NCERT Textbook: Flow of Control | Computer Science for Class 11 - Humanities/Arts PDF Download

 Page 1


6.1  Introduct Ion In Figure 6.1, we see a bus carrying the children to 
school. There is only one way to reach the school. The 
driver has no choice, but to follow the road one milestone 
after another to reach the school. We learnt in Chapter 
5 that this is the concept of sequence, where Python 
executes one statement after another from beginning to 
the end of the program. These are the kind of programs 
we have been writing till now.
Figure 6.1: Bus carrying students to school
Let us consider a program 6-1 that executes in 
sequence, that is, statements are executed in an order 
in which they are written.
The order of execution of the statements in a program 
is known as ??ow of control. The ??ow of control can be 
implemented using control structures. Python supports 
two types of control structures—selection and repetition.
“Don't you hate code that's 
not properly indented? 
Making it [indenting] part of 
the syntax guarantees that all 
code is properly indented.” 
–  G. van Rossum
Chapter 6
Flow of Control
In this chapter
 » Introduction to Flow 
of Control
 » Selection
 » Indentation
 » Repetition
 » Break and Continue 
Statements
 » Nested Loops
Ch 6.indd   121 08-Apr-19   12:37:51 PM
Reprint 2025-26
Page 2


6.1  Introduct Ion In Figure 6.1, we see a bus carrying the children to 
school. There is only one way to reach the school. The 
driver has no choice, but to follow the road one milestone 
after another to reach the school. We learnt in Chapter 
5 that this is the concept of sequence, where Python 
executes one statement after another from beginning to 
the end of the program. These are the kind of programs 
we have been writing till now.
Figure 6.1: Bus carrying students to school
Let us consider a program 6-1 that executes in 
sequence, that is, statements are executed in an order 
in which they are written.
The order of execution of the statements in a program 
is known as ??ow of control. The ??ow of control can be 
implemented using control structures. Python supports 
two types of control structures—selection and repetition.
“Don't you hate code that's 
not properly indented? 
Making it [indenting] part of 
the syntax guarantees that all 
code is properly indented.” 
–  G. van Rossum
Chapter 6
Flow of Control
In this chapter
 » Introduction to Flow 
of Control
 » Selection
 » Indentation
 » Repetition
 » Break and Continue 
Statements
 » Nested Loops
Ch 6.indd   121 08-Apr-19   12:37:51 PM
Reprint 2025-26
Computer SCien Ce – Cla SS xi 122
Program 6-1 Program to print the difference of two 
numbers.
#Program 6-1
#Program to print the di??erence of two input numbers
num1 = int(input("Enter ??rst number: "))
num2 = int(input("Enter second number: "))
di?? = num1 - num2
print("The di??erence of",num1,"and",num2,"is",di??)
Output:
Enter ??rst number 5
Enter second number 7
The di??erence of 5 and 7 is -2 
6.2 Select Ion Now suppose we have `10 to buy a pen. On visiting the 
stationery shop, there are a variety of pens priced at 
`10 each. Here, we have to decide which pen to buy. 
Similarly, when we use the direction services of a digital 
map, to reach from one place to 
another, we notice that sometimes 
it shows more than one path like 
the least crowded path, shortest 
distance path, etc. We decide 
the path as per our priority. A 
decision involves selecting from 
one of the two or more possible 
options. In programming, this 
concept of decision making or 
selection is implemented with the 
help of if..else statement.
Now, suppose we want to 
display the positive difference of 
the two numbers num1 and num2 
given at program 6-1. For that, 
we need to modify our approach. 
Look at the ??owchart shown in 
Figure 6.2 that subtracts the 
smaller number from the bigger 
number so that we always get a positive difference. This 
selection is based upon the values that are input for the 
two numbers num1 and num2.
Figure 6.2: Flow chart depicting decision making
Ch 6.indd   122 08-Apr-19   12:37:52 PM
Reprint 2025-26
Page 3


6.1  Introduct Ion In Figure 6.1, we see a bus carrying the children to 
school. There is only one way to reach the school. The 
driver has no choice, but to follow the road one milestone 
after another to reach the school. We learnt in Chapter 
5 that this is the concept of sequence, where Python 
executes one statement after another from beginning to 
the end of the program. These are the kind of programs 
we have been writing till now.
Figure 6.1: Bus carrying students to school
Let us consider a program 6-1 that executes in 
sequence, that is, statements are executed in an order 
in which they are written.
The order of execution of the statements in a program 
is known as ??ow of control. The ??ow of control can be 
implemented using control structures. Python supports 
two types of control structures—selection and repetition.
“Don't you hate code that's 
not properly indented? 
Making it [indenting] part of 
the syntax guarantees that all 
code is properly indented.” 
–  G. van Rossum
Chapter 6
Flow of Control
In this chapter
 » Introduction to Flow 
of Control
 » Selection
 » Indentation
 » Repetition
 » Break and Continue 
Statements
 » Nested Loops
Ch 6.indd   121 08-Apr-19   12:37:51 PM
Reprint 2025-26
Computer SCien Ce – Cla SS xi 122
Program 6-1 Program to print the difference of two 
numbers.
#Program 6-1
#Program to print the di??erence of two input numbers
num1 = int(input("Enter ??rst number: "))
num2 = int(input("Enter second number: "))
di?? = num1 - num2
print("The di??erence of",num1,"and",num2,"is",di??)
Output:
Enter ??rst number 5
Enter second number 7
The di??erence of 5 and 7 is -2 
6.2 Select Ion Now suppose we have `10 to buy a pen. On visiting the 
stationery shop, there are a variety of pens priced at 
`10 each. Here, we have to decide which pen to buy. 
Similarly, when we use the direction services of a digital 
map, to reach from one place to 
another, we notice that sometimes 
it shows more than one path like 
the least crowded path, shortest 
distance path, etc. We decide 
the path as per our priority. A 
decision involves selecting from 
one of the two or more possible 
options. In programming, this 
concept of decision making or 
selection is implemented with the 
help of if..else statement.
Now, suppose we want to 
display the positive difference of 
the two numbers num1 and num2 
given at program 6-1. For that, 
we need to modify our approach. 
Look at the ??owchart shown in 
Figure 6.2 that subtracts the 
smaller number from the bigger 
number so that we always get a positive difference. This 
selection is based upon the values that are input for the 
two numbers num1 and num2.
Figure 6.2: Flow chart depicting decision making
Ch 6.indd   122 08-Apr-19   12:37:52 PM
Reprint 2025-26
Flow o F Control 123
The syntax of if statement is:
if condition:
     statement(s)
In the following example, if the age entered by the 
user is greater than 18, then print that the user is 
eligible to vote. If the condition is true, then the indented 
statement(s) are executed. The indentation implies that 
its execution is dependent on the condition. There is no 
limit on the number of statements that can appear as a 
block under the if statement. 
Example 6.1 
age = int(input("Enter your age "))
if age >= 18:
    print("Eligible to vote")
A variant of if statement called if..else statement 
allows us to write two alternative paths and the control 
condition determines which path gets executed. The 
syntax for if..else statement is as follows.
if condition:
     statement(s)
else:
     statement(s)
Let us now modify the example on voting with the 
condition that if the age entered by the user is greater 
than 18, then to display that the user is eligible to vote. 
Otherwise display that the user is not eligible to vote.
age = int(input("Enter your age: "))
if age >= 18:
     print("Eligible to vote")
else:
     print("Not eligible to vote")
Now let us use the same concept to modify program 
6-1, so that it always gives a positive difference as the 
output. From the ??ow chart in Figure 6.2, it is clear that 
we need to decide whether num1 > num2 or not and 
take action accordingly.
We have to specify two blocks of statements since 
num1 can be greater than num2 or vice-versa as shown 
in program 6-2.
Many a times there are situations that require 
multiple conditions to be checked and it may lead to 
many alternatives. In such cases we can chain the 
conditions using if..elif (elif means else..if).
n ote S
Ch 6.indd   123 08-Apr-19   12:37:52 PM
Reprint 2025-26
Page 4


6.1  Introduct Ion In Figure 6.1, we see a bus carrying the children to 
school. There is only one way to reach the school. The 
driver has no choice, but to follow the road one milestone 
after another to reach the school. We learnt in Chapter 
5 that this is the concept of sequence, where Python 
executes one statement after another from beginning to 
the end of the program. These are the kind of programs 
we have been writing till now.
Figure 6.1: Bus carrying students to school
Let us consider a program 6-1 that executes in 
sequence, that is, statements are executed in an order 
in which they are written.
The order of execution of the statements in a program 
is known as ??ow of control. The ??ow of control can be 
implemented using control structures. Python supports 
two types of control structures—selection and repetition.
“Don't you hate code that's 
not properly indented? 
Making it [indenting] part of 
the syntax guarantees that all 
code is properly indented.” 
–  G. van Rossum
Chapter 6
Flow of Control
In this chapter
 » Introduction to Flow 
of Control
 » Selection
 » Indentation
 » Repetition
 » Break and Continue 
Statements
 » Nested Loops
Ch 6.indd   121 08-Apr-19   12:37:51 PM
Reprint 2025-26
Computer SCien Ce – Cla SS xi 122
Program 6-1 Program to print the difference of two 
numbers.
#Program 6-1
#Program to print the di??erence of two input numbers
num1 = int(input("Enter ??rst number: "))
num2 = int(input("Enter second number: "))
di?? = num1 - num2
print("The di??erence of",num1,"and",num2,"is",di??)
Output:
Enter ??rst number 5
Enter second number 7
The di??erence of 5 and 7 is -2 
6.2 Select Ion Now suppose we have `10 to buy a pen. On visiting the 
stationery shop, there are a variety of pens priced at 
`10 each. Here, we have to decide which pen to buy. 
Similarly, when we use the direction services of a digital 
map, to reach from one place to 
another, we notice that sometimes 
it shows more than one path like 
the least crowded path, shortest 
distance path, etc. We decide 
the path as per our priority. A 
decision involves selecting from 
one of the two or more possible 
options. In programming, this 
concept of decision making or 
selection is implemented with the 
help of if..else statement.
Now, suppose we want to 
display the positive difference of 
the two numbers num1 and num2 
given at program 6-1. For that, 
we need to modify our approach. 
Look at the ??owchart shown in 
Figure 6.2 that subtracts the 
smaller number from the bigger 
number so that we always get a positive difference. This 
selection is based upon the values that are input for the 
two numbers num1 and num2.
Figure 6.2: Flow chart depicting decision making
Ch 6.indd   122 08-Apr-19   12:37:52 PM
Reprint 2025-26
Flow o F Control 123
The syntax of if statement is:
if condition:
     statement(s)
In the following example, if the age entered by the 
user is greater than 18, then print that the user is 
eligible to vote. If the condition is true, then the indented 
statement(s) are executed. The indentation implies that 
its execution is dependent on the condition. There is no 
limit on the number of statements that can appear as a 
block under the if statement. 
Example 6.1 
age = int(input("Enter your age "))
if age >= 18:
    print("Eligible to vote")
A variant of if statement called if..else statement 
allows us to write two alternative paths and the control 
condition determines which path gets executed. The 
syntax for if..else statement is as follows.
if condition:
     statement(s)
else:
     statement(s)
Let us now modify the example on voting with the 
condition that if the age entered by the user is greater 
than 18, then to display that the user is eligible to vote. 
Otherwise display that the user is not eligible to vote.
age = int(input("Enter your age: "))
if age >= 18:
     print("Eligible to vote")
else:
     print("Not eligible to vote")
Now let us use the same concept to modify program 
6-1, so that it always gives a positive difference as the 
output. From the ??ow chart in Figure 6.2, it is clear that 
we need to decide whether num1 > num2 or not and 
take action accordingly.
We have to specify two blocks of statements since 
num1 can be greater than num2 or vice-versa as shown 
in program 6-2.
Many a times there are situations that require 
multiple conditions to be checked and it may lead to 
many alternatives. In such cases we can chain the 
conditions using if..elif (elif means else..if).
n ote S
Ch 6.indd   123 08-Apr-19   12:37:52 PM
Reprint 2025-26
Computer SCien Ce – Cla SS xi 124
Program 6-2 Program to print the positive difference 
of two numbers.
#Program 6-2
#Program to print the positive di??erence of two numbers
num1 = int(input("Enter ??rst number: "))
num2 = int(input("Enter second number: "))
if num1 > num2:
     di?? = num1 - num2
else:
     di?? = num2 - num1
print("The di??erence of",num1,"and",num2,"is",di??)
Output:
Enter ??rst number: 5
Enter second number: 6
The di??erence of 5 and 6 is 1
The syntax for a selection structure using elif is as 
shown below.
if condition:
     statement(s)
elif condition:
     statement(s)
elif condition:
     statement(s)
else:
     statement(s)
Example 6.2 Check whether a number is positive, 
negative, or 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 Display the appropriate message as per 
the colour of signal at the road crossing.
signal = input("Enter the colour: ")
if signal == "red" or signal == "RED":                
     print("STOP")
elif signal == "orange" or signal == 
"ORANGE":    
Ch 6.indd   124 08-Apr-19   12:37:52 PM
Reprint 2025-26
Page 5


6.1  Introduct Ion In Figure 6.1, we see a bus carrying the children to 
school. There is only one way to reach the school. The 
driver has no choice, but to follow the road one milestone 
after another to reach the school. We learnt in Chapter 
5 that this is the concept of sequence, where Python 
executes one statement after another from beginning to 
the end of the program. These are the kind of programs 
we have been writing till now.
Figure 6.1: Bus carrying students to school
Let us consider a program 6-1 that executes in 
sequence, that is, statements are executed in an order 
in which they are written.
The order of execution of the statements in a program 
is known as ??ow of control. The ??ow of control can be 
implemented using control structures. Python supports 
two types of control structures—selection and repetition.
“Don't you hate code that's 
not properly indented? 
Making it [indenting] part of 
the syntax guarantees that all 
code is properly indented.” 
–  G. van Rossum
Chapter 6
Flow of Control
In this chapter
 » Introduction to Flow 
of Control
 » Selection
 » Indentation
 » Repetition
 » Break and Continue 
Statements
 » Nested Loops
Ch 6.indd   121 08-Apr-19   12:37:51 PM
Reprint 2025-26
Computer SCien Ce – Cla SS xi 122
Program 6-1 Program to print the difference of two 
numbers.
#Program 6-1
#Program to print the di??erence of two input numbers
num1 = int(input("Enter ??rst number: "))
num2 = int(input("Enter second number: "))
di?? = num1 - num2
print("The di??erence of",num1,"and",num2,"is",di??)
Output:
Enter ??rst number 5
Enter second number 7
The di??erence of 5 and 7 is -2 
6.2 Select Ion Now suppose we have `10 to buy a pen. On visiting the 
stationery shop, there are a variety of pens priced at 
`10 each. Here, we have to decide which pen to buy. 
Similarly, when we use the direction services of a digital 
map, to reach from one place to 
another, we notice that sometimes 
it shows more than one path like 
the least crowded path, shortest 
distance path, etc. We decide 
the path as per our priority. A 
decision involves selecting from 
one of the two or more possible 
options. In programming, this 
concept of decision making or 
selection is implemented with the 
help of if..else statement.
Now, suppose we want to 
display the positive difference of 
the two numbers num1 and num2 
given at program 6-1. For that, 
we need to modify our approach. 
Look at the ??owchart shown in 
Figure 6.2 that subtracts the 
smaller number from the bigger 
number so that we always get a positive difference. This 
selection is based upon the values that are input for the 
two numbers num1 and num2.
Figure 6.2: Flow chart depicting decision making
Ch 6.indd   122 08-Apr-19   12:37:52 PM
Reprint 2025-26
Flow o F Control 123
The syntax of if statement is:
if condition:
     statement(s)
In the following example, if the age entered by the 
user is greater than 18, then print that the user is 
eligible to vote. If the condition is true, then the indented 
statement(s) are executed. The indentation implies that 
its execution is dependent on the condition. There is no 
limit on the number of statements that can appear as a 
block under the if statement. 
Example 6.1 
age = int(input("Enter your age "))
if age >= 18:
    print("Eligible to vote")
A variant of if statement called if..else statement 
allows us to write two alternative paths and the control 
condition determines which path gets executed. The 
syntax for if..else statement is as follows.
if condition:
     statement(s)
else:
     statement(s)
Let us now modify the example on voting with the 
condition that if the age entered by the user is greater 
than 18, then to display that the user is eligible to vote. 
Otherwise display that the user is not eligible to vote.
age = int(input("Enter your age: "))
if age >= 18:
     print("Eligible to vote")
else:
     print("Not eligible to vote")
Now let us use the same concept to modify program 
6-1, so that it always gives a positive difference as the 
output. From the ??ow chart in Figure 6.2, it is clear that 
we need to decide whether num1 > num2 or not and 
take action accordingly.
We have to specify two blocks of statements since 
num1 can be greater than num2 or vice-versa as shown 
in program 6-2.
Many a times there are situations that require 
multiple conditions to be checked and it may lead to 
many alternatives. In such cases we can chain the 
conditions using if..elif (elif means else..if).
n ote S
Ch 6.indd   123 08-Apr-19   12:37:52 PM
Reprint 2025-26
Computer SCien Ce – Cla SS xi 124
Program 6-2 Program to print the positive difference 
of two numbers.
#Program 6-2
#Program to print the positive di??erence of two numbers
num1 = int(input("Enter ??rst number: "))
num2 = int(input("Enter second number: "))
if num1 > num2:
     di?? = num1 - num2
else:
     di?? = num2 - num1
print("The di??erence of",num1,"and",num2,"is",di??)
Output:
Enter ??rst number: 5
Enter second number: 6
The di??erence of 5 and 6 is 1
The syntax for a selection structure using elif is as 
shown below.
if condition:
     statement(s)
elif condition:
     statement(s)
elif condition:
     statement(s)
else:
     statement(s)
Example 6.2 Check whether a number is positive, 
negative, or 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 Display the appropriate message as per 
the colour of signal at the road crossing.
signal = input("Enter the colour: ")
if signal == "red" or signal == "RED":                
     print("STOP")
elif signal == "orange" or signal == 
"ORANGE":    
Ch 6.indd   124 08-Apr-19   12:37:52 PM
Reprint 2025-26
Flow o F Control 125
     print("Be Slow")
elif signal == "green" or signal == "GREEN":                   
     print("Go!")
Number of elif is dependent on the number of 
conditions to be checked. If the ??rst condition is false, 
then the next condition is checked, and so on. If one of 
the conditions is true, then the corresponding indented 
block executes, and the if statement terminates.
Let us write a program to create a simple calculator 
to perform basic arithmetic operations on two numbers. 
The program should do the following:
• Accept two numbers from the user.
• Ask user to input any of the operator (+, -, *, /). 
An error message is displayed if the user enters 
anything else.
• Display only positive difference in case of the 
operator "-".
• Display a message “Please enter a value other 
than 0” if the user enters the second number as 
0 and operator ‘/’ is entered.
Program 6-3 Write a program to create a simple 
calculator  performing only four basic 
operations.
#Program to create a four function calculator
result = 0
val1 = ??oat(input("Enter value 1: "))
val2 = ??oat(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)
Ch 6.indd   125 08-Apr-19   12:37:52 PM
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