Flow of Control Class 11: Conditional Statements, Loops, break and continue in Python

What Flow of Control is about

Weightage: This chapter is part of Unit II, Computational Thinking and Programming - 1, which carries 45 of the 70 theory marks in the school annual exam together with Introduction to Problem Solving, Getting Started with Python, Strings, Lists, Tuples and Dictionaries and Introduction to Python Modules.Source: CBSE Computer Science (083) syllabus 2026-27, Class XI distribution of marks, Unit II Computational Thinking and Programming - 1

Key concepts

Sequential flow and indentation

NCERT section NCERT 6.1, 6.3

By default statements run one after another from top to bottom: this is sequential flow. Conditional flow chooses between blocks; iterative flow repeats a block.

Python marks a block (the statements controlled by an if, else, for or while) by indentation: the same number of leading spaces for every statement in the block (4 spaces is the usual choice). The line that starts the block ends with a colon (:). Lines after the block return to the earlier indentation. Wrong or inconsistent indentation gives an IndentationError (a syntax error), or, worse, a logical error when a statement ends up inside or outside a block by mistake. Other languages use braces { } for blocks; Python uses indentation alone.

Worked example

How many times is 'Done' printed in each case?
(a)
for i in range(3):
print(i)
print('Done')
(b)
for i in range(3):
print(i)
print('Done')

Try it first, then show the solutionSolution

1. In (a) print('Done') is indented, so it is inside the loop: it runs on each of the 3 passes.
2. In (b) it is outside the loop: it runs once after the loop ends.

Answer: (a) 3 times (b) once.

if, if-else and if-elif-else

NCERT section NCERT 6.2

if: runs a block only when a condition is True.

if condition:
    statements

if-else: runs one of two blocks: the if block when the condition is True, the else block otherwise.
if-elif-else: tests conditions in order; the block of the first True condition runs and the rest are skipped; else runs if none is True. There can be any number of elif parts, and else is optional.

The order of elif conditions matters: put the most specific (or highest) range first. Conditions can be combined with and, or and not. An if can also contain another if (nested if). In a flowchart each condition is a diamond with True and False exits.

Worked example

Find the output for marks = 85:

marks = 85
if marks >= 33:
    print('Pass')
if marks >= 90:
    print('A1')
elif marks >= 80:
    print('A2')
elif marks >= 70:
    print('B1')
else:
    print('Below B1')

Try it first, then show the solutionSolution

1. The first if is a separate statement: 85 >= 33, so 'Pass' is printed.
2. In the if-elif chain, 85 >= 90 is False; 85 >= 80 is True, so 'A2' is printed and the rest is skipped.

Answer: Pass
A2

Simple decision programs: absolute value, sorting three numbers, divisibility

NCERT section NCERT 6.2 and syllabus suggested programs

These three programs are named in the syllabus.

• Absolute value: if n < 0, make it -n; otherwise keep n. (Python also has the built-in abs().)
• Sorting three numbers: compare pairs and swap so that the smallest moves first. Three comparisons are enough: (a, b), then (a, c), then (b, c).
• Divisibility: n is divisible by d when n % d == 0. Combine tests with and/or, e.g. divisible by both 3 and 5 means n % 3 == 0 and n % 5 == 0 (that is, by 15).

Worked example

Write a program that sorts a = 42, b = 7, c = 19 into ascending order using only if statements and swaps, and give the output.

Try it first, then show the solutionSolution

1. a, b, c = 42, 7, 19
   if a > b:
       a, b = b, a
   if a > c:
       a, c = c, a
   if b > c:
       b, c = c, b
   print(a, b, c)

2. After (a, b): 7, 42, 19. (a, c): 7 < 19, no swap. (b, c): 42 > 19, swap: 7, 19, 42.

Answer: 7 19 42

The for loop and range()

NCERT section NCERT 6.4.1 (A), (B)

A for loop takes each item of a sequence (string, list, tuple, range) in turn and runs its block once per item:

for variable in sequence:
    statements

range([start,] stop[, step]) produces integers from start (default 0) up to but not including stop, increasing by step (default 1). The step may be negative to count down, but never 0.

• range(5) → 0, 1, 2, 3, 4
• range(2, 10, 3) → 2, 5, 8
• range(10, 0, -2) → 10, 8, 6, 4, 2
• range(5, 1) → nothing (start is already past stop)

The number of values in range(start, stop, step) with a positive step is the number of terms of start, start + step, ... that stay below stop. Use a for loop when the number of repetitions is known in advance.

Worked example

Give the output:

print(list(range(3, 15, 4)))
print(list(range(9, 0, -3)))
for ch in 'Py':
    print(ch * 2, end=' ')

Try it first, then show the solutionSolution

1. 3, 7, 11 (15 is not reached).
2. 9, 6, 3 (0 is excluded).
3. For each character, print it twice followed by a space.

Answer: [3, 7, 11]
[9, 6, 3]
PP yy

The while loop

NCERT section NCERT 6.4.2

A while loop repeats its block as long as a condition is True. The condition is tested before each pass, so the block may run zero times.

while condition:
    statements

Something inside the loop must eventually make the condition False, or the loop never ends: an infinite loop, which is a logical error. Use while when the number of repetitions is not known in advance: reading input until a sentinel value, processing the digits of a number, repeating until a target is reached.

Worked example

Write a while loop to reverse the digits of 4072 and give its output.

Try it first, then show the solutionSolution

1. n = 4072
   rev = 0
   while n > 0:
       rev = rev * 10 + n % 10
       n = n // 10
   print(rev)

2. Trace: (n, rev) = (4072, 0) → (407, 2) → (40, 27) → (4, 270) → (0, 2704).

Answer: 2704

break and continue

NCERT section NCERT 6.5

break ends the innermost loop at once; control moves to the first statement after that loop. Use it to stop searching once the answer is found.

continue skips the rest of the current pass and goes straight to the next pass (the next item in a for loop, or the condition test in a while loop). Use it to ignore some items.

In a while loop, make sure continue does not skip the statement that updates the loop variable, or the loop may become infinite.

Worked example

Give the output:

for n in range(1, 10):
    if n % 3 == 0:
        continue
    if n == 8:
        break
    print(n, end=' ')

Try it first, then show the solutionSolution

1. 1, 2 printed; 3 skipped (continue); 4, 5 printed; 6 skipped; 7 printed.
2. At 8 the loop breaks, so 8 and 9 are never printed.

Answer: 1 2 4 5 7

Nested loops and patterns

NCERT section NCERT 6.6

A loop inside another loop is a nested loop. For every single pass of the outer loop, the inner loop runs completely. If the outer loop runs m times and the inner n times, the inner block runs m × n times.

For patterns, the outer loop usually controls the rows and the inner loop the columns (what is printed on each row). Print items on the same row with end='' or end=' ' and move to the next row with an empty print().

Worked example

Write a program to print:
1
1 2
1 2 3
1 2 3 4

Try it first, then show the solutionSolution

1. Row i (1 to 4) prints the numbers 1 to i.

2. for i in range(1, 5):
       for j in range(1, i + 1):
           print(j, end=' ')
       print()

Answer: 1
1 2
1 2 3
1 2 3 4 (each row ends with a space, invisible on screen)

Series, factorial and other loop programs

NCERT section Syllabus suggested programs; NCERT 6.4

Common loop patterns:

• Accumulator (sum or product): start with total = 0 (for a sum) or product = 1 (for a product), then update in the loop.
• Factorial of a positive n: n! = 1 × 2 × ... × n; 0! = 1.
• Series: write the general term with the loop variable, e.g. 1 + x + x² + ... + xⁿ has term xi; 1 - 2 + 3 - 4 ... has term i * (-1)(i + 1).
• Counter: count = 0, then count += 1 when a condition holds.
• Prime test: count the divisors, or break as soon as one is found.

Worked example

Write a program to find 6! and the sum 1 + 1/2 + 1/3 + ... + 1/6 (to 3 decimal places).

Try it first, then show the solutionSolution

1. n = 6
   fact = 1
   s = 0
   for i in range(1, n + 1):
       fact = fact * i
       s = s + 1 / i
   print(fact, round(s, 3))

2. fact: 1, 2, 6, 24, 120, 720. s = 1 + 0.5 + 0.333 + 0.25 + 0.2 + 0.1667 = 2.45.

Answer: 720 2.45

Flow of Control 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

• for and while Loops, break, continue and Nested Loop Patterns
• if, elif and else in Python: Decision Programs and Output Tracing

Flow of Control: syntax and key facts

range()

range(start, stop, step): start, start+step, ... while < stop (step > 0) or > stop (step < 0)

stop is never included; step cannot be 0.

Number of passes

len(range(a, b)) = b - a for b > a; nested m × n

range(1, n + 1) gives n passes.

if-elif-else

Only the first True branch runs

Order the conditions from most to least specific.

break / continue

break: leave the innermost loop; continue: skip to the next pass

Neither affects an outer loop.

Accumulators

sum starts at 0; product (factorial) starts at 1; count starts at 0

Initialise before the loop, not inside it.

Digit loop

digit = n % 10; n = n // 10 while n > 0

Gives digits from the right: reverse, sum, count, palindrome.

Flow of Control: diagrams to practise

Practise each one on paper: draw it from the checklist, then tick off every label.

How the school exam asks Flow of Control

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

Tricks to remember Flow of Control

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

• Colon then indent

  Every if, elif, else, for and while line ends with a colon and is followed by an indented block.

  Use it when: Spotting syntax errors.

• range stops short

  range(1, 10) is like a lift that stops one floor before 10.

  Use it when: Counting values and last values of range().

• break breaks out, continue carries on

  break leaves the loop; continue jumps to the next round.

  Use it when: Output questions with both.

• Outer = rows, inner = columns

  In patterns the outer loop prints lines and the inner loop fills each line.

  Use it when: Writing pattern programs.

• Sum 0, product 1

  Start a sum at 0 and a product (factorial) at 1.

  Use it when: Accumulators in series and factorial programs.

Flow of Control: exam-level solved questions

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

for i in range(1, 4):
    for j in range(1, 4):
        if j == i:
            break
        print(i, j)

i = 0
total = 0
while i < 10:
    i += 1
    if i % 4 == 0:
        continue
    total += i
print(i, total)

Case-study practice (Section E style)

Read the situation, then answer every part. In a school exam set on the board pattern this is a 4-mark question split into 1 + 2 + 1 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 Flow of Control

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

• ✗Mistake: Including stop in range()

  range never produces the stop value.

  Wrong working

  for i in range(1, 5): prints 1 2 3 4 5

  Right working

  for i in range(1, 5): prints 1 2 3 4

  ✓Do this: Use range(1, n + 1) when n must be included.

  Usually costs: 1 mark

• ✗Mistake: Using separate ifs where elif is needed

  Separate ifs are all tested, so more than one block can run.

  Wrong working

  if m >= 80: print('A')
  if m >= 60: print('B') # m = 85 prints A and B

  Right working

  if m >= 80: print('A')
  elif m >= 60: print('B') # prints only A

  ✓Do this: Use elif for mutually exclusive choices.

  Usually costs: 1 mark

• ✗Mistake: Initialising the accumulator inside the loop

  It is reset on every pass, so only the last value survives.

  Wrong working

  for i in range(1, 6):
  s = 0
  s = s + i

  Right working

  s = 0
  for i in range(1, 6):
  s = s + i

  ✓Do this: Initialise totals, products and counters before the loop.

  Usually costs: 1 mark

• ✗Mistake: Forgetting to update the while variable

  The condition never becomes False: infinite loop.

  Wrong working

  i = 1
  while i <= 5:
  print(i)

  Right working

  i = 1
  while i <= 5:
  print(i)
  i += 1

  ✓Do this: Every while loop needs an initialisation, a condition and an update.

  Usually costs: 1 mark

• ✗Mistake: Thinking break exits all nested loops

  break leaves only the innermost loop in which it appears.

  Wrong working

  Saying the outer loop also stops

  Right working

  The outer loop goes on to its next pass

  ✓Do this: Trace the outer loop separately after a break.

  Usually costs: 1 mark

NCERT exercise guide: Flow of Control

What each exercise in the NCERT chapter tests, how to approach it, and a question of the same kind solved step by step. Questions here are written in our own words.

Practice questions on Flow of Control

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 · MCQExam level

   What will range(10, 1, -3) produce? (a) 10 7 4 1 (b) 10 7 4 (c) 7 4 1 (d) 10 7 4 1 -2

   Hint

   Stop value 1 is excluded.

   Answer

   (b) 10 7 4

2. Q2 · 1 mark · Very short answerHarder exam level

   How many times is 'Hi' printed?
   for i in range(3):
   for j in range(i):
   print('Hi')

   Hint

   Inner passes: 0, 1, 2.

   Answer

   3 times.

3. Q3 · 3 marks · Short answerExam level

   Write a program to input a year and print whether it is a leap year.

   Hint

   Divisible by 400, or by 4 but not by 100.

   Answer

   y = int(input('Year: '))
   if y % 400 == 0 or (y % 4 == 0 and y % 100 != 0):
   print('Leap year')
   else:
   print('Not a leap year')

4. Q4 · 2 marks · Short answerHarder exam level

   Predict the output:

   x = 1
   while x < 100:
       x = x * 3
       if x % 2 == 0:
           break
   print(x)

   Hint

   Powers of 3 are odd.

   Answer

   243 (3, 9, 27, 81, 243; no even value, so the loop ends when x ≥ 100).

5. Q5 · 2 marks · Short answerExam level

   Find the errors and rewrite:
   for i in range(1, 6)
   if i % 2 = 0
   print(i)

   Hint

   Colons, ==, indentation.

   Answer

   for i in range(1, 6):
   if i % 2 == 0:
   print(i)

6. Q6 · 2 marks · Short answerHarder exam level

   Predict the output:

   for i in 'CODE':
       if i in 'AEIOU':
           continue
       print(i.lower(), end='')

   Hint

   Vowels are skipped.

   Answer

   cd

7. Q7 · 3 marks · Short answerHarder exam level

   Write a program to print the sum of the digits of 90517 and the number of odd digits in it.

   Hint

   Use n % 10 and n // 10.

   Answer

   n = 90517
   s = c = 0
   while n > 0:
   d = n % 10
   s += d
   if d % 2 == 1:
   c += 1
   n //= 10
   print(s, c)
   Output: 22 4

8. Q8 · 1 mark · MCQHarder exam level

   Assertion (A): The statement in a while loop's block may never execute. Reason (R): The condition of a while loop is checked before each pass. Choose (a) both true, R explains A (b) both true, R does not explain A (c) A true, R false (d) A false, R true.

   Hint

   What if the condition is False at the start?

   Answer

   (a)

9. Q9 · 3 marks · Short answerHOTS / competency

   Write a program to print the first 10 terms of the Fibonacci series 0, 1, 1, 2, 3, ... and the sum of its even terms.

   Hint

   Even terms among 0, 1, 1, 2, 3, 5, 8, 13, 21, 34.

   Answer

   a, b = 0, 1
   s = 0
   for i in range(10):
   print(a, end=' ')
   if a % 2 == 0:
   s += a
   a, b = b, a + b
   print()
   print(s)
   Output: 0 1 1 2 3 5 8 13 21 34, then 44

10. Q10 · 2 marks · Short answerHOTS / competency

    Why does this loop never end, and how would you fix it?
    i = 10
    while i != 0:
    i -= 3

    Hint

    Values: 10, 7, 4, 1, -2 ...

    Answer

    i goes 10, 7, 4, 1, -2, -5 ... and never equals 0, so the condition i != 0 stays True for ever. Use while i > 0 instead.

11. Q11 · 3 marks · Short answerHOTS / competency

    Write a program that keeps reading marks until -1 is entered and then prints the highest mark entered (assume at least one mark).

    Hint

    Sentinel-controlled while loop.

    Answer

    m = int(input())
    high = m
    while m != -1:
    if m > high:
    high = m
    m = int(input())
    print('Highest:', high)

12. Q12 · 1 mark · Very short answerExam level

    Write the output of print(sum(range(1, 101, 2))).

    Hint

    Sum of the first 50 odd numbers.

    Answer

    2500

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.What does range(4) produce?Show answer

   0, 1, 2, 3.

2. 2.What is the default step of range()?Show answer

   1.

3. 3.Which loop is used when the number of repetitions is not known in advance?Show answer

   The while loop.

4. 4.What does continue do?Show answer

   Skips the rest of the current pass and moves to the next pass.

5. 5.What error does inconsistent indentation give?Show answer

   IndentationError (a syntax error).

6. 6.How many times does the inner block run in two nested loops of 3 and 4 passes?Show answer

   12.

7. 7.What is 0! ?Show answer

   1.

Quick revision checklist

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

• ☐Blocks are marked by indentation after a line ending with a colon.
• ☐if-elif-else runs only the first True branch.
• ☐for item in sequence; range(start, stop, step), stop excluded.
• ☐while repeats while the condition is True; update the variable to avoid an infinite loop.
• ☐break leaves the innermost loop; continue skips to the next pass.
• ☐Nested loops: inner runs fully for each outer pass; outer = rows, inner = columns.
• ☐Sum starts at 0, factorial at 1; n % d == 0 tests divisibility.

Flow of Control: frequently asked questions