Chapter 6 : Flow of Control | class 11th | revision notes computer science

Flow of Control

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Topics:

• Flow of control: Introduction, use of indentation, sequential flow, conditional and iterative flow control
• Conditional statements: if, if-else, if-elif-else, flowcharts, simple programs: e.g.: absolute value, sort 3 numbers and divisibility of a number
• Iterative statements: for loop, range function, while loop, flowcharts, nested loops, suggested programs: generating pattern, summation of series, finding the factorial of a positive number.
• Jump Statement – break and continue statements

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Introduction

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.

Indentation

Leading whitespace (spaces and tabs) at the beginning of a statement is called indentation.

Python uses indentation for block as well as for nested block structures.

In Python, the same level of indentation associates statements into a single block of code. The interpreter checks indentation levels very strictly and throws up syntax errors if indentation is not correct. It is a common practice to use a single tab for each level of indentation.

Types of Flow of Control

There are three types of flow of control –

a) Sequential flow of control

In Sequential flow of control execution of statement takes place in a sequence i.e. top to bottom approach.

num = int (input ("Enter Number " ))

num = num * 5

print (num) 

b) Conditional flow of control

Conditional flow of control is use to execute set of statements on the basis of the conditions.

num = int(input("Enter a number : "))
if num % 5 == 0:
   print(num, "is divisible by 5")
else:
   print(num, "is not divisible by 5")

c) Iterative flow of control

iterative flow of control means repetition. It execute the set of statements till the condition is true.

Conditional statements:

The if Statement

An if statement tests a particular condition; if the condition evaluates to true, then set of statements executed otherwise does not.

 if <conditional expression > :
   statement
   [statements] 

# Write a program to check given character is an alphabet ‘A’.

ch = input("Enter a character : ")
if ch == 'A' :
   print("You entered alphabet A")
if ch != 'A' :
    print("You entered alphabet A")

The if-else Statement

An if statement tests a particular condition; if the condition evaluates to true, then it carries out the  statements indented below if and in case condition evaluate to false, it carries out statements indented below else.

 if <conditional expression > :
    statement
    [statements]
 else :
    statement
    [statements] 

# Write a program to check given character is an upper alphabet.

ch = input("Enter a character : ")
if ch >= 'A' and ch <= 'Z':
   print("You have entered Upper alphabet")
else:
    print("You have entered other than Upper Alphabet")

The if-elif-else

An if-elif statement provide a facility to tests a condition with else ;

Syntax – 1

 if <conditional expression > :
   statement
   [statements]
 elif <conditional-expression> :
   statement
   [statements] 

Syntax – 2

 if <conditional expression > :
   statement
   [statements]
 elif <conditional-expression>:
   statement
   [statements]
 else:
   statement
   [statements] 

# Write a program to check given character is an upper alphabet, lower alphabet, digits or other symbol.

ch = input("Enter a character : ")
if ch >= 'A' and ch <= 'Z':
   print("You have entered Upper alphabet")
elif ch >= 'a' and ch <= 'z':
   print("You have entered Lower alphabet")
elif ch >= '0' and ch <= '9':
   print("You have entered Digit")
else:
    print("You have entered Symbol")

Nested if statements

An if inside the another if, called Nested if’s statement.

An if-else inside the another if or else called nested if-else.

 if <conditional expression > :
   if <condition>:
       statements
   else:
       statements
 else :
   if <condition>:
       statements
   else:
       statements 

Sample Programs :

# Absolute value

num = int(input("Enter a Number : "))
if num > 0 :
   print("Absolute Value is ", num)
elif num < 0:
   print("Absolute Value is ", num * -1)
else:
    print("You have entered ", num)

# Sort 3 numbers

num1 = int(input("Enter a Number 1 : "))
num2 = int(input("Enter a Number 2 : "))
num3 = int(input("Enter a Number 3 : "))
if num1 < num2 and num1 < num3:
   if num2 < num3:
       print(num1, num2, num3)
   else:
       print(num1, num3, num2)
elif num2 < num1 and num2 < num3:
   if num1 < num3:
       print(num2, num1, num3)
   else:
       print(num2, num3, num1)
else:
   if num2 < num1:
       print(num3, num2, num1)
   else:
       print(num3, num1, num2)

# Divisibility of a number

num1 = int(input("Enter a Number 1 : "))
num2 = int(input("Enter a Number 2 : "))
if num1 % num2 == 0 :
   print(num1 "is divisible by", num2)
elif num2 % num1 == 0:
   print(num2, "is divisible by", num1)
else:
    print(num1, 'and', num2,'neighter factor nor multiples')

Iterative / Repetitive Statements / Looping

range() function

for loop

while loop,

Flow Charts

Nested loops,

Suggested Programs:

# Python programs for generating pattern

Pattern-1

*

* *

* * *

* * * *

* * * * *

# Python Programs for summation of series.

# Finding the factorial of a positive number.

Jump Statement – break and continue statements

break statement

continue statement