Chapter 13 — Flow of Control (Introduction)

Easy-language Notes · Part 1 of 2 (three patterns · indentation · sequential)

REVISION NOTES
Class XI · Computer Science (083)
Sheet 1 (Front)

1. The Three Patterns of Flow

The flow of control is the order in which Python runs your statements. By default lines run top-to-bottom; with the right keywords we can choose which lines to run, or repeat them.

A program is like a recipe. By default you follow steps in order. Sometimes the recipe says "if dough is dry, add water" — that is choosing. Sometimes "knead for 5 minutes" — that is repeating.

Pattern	Behaviour	Keywords
Sequential	Lines run once, top to bottom	(default, no keyword)
Conditional	Choose which block runs	`if`, `elif`, `else`
Iterative	Repeat a block several times	`for`, `while`

Every program is built from these three patterns combined — an idea sometimes called structured programming. A 1966 result by Bohm and Jacopini proved any algorithm can be written using just these three.

Why this matters

Knowing the type tells you which keyword to reach for.
Each pattern has a different shape on a flowchart — useful in design questions.
Mixing them correctly is what makes programs do something useful rather than just print fixed text.

In an exam, look at the first keyword of the line: if → conditional; for / while → iterative; anything else → sequential.

2. Indentation Groups Statements

Most languages use { } to mark a block. Python uses indentation — the leading spaces — to show which lines belong inside an if, a loop, or a function.

Rules

Use a consistent count for every level. Standard = 4 spaces.
Lines with the same indent are in the same block.
The line opening a block ends with a colon.
Never mix tabs and spaces in the same file.
Going back to a smaller indent ends the inner block.

if marks >= 33:
    print("Pass")        # inside if
    print("Well done")   # still inside
print("End")              # OUTSIDE, always

The first two indented lines run only when the condition is True. The fourth line is back at the outer level, so it always runs.

Common Indentation Mistakes

Mistake	Result
No indent after a colon line	IndentationError
Mixed 2-space and 4-space lines	IndentationError
Tabs and spaces mixed	TabError
Forgotten colon at end of `if`	SyntaxError
Random extra indent at outer level	IndentationError

Indentation is not just style in Python — it changes meaning. Two lines that look the same but have different indents belong to different blocks. Configure your editor to "show whitespace" so invisible mismatches don't bite you.

3. Sequential Flow

The default behaviour: every statement runs once, from top to bottom. No loops, no decisions.

Sequential flow is like reading a story page by page — you don't skip ahead and you don't read the same page twice. Each instruction runs exactly once.

The IPO Shape

Most short programs follow Input → Process → Output:

Input — read values from the user.
Process — calculate or transform.
Output — show the result.

A pure-sequential program just walks through these steps once and stops.

Area of a rectangle:

l = int(input("L: "))
b = int(input("B: "))
area = l * b
print("Area =", area)

Four lines, four steps — in order. Pure sequential flow.

Simple interest:

p, r, t = 5000, 8.5, 3
si = (p * r * t) / 100
print("SI =", si)

Celsius to Fahrenheit:

c = float(input("C: "))
f = c * 9/5 + 32
print(c, "C =", f, "F")

Three lines — input, process, output.

When sequential is enough

The task is a fixed formula with known inputs.
No decision is needed.
Nothing has to be repeated.

Even programs with if or loops are mostly sequential between those blocks. Sequential is the default; the others are special cases that depart from it.

4. Conditional Flow (Selection)

A conditional statement lets the program pick one of several blocks based on a True/False test. Python uses three keywords: if, elif, else.

Form	Use when…
`if`	One block, run only when condition holds
`if-else`	Exactly two paths, choose one
`if-elif-else`	Three or more paths, only one runs

Pass / Fail (two-path):

marks = 47
if marks >= 33:
    print("Pass")
else:
    print("Fail")

Grade ladder (multi-path):

m = 67
if m >= 90: g = "A"
elif m >= 75: g = "B"
elif m >= 60: g = "C"
else: g = "D"
print(g)        # C

Python tests top-to-bottom; the first True branch runs, the rest are skipped.

If-elif vs separate ifs

Three separate if statements run their blocks independently — multiple can be True. An if-elif-else ladder runs at most one block. Pick based on whether the conditions are mutually exclusive.

Each if/elif/else line ends with a colon. Only one branch runs per execution. Order matters — put the most specific condition first.

Chapter 13 — Flow of Control (Introduction)

Easy-language Notes · Part 2 of 2 (conditional · iterative · comparison)

REVISION NOTES
Class XI · Computer Science (083)
Sheet 1 (Back)

5. Iterative Flow (Loops)

An iterative statement repeats a block several times. Python provides for and while.

`for` — counted repetition

Use when you know how many times to run, or want to walk through every item of a sequence.

for i in range(1, 6):
    print(i, i*i)

Prints 1 1, 2 4, 3 9, 4 16, 5 25.

range(start, stop, step) generates numbers from start up to but not including stop:

range(5) → 0, 1, 2, 3, 4
range(1, 6) → 1, 2, 3, 4, 5
range(0, 10, 2) → 0, 2, 4, 6, 8
range(10, 0, -1) → 10 down to 1

`while` — condition-controlled repetition

Use when you don't know the count in advance.

n = 5
while n > 0:
    print(n)
    n -= 1

Prints 5, 4, 3, 2, 1. The loop ends when n > 0 becomes False.

For vs While

When to use	for	while
Count is known up front	Yes	OK
Walking through list / string	Yes	No
Repeat until something changes	No	Yes
Risk of running forever	No	Yes

An infinite loop happens when the while condition never becomes False. Always make sure the variable being tested actually changes inside the loop.

6. Putting It Together

Real programs combine all three patterns. A small login script is a good demo:

attempts = 0
while attempts < 3:           # iterative
    pwd = input("PWD: ")
    if pwd == "vm123":        # conditional
        print("Granted")
        break
    attempts += 1
else:
    print("Locked")               # sequential

The while drives repetition, if chooses what runs each round, the rest is plain sequential code.

Comparison at a Glance

Aspect	Sequential	Conditional	Iterative
Block runs	Once	0 or 1 time	0 or many
Decided by	Position	Condition	Condition / count
Keywords	(none)	if/elif/else	for, while
Risk	Forgotten step	Wrong condition	Infinite loop

Quick Self-Test

For each phrase, identify the flow type:

"Read marks, compute total, print." → sequential
"If marks < 33 print Fail else Pass." → conditional
"Print Hello five times." → iterative
"Repeat asking until user types Y." → iterative

Three questions break down any task: Do I always do this? → sequential. Should I sometimes skip this? → conditional. Should I redo this? → iterative.

Chapter 13 Worksheet — Flow of Control (Introduction)

Total: 50 marks | Time: 75 min

Name: Class/Sec: Roll No.: Date:

Section A — Multiple Choice (1 × 10 = 10)

The default flow of control in Python is:
(a) Conditional(b) Iterative(c) Sequential(d) Random
Python groups statements into a block using:
(a) Curly braces(b) Indentation(c) Semicolons(d) BEGIN/END
The standard indentation is:
(a) 1 space(b) 2 spaces(c) 4 spaces(d) 8 spaces
Which keyword starts a counted loop?
(a) if(b) for(c) elif(d) repeat
range(2, 10, 3) produces:
(a) 2, 5, 8(b) 2, 5, 8, 11(c) 2, 4, 6, 8(d) 3, 6, 9
An infinite loop is most likely with:
(a) for(b) if(c) while(d) elif
Which is NOT a flow-of-control pattern?
(a) Sequential(b) Iterative(c) Conditional(d) Recursive
Which line opens a block?
(a) if x > 0;(b) if x > 0:(c) if x > 0 then(d) if x > 0 {
How many branches of if-elif-else run per execution?
(a) Zero(b) Exactly one(c) All matching(d) The first two
Forgotten indent after for gives:
(a) ValueError(b) NameError(c) IndentationError(d) Nothing

Section B — Find the Output (1 × 8 = 8)

```
for i in range(3):
    print(i)
```
Output:

for i in range(1, 6, 2):
    print(i, end=" ")

Output:

n = 3
while n > 0:
    print(n, end=" ")
    n -= 1

Output:

x = 7
if x % 2 == 0:
    print("even")
else:
    print("odd")

Output:

total = 0
for i in range(1, 5):
    total += i
print(total)

Output:

m = 80
if m >= 90: g = "A"
elif m >= 75: g = "B"
else: g = "C"
print(g)

Output:

for ch in "abc":
    print(ch * 2, end="-")

Output:

i = 1
while i < 20:
    print(i, end=" ")
    i *= 2

Output:

Section C — Find the Error (1 × 6 = 6)

Each snippet has exactly one error. Mark the line number and write the corrected line.

```
1  if x > 0
2      print("+ve")
```
Wrong line: Correction:
```
1  for i in range(5):
2  print(i)
```
Wrong line: Correction:

1  n = 5
2  while n > 0:
3      print(n)

Wrong line: Correction:

1  age = 20
2  if age >= 18: print("vote")
3  elseif age < 18: print("wait")

Wrong line: Correction:

1  m = 50
2  if m >= 33: print("Pass")
3  else m < 33: print("Fail")

Wrong line: Correction:

1  for i in range(1, 5):
2      print(i)
3   print("done")

Wrong line: Correction:

Section D — Short Answer (2 × 5 = 10)

List the three patterns of flow of control with one Python keyword for each.
Why is indentation important in Python? Give one example of an error caused by wrong indentation.
Differentiate between a for loop and a while loop with one example each.
What is meant by the Input → Process → Output structure? Show it for a program that prints simple interest.
What is an infinite loop? Write one example and show how to avoid it.

Chapter 13 Worksheet — (continued)

Sheet 2 (Back)

Section E — Code Tasks (16 marks)

Each program should read input where required and produce a clear output. Use a separate sheet if you need more space.

(2) Write a Python program that reads the radius of a circle and prints its area and circumference. Use A = 3.14 * r * r and C = 2 * 3.14 * r. (Pure sequential.)
(3) Read a number and use if-elif-else to print whether it is positive, negative, or zero.
(3) Use a for loop to print the multiplication table of any number entered by the user, from 1 to 10. Output format: 5 x 1 = 5, 5 x 2 = 10, …
(2) Use a while loop to print the numbers from 10 down to 1 on one line, separated by spaces, followed by "Lift off!".
(3) Read an integer n and print the sum of all even numbers from 1 to n (inclusive). Use a for loop.
(3) Identify the flow-of-control pattern (sequential / conditional / iterative) used in each line below, and give a short reason.
```
(i)   total = a + b
(ii)  if n % 2 == 0: print("Even")
(iii) for i in range(5): print(i)
```

Chapter 13 — Flow of Control (Introduction)

1. The Three Patterns of Flow

Why this matters

2. Indentation Groups Statements

Rules

Common Indentation Mistakes

3. Sequential Flow

The IPO Shape

When sequential is enough

4. Conditional Flow (Selection)

If-elif vs separate ifs

Chapter 13 — Flow of Control (Introduction)

5. Iterative Flow (Loops)

for — counted repetition

while — condition-controlled repetition

For vs While

6. Putting It Together

Comparison at a Glance

Quick Self-Test

Chapter 13 Worksheet — Flow of Control (Introduction)

Section A — Multiple Choice (1 × 10 = 10)

Section B — Find the Output (1 × 8 = 8)

Section C — Find the Error (1 × 6 = 6)

Section D — Short Answer (2 × 5 = 10)

Chapter 13 Worksheet — (continued)

Section E — Code Tasks (16 marks)

`for` — counted repetition

`while` — condition-controlled repetition