Flow of Control – NCERT Class 11 Computer Science Chapter 6 – Conditional and Looping Constructs in Python

Explains how Python manages the order of execution of statements using control structures. Covers conditional statements (if, if-else, if-elif), indentation rules, and looping constructs (for and while). Introduces range() function, nested loops, and the use of break and continue statements with various examples. Highlights flowcharts, iteration logic, and pattern generation for better understanding of program control flow.

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Categories: NCERT, Class XI, Computer Science, Python, Flow of Control, Conditional Statements, Looping, Chapter 6

Tags: Flow of Control, Conditional Statements, If Else, Elif, Loops, For Loop, While Loop, Range Function, Break Statement, Continue Statement, Nested Loops, Indentation, Decision Making, Iteration, NCERT Class 11, Computer Science, Chapter 6

Flow of Control: NCERT Class 11 Chapter 6 - Enhanced Study Guide, Precise Notes, Diagrams & Quiz 2025

Enhanced Full Chapter Summary & Precise Notes from NCERT PDF (22 Pages)

Overview & Key Concepts

Exact Definition: "The order of execution of the statements in a program is known as flow of control. The flow of control can be implemented using control structures. Python supports two types of control structures—selection and repetition."

Introduction: Sequence from Ch 5; Bus analogy (Fig 6.1); Quote: G. van Rossum on indentation.
Chapter Structure: Selection (if/else/elif), Indentation, Repetition (for/while), Break/Continue, Nested Loops.
2025 Relevance: Control in AI loops (e.g., TensorFlow training); Indentation in VS Code; Nested for data processing.

6.1 Introduction to Flow of Control

Precise: Sequential execution; Control structures: Selection/Repetition. Expanded: Program 6-1 (difference); Flow like bus route.

Precise Fig 6.1: Bus to School (SVG)

Program 6-1: Difference of Two Numbers

#Program 6-1
#Program to print the difference of two input numbers
num1 = int(input("Enter first number: "))
num2 = int(input("Enter second number: "))
diff = num1 - num2
print("The difference of",num1,"and",num2,"is",diff)

Output:
Enter first number 5
Enter second number 7
The difference of 5 and 7 is -2

6.2 Selection

Precise: Decision making (if/else/elif); Positive difference (Prog 6-2); Flowchart Fig 6.2. Expanded: Nested if in calculator (Prog 6-3).

Precise Fig 6.2: Decision Flowchart (SVG)

Program 6-2: Positive Difference

#Program 6-2
#Program to print the positive difference of two numbers
num1 = int(input("Enter first number: "))
num2 = int(input("Enter second number: "))
if num1 > num2:
     diff = num1 - num2
else:
     diff = num2 - num1
print("The difference of",num1,"and",num2,"is",diff)

Output:
Enter first number: 5
Enter second number: 6
The difference of 5 and 6 is 1

Program 6-3: Simple Calculator

#Program to create a four function calculator
result = 0
val1 = float(input("Enter value 1: "))
val2 = float(input("Enter value 2: "))
op = input("Enter any one of the operator (+,-,*,/): ")
if op == "+":
    result = val1 + val2
elif op == "-":
    if val1 > val2:
        result = val1 - val2
    else:
        result = val2 - val1
elif op == "*":
        result = val1 * val2
elif op == "/":
    if val2 == 0:
        print("Error! Division by zero is not allowed. Program terminated")
    else:
        result = val1/val2
else:
    print("Wrong input,program terminated")
print("The result is ",result)

Output:
Enter value 1: 84
Enter value 2: 4
Enter any one of the operator (+,-,*,/): /
The result is 21.0

Example 6.1: if Syntax

age = int(input("Enter your age "))
if age >= 18:
    print("Eligible to vote")

Example 6.2: if-elif-else (Positive/Negative/Zero)

number = int(input("Enter a number: "))
if number > 0:
     print("Number is positive")
elif number < 0:
     print("Number is negative")
else:
     print("Number is zero")

Example 6.3: Traffic Signal

signal = input("Enter the colour: ")
if signal == "red" or signal == "RED":                
     print("STOP")
elif signal == "orange" or signal == "ORANGE":    
     print("Be Slow")
elif signal == "green" or signal == "GREEN":          
     print("Go!")

6.3 Indentation

Precise: Whitespace for blocks; Strict check; Single tab common. Expanded: Prog 6-4 shows blocks.

Program 6-4: Larger of Two Numbers

#Program 6-4
#Program to find larger of the two numbers
num1 = 5
num2 = 6
if num1 > num2:         
     #Block1
     print("first number is larger")         
     print("Bye")
else:   
     #Block2
     print("second number is larger")   
     print("Bye Bye")

Output:
second number is larger
Bye Bye

6.4 Repetition

Precise: Loops for iteration; Butterfly cycle (Fig 6.3); For/While. Expanded: Prog 6-5 (sequence); Range() function.

Precise Fig 6.3: Iterative Process (SVG)

Program 6-5: First Five Natural Numbers

#Program 6-5
#Print first five natural numbers
print(1)
print(2)
print(3)
print(4)
print(5)

Output:
1
2
3
4
5

Program 6-6: Print 'PYTHON' Characters

#Program 6-6
#Print the characters in word PYTHON using for loop
for letter in 'PYTHON':
     print(letter)

Output:
P
Y
T
H
O
N

Program 6-7: Print Sequence [10,20,30,40,50]

#Program 6-7
#Print the given sequence of numbers using for loop
count = [10,20,30,40,50]
for num in count:
     print(num)

Output:
10
20
30
40
50

Program 6-8: Even Numbers

#Program 6-8
#Print even numbers in the given sequence
numbers = [1,2,3,4,5,6,7,8,9,10]
for num in numbers:
     if (num % 2) == 0:
          print(num,'is an even Number')

Output:
2 is an even Number
4 is an even Number
6 is an even Number
8 is an even Number
10 is an even Number

Example 6.4: range() Function

>>> list(range(10)) 
[0, 1, 2, 3, 4, 5, 6, 7, 8, 9]
>>> list(range(2, 10))  
[2, 3, 4, 5, 6, 7, 8, 9]       
>>> list(range(0, 30, 5))  
[0, 5, 10, 15, 20, 25]   
>>> list (range (0, -9, -1))  
[0, -1, -2, -3, -4, -5, -6, -7, -8]

Program 6-9: Multiples of 10

#Program 6-9
#Print multiples of 10 for numbers in a given range
for num in range(5):
     if num > 0:
          print(num * 10)

Output:
10
20
30
40

Precise Fig 6.4: For Loop Flowchart (SVG)

Program 6-10: First 5 Natural (While)

#Program 6-10
#Print first 5 natural numbers using while loop
count = 1
while count <= 5:
     print(count)
     count += 1

Output:
1
2
3
4
5

Program 6-11: Factors of Number

#Program 6-11
#Find the factors of a number using while loop
num = int(input("Enter a number to find its factor: "))
print (1, end=' ')   #1 is a factor of every number
factor = 2  
while factor <= num/2 :
     if num % factor == 0:
          print(factor, end=' ') 
     factor += 1
print (num, end=' ') #every number is a factor of itself

Output:
Enter a number to find its factors : 6
1 2 3 6

Precise Fig 6.5: While Loop Flowchart (SVG)

6.5 Break and Continue

Precise: Break: Exit loop; Continue: Skip iteration. Expanded: Prog 6-12 (break at 8); Prog 6-13 (sum positives); Prog 6-14 (prime check).

Precise Fig 6.6: Break Flowchart (SVG)

Program 6-12: Break Demo

#Program 6-12
#Program to demonstrate the use of break statement in loop
num = 0
for num in range(10):
     num = num + 1
     if num == 8:
          break
     print('Num has value ' + str(num))
print('Encountered break!! Out of loop')

Output:
Num has value 1
Num has value 2
Num has value 3
Num has value 4
Num has value 5
Num has value 6
Num has value 7
Encountered break!! Out of loop

Program 6-13: Sum Positives

#Program 6-13
#Find the sum of all the positive numbers entered by the user 
#till the user enters a negative number. 
entry = 0
sum1 = 0
print("Enter numbers to find their sum, negative number ends the loop:")
while True: 
     entry = int(input())
     if (entry < 0):
          break
     sum1 += entry
print("Sum =", sum1)

Output:
Enter numbers to find their sum, negative number ends the loop:
3
4
5
-1
Sum = 12

Program 6-14: Prime Check

#Program 6-14
#Write a Python program to check if a given number is prime or not.
num = int(input("Enter the number to be checked: "))
flag = 0       
 
if num > 1 :
     for i in range(2, int(num / 2)):
          if (num % i == 0):
               flag = 1     
               break       

     if flag == 1:
          print(num , "is not a prime number")
     else:
          print(num , "is a prime number")
else :
          print("Entered number is <= 1, execute again!")

Output 1:
Enter the number to be checked: 20
20 is not a prime number
Output 2:
Enter the number to check: 19
19 is a prime number

Precise Fig 6.7: Continue Flowchart (SVG)

Program 6-15: Continue Demo

#Program 6-15
#Prints values from 0 to 6 except 3
num = 0
for num in range(6):
     num = num + 1
     if num == 3:
          continue
     print('Num has value ' + str(num))
print('End of loop')

Output:
Num has value 1
Num has value 2
Num has value 4
Num has value 5
Num has value 6
End of loop

6.6 Nested Loops

Precise: Loop inside loop; No limit on levels. Expanded: Prog 6-16 (nested for); Prog 6-17 (pattern); Prog 6-18 (primes 2-50); Prog 6-19 (factorial).

Program 6-16: Nested For

#Program 6-16
#Demonstrate working of nested for loops
for var1 in range(3):
    print( "Iteration " + str(var1 + 1) + " of outer loop")
    for var2 in range(2):    #nested loop
        print(var2 + 1)
    print("Out of inner loop")
print("Out of outer loop")

Output:
Iteration 1 of outer loop
1
2
Out of inner loop
Iteration 2 of outer loop
1
2
Out of inner loop
Iteration 3 of outer loop
1
2
Out of inner loop
Out of outer loop

Program 6-17: Pattern

#Program 6-17
#Program to print the pattern for a number input by the user
num = int(input("Enter a number to generate its pattern = "))
for i in range(1,num + 1):
     for j in range(1,i + 1):
          print(j, end = " ")
     print()

Output:
Enter a number to generate its pattern = 5
1
1 2
1 2 3
1 2 3 4
1 2 3 4 5

Program 6-18: Primes 2-50

#Program 6-18
#Use of nested loops to find the prime numbers between 2 to 50
num = 2
for i in range(2, 50):
     j= 2
     while ( j <= (i/2)):
          if (i % j == 0):     
               break           
          j += 1
     if ( j > i/j) :           
          print ( i, "is a prime number")
print ("Bye Bye!!")

Output:
2 is a prime number
3 is a prime number
5 is a prime number
7 is a prime number
11 is a prime number
13 is a prime number
17 is a prime number
19 is a prime number
23 is a prime number
29 is a prime number
31 is a prime number
37 is a prime number
41 is a prime number
43 is a prime number
47 is a prime number
Bye Bye!!

Program 6-19: Factorial

#Program 6-19
#The following program uses a for loop nested inside an if..else 
#block to calculate the factorial of a given number
num = int(input("Enter a number: "))
fact = 1
# check if the number is negative, positive or zero
if num < 0:
    print("Sorry, factorial does not exist for negative numbers")
elif num == 0:
    print("The factorial of 0 is 1")
else:
    for i in range(1, num + 1): 
        fact = fact * i
    print("factorial of ", num, " is ", fact)

Output:
Enter a number: 5
Factorial of 5 is 120

Enhanced Features (2025)

Full PDF integration, expanded programs (6-1 to 6-19), SVGs (Figs 6.1-6.7), detailed tables/examples, 30 Q&A updated, 10-Q quiz. Focus: Hands-on control flow.

Exam Tips

Write programs (positive diff/prime/pattern); Explain syntax/flowcharts; Differentiate for/while; Use break/continue in code; Nested loop examples.

30 Questions & Answers - NCERT Precise (Updated with Full Chapter)

Expanded: 10 (1-mark), 10 (4-marks), 10 (8-marks) based on full chapter; Mark-based with precise lines.

1-Mark Questions (10 Questions, One-Line Answers)

Q1: What is flow of control?

Ans: Order of statement execution.

Q2: Types of control structures?

Ans: Selection and repetition.

Q3: Syntax for if?

Ans: if condition: statements.

Q4: Purpose of elif?

Ans: Chain multiple conditions.

Q5: Indentation uses?

Ans: Define code blocks.

Q6: For loop iterates over?

Ans: Sequence or range.

Q7: While loop condition checked?

Ans: Before body execution.

Q8: Break does what?

Ans: Exits the loop.

Q9: Continue skips?

Ans: Rest of current iteration.

Q10: Nested loop is?

Ans: Loop inside another loop.

4-Mark Questions (10 Questions, Five-Line Answers)

Q11: Explain selection with syntax.

Ans: Decision making; if: condition true execute block. if-else: Two paths. if-elif-else: Multiple alternatives. Nested for sub-conditions (Prog 6-3). Fig 6.2 flowchart.

Q12: Differentiate for and while.

Ans: For: Known iterations, over sequence/range. While: Condition-based, test first. For: Exhausts items. While: Update to avoid infinite. Progs 6-6/6-10.

Q13: Describe indentation rules.

Ans: Whitespace (tab/spaces) for blocks; Same level groups statements. Strict syntax errors. Common: 1 tab/level. Prog 6-4 example.

Q14: Explain range() function.

Ans: range(start, stop, step); Integers sequence excluding stop. Default start=0, step=1. Positive/negative step. Ex 6.4 examples; Used in for loops.

Q15: Purpose of break/continue.

Ans: Break: Terminate loop early. Continue: Skip to next iteration. Flexibility in flow. Progs 6-12/6-15. Figs 6.6/6.7.

Q16: Nested loops application.

Ans: Inner repeats per outer; Patterns/primes/factorial. Prog 6-17 (triangle); No nesting limit. Mix for/while.

Q17: Infinite loop issue.

Ans: Condition never false; Logical error. Ensure update (count +=1). While without increment example.

Q18: Positive difference program.

Ans: if num1 > num2: diff=num1-num2 else: diff=num2-num1. Prog 6-2; Always absolute value.

Q19: Prime number check.

Ans: For i in 2 to num/2: if divisible break. Flag=1 not prime. Prog 6-14; Outputs for 20/19/2/1.

Q20: Pattern printing.

Ans: Nested for: Outer rows, inner columns. Prog 6-17; 1 to n triangle.

8-Mark Questions (10 Questions, Ten-Line Answers)

Q21: Explain introduction and selection.

Ans: Sequence to control; Selection for decisions (pen/path). if/else/elif syntax/examples (Ex 6.1-6.3). Prog 6-2 positive diff. Nested in calculator (Prog 6-3). Flowchart Fig 6.2. Van Rossum quote.

Q22: Detail indentation and repetition intro.

Ans: Indentation: Blocks without braces; Strict. Prog 6-4. Repetition: Iteration like bills/butterfly (Fig 6.3). Avoid 100k prints; Logic: Init-print-increment. Prog 6-5 sequence.

Q23: Describe for loop with range.

Ans: Iterates sequence; Syntax; Flow Fig 6.4. Progs 6-6/6-7/6-8 evens. range(): Parameters/examples (Ex 6.4). Prog 6-9 multiples. Known iterations.

Q24: Discuss while loop and examples.

Ans: Condition true repeat; Syntax; Flow Fig 6.5. Progs 6-10 naturals, 6-11 factors. Update to exit; Infinite risk. end=' ' for same line.

Q25: Elaborate break/continue with programs.

Ans: Break: Exit (Prog 6-12, 6-13 sum, 6-14 prime). Continue: Skip (Prog 6-15). Flow Figs 6.6/6.7. Flexibility; No infinite.

Q26: Explain nested loops with applications.

Ans: Loop in loop; Prog 6-16 demo. Patterns (6-17), primes (6-18 while inner), factorial (6-19). Unlimited levels; Mix types.

Q27: How control structures build programs?

Ans: Sequence base; Selection branches; Repetition iterates. Combined: Calculator/nested. Indentation enforces. 2025: Essential for algorithms/AI.

Q28: Difference between selection/repetition.

Ans: Selection: Choose path (if). Repetition: Repeat block (loops). Both alter sequence. Examples: Voting (select), naturals (repeat).

Q29: Role of indentation in Python.

Ans: Defines scope/blocks; No braces. Same indent level groups. Errors if wrong. Prog 6-4; Van Rossum: Guarantees proper indent.

Q30: Apply chapter to exercises like patterns/primes.

Ans: Nested loops for patterns (Ex 9); Break in primes. If-elif for grades (Ex 10). Case study: SMIS menu with selection/repetition.

Tip: Include programs/flowcharts in answers; Practice patterns/primes.

Concept	Precise Description	Example	Process/2025 Note
Selection	if/else/elif decisions	Positive diff (Prog 6-2)	Branch flow
Indentation	Block definition	1 tab/level (Prog 6-4)	Strict syntax
For Loop	Over range/sequence	Print 'PYTHON' (6-6)	Known iterations
While Loop	Condition repeat	Factors (6-11)	Avoid infinite
Break	Exit loop	At 8 (6-12)	Early terminate
Continue	Skip iteration	Skip 3 (6-15)	Next cycle
Nested Loops	Loop in loop	Pattern (6-17)	Patterns/primes
Range	Integer sequence	range(0,10) [0-9]	For generation

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