for, while and do-while Loops in Java with Output Tracing (ICSE Class 9)

Understand for, while and do-while Loops with Output Tracing and Loop Conversion

for, while and do-while Loops with Output Tracing and Loop Conversion: key facts

Parts of a loop

• •Initialisation
• •test condition
• •body
• •update

All four are needed to avoid an infinite loop.

Entry versus exit

for and while: condition first, may run zero times | do-while: condition last, runs at least once

do-while ends with a semicolon.

for to while

for (a; b; c) body | becomes a; while (b) { body; c; }

Initialisation before the loop, update at the end.

Iterations

i from a, step s, while i <= b | values a, a + s, a + 2s, ...

Count the values that satisfy the condition.

How to answer for, while and do-while Loops with Output Tracing and Loop Conversion questions

1. 1Write the starting values of all variables.
2. 2Test the condition; if it is false, stop (for an entry-controlled loop, the body is skipped).
3. 3Run the body, noting the output and changed variables.
4. 4Apply the update and test again.
5. 5When the condition fails, write the final values and the printed output.

for, while and do-while Loops with Output Tracing 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 for, while and do-while Loops with Output Tracing and Loop Conversion

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

• do-while does once

  do-while runs the body at least once even if the condition is false at the start.

  Use it when: Entry versus exit control.

• I-T-U

  Initialise, Test, Update.

  Use it when: Loop parts.

• Semicolon at the end of do-while

  while (condition); closes a do-while loop.

  Use it when: Syntax.

for, while and do-while Loops with Output Tracing and Loop Conversion: solved examples

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

int s = 0;
for (int i = 2; i <= 10; i += 3)
s = s + i;
System.out.println(s);

int n = 2;
while (n < 50)
n = n * 3;
System.out.println(n);

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

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

for (int i = 1; i <= 4; i++)
{
if (i % 2 == 0)
System.out.print(i * 10 + " ");
else
System.out.print(i + " ");
}

Where students lose marks in for, while and do-while Loops with Output Tracing and Loop Conversion

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

• ✗Mistake: Forgetting the update

  The loop never moves.

  Wrong working

  int i = 1; while (i <= 5) System.out.println(i);

  Right working

  add i++; inside the loop

  ✓Do this: Check the update every time.

  Usually costs: 2 marks

• ✗Mistake: Missing semicolon after do-while

  Java reports a syntax (compile-time) error because the do-while statement is not terminated, so the program does not compile and the marks are lost.

  Wrong working

  while (k < 10)

  Right working

  while (k < 10);

  ✓Do this: End a do-while with a semicolon.

  Usually costs: 1 mark

• ✗Mistake: Counting iterations wrongly

  The last value is mishandled.

  Wrong working

  i = 2; i <= 10; i += 3 runs 4 times

  Right working

  It runs 3 times: 2, 5, 8

  ✓Do this: List the values.

  Usually costs: 1 mark

Practice questions on for, while and do-while Loops with Output Tracing 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

   A menu must be shown at least once and repeated until the user chooses Exit. Which loop fits best?
   (a) for (b) while (c) do-while (d) none of these

   Hint

   The body must run first.

   Answer

   (c) do-while

2. Q2 · 2 marks · Very short answerBoard level

   Write a for loop that prints the numbers 5, 4, 3, 2, 1 on separate lines.

   Hint

   Count down.

   Answer

   for (int i = 5; i >= 1; i--)
   System.out.println(i);

3. Q3 · 3 marks · Short answerHarder board level

   Rewrite using a while loop and find s:

   int s = 0;
   for (int i = 1; i <= 4; i++)
   s += i * 2;

   Hint

   Initialise before, update at the end.

   Answer

   int s = 0;
   int i = 1;
   while (i <= 4)
   {
   s += i * 2;
   i++;
   }
   s = 2 + 4 + 6 + 8 = 20

4. Q4 · 3 marks · Short answerHarder board level

   Find the output:

   int x = 1;
   while (x <= 3)
   {
   System.out.print(x + " " + x * x + " ");
   x++;
   }

   Hint

   Print x and its square in each pass.

   Answer

   1 1 2 4 3 9

5. Q5 · 4 marks · Short answerHOTS / competency

   (i) How many times does the body run in for (int i = 0; i < 20; i += 5)? (ii) How many times in for (int i = 20; i > 0; i -= 6)? (iii) Why does for (int i = 1; i <= 5; i--) never end normally?

   Hint

   List the values of i.

   Answer

   (i) i = 0, 5, 10, 15: 4 times. (ii) i = 20, 14, 8, 2: 4 times. (iii) i decreases, so i <= 5 stays true; the loop is infinite (it ends only when the value overflows).

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 four parts of a loop.Show answer

   Initialisation, test condition, body and update.

2. 2.Which loop always runs once?Show answer

   do-while

3. 3.Which loops are entry controlled?Show answer

   for and while

4. 4.What ends a do-while statement?Show answer

   A semicolon.

5. 5.What is an infinite loop?Show answer

   A loop whose condition never becomes false.

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

   A loop without a body that only counts, to slow a program.

for, while and do-while Loops with Output Tracing and Loop Conversion: frequently asked questions