break stops all.

   Answer

   7
   3

4. Q4 · 3 marks · Short answerHarder board level

   Convert to a for loop and give the output:

   int i = 10;
   while (i > 0) {
       System.out.print(i + " ");
       i -= 3;
   }

   Hint

   Initialization and update move into the header.

   Answer

   for (int i = 10; i > 0; i -= 3)
   System.out.print(i + " ");
   Output: 10 7 4 1

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

   Assertion (A): In a for loop, the update expression is skipped when continue is executed.
   Reason (R): continue skips the remaining statements of the current pass of the loop.
   (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

   Where does control go after continue in a for loop?

   Answer

   (d) A is false: in a for loop, continue jumps to the update expression and then the test. R is true.

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 loop runs at least once?Show answer

   do-while.

2. 2.What does break do?Show answer

   Ends the loop immediately.

3. 3.What does continue do?Show answer

   Skips the rest of the current pass.

4. 4.What is a delay loop?Show answer

   A loop with an empty body used only to waste time.

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

   The last digit of n.

6. 6.What makes a loop infinite?Show answer

   A condition that never becomes false, for example a missing update.

Loop Tracing, break, continue and Loop Conversion: frequently asked questions