LOOPS TOPIC FOR CLASS XI PYTHON SYLLABUS

What is Loop?
 As we have already studied there are many
situation where we need to repeat same set
of tasks again and again. Like while writing
table of any number we multiply same
number from 1 to 10, multiplying number
from n to 1 to find the factorial, In real life,
to prepare parathas, suppose to prepare
your bag with all items, daily coming to
school again and again, etc. Please explore
more real life conditions where repetition of
task was done by you.

Pseudocode Reference
A. start
B. Input number(n) to print table
C. Let count=1
D. Display n * I
E. Add 1 to count
F. if count==10:
A. Stop
G. else:
A. Repeat from Step C
H. Stop

Python Loop Statements
 To carry out repetition of statements
Python provide 2 loop statements
◦ Conditional loop (while)
◦ Counting loop (for)

range() function
 Before we proceed to for loop let us
understand range() function which we
will use in for loop to repeat the
statement to n number of times.
 Syntax:
◦ range(lower_limit, upper_limit)
 The range function generate set of values
from lower_limit to upper_limit-1

range() function
 For e.g.
 range(1,10) will generate set of values
from 1-9
 range(0,7) will generate [0-6]
 Default step value will be +1 i.e.
 range(1,10) means (1,2,3,4,5,6,7,8,9)

range() function
 To change the step value we can use third
parameter in range() which is step value
 For e.g.
 range(1,10,2) now this will generate
value [1,3,5,7,9]
 Step value can be in –ve also to generate
set of numbers in reverse order.
 range(10,0) will generate number as
[10,9,8,7,6,5,4,3,2,1]

range() function
 To create list from 0 we can use
 range(10) it will generate
[0,1,2,3,4,5,6,7,8,9]

Operators in and not in
 The operator in and not in is used in for
loop to check whether the value is in the
range / list or not
 For e.g.
>>> 5 in [1,2,3,4,5]
True
>>> 5 in [1,2,3,4]
False

>>>‟a‟ in „apple‟
True
>>>‟national‟ in „international‟
True

Program to check whether any
word is a part of sentence or not
line = input("Enter any statement")
word = input("Enter any word")
if word in line:
print("Yes ", word, "is a part of ",line)
else:
print("No", word ," is not part of ",line)

for loop
 for loop in python is used to create a
loop to process items of any sequence
like List,Tuple, Dictionary, String
 It can also be used to create loop of fixed
number of steps like 5 times, 10 times, n
times etc using range() function.

Example – for loop with List
School=["Principal","PGT","TGT","PRT"]
for sm in School:
print(sm)
Example – for loop with Tuple
Code=(10,20,30,40,50,60)
for cd in Code:
print(cd)

Let us understand how for loop works
Code=(10,20,30,40,50,60)
for cd in Code:
print(cd)

First it will create Tuple „Code‟
Code=(10,20,30,40,50,60)
for cd in Code:
print(cd)

Then for loop starts, it will pick values one
by one from Code and assign it to cd
Code=(10,20,30,40,50,60)
for cd in Code:
print(cd)

Code=(10,20,30,40,50,60)
for cd in Code:
print(cd)
cd = 10

Code=(10,20,30,40,50,60)
for cd in Code:
print(cd)
cd = 10
OUTPUT
10

Code=(10,20,30,40,50,60)
for cd in Code:
print(cd)
cd = 20
OUTPUT
10

Code=(10,20,30,40,50,60)
for cd in Code:
print(cd)
cd = 20
OUTPUT
10
20
AND SO ON…
FOR LOOP WILL
AUTOMATICALLY ENDS
WHEN LAST ITEM OF
LIST IS PROCESSED

for loop with string
for ch in „Plan‟:
print(ch)
The above loop product output
P
l
a
n

for with range()
Let us create a loop to print all the natural number from 1
to 100
for i in range(1,101):
print(i,end='t')
** here end=„t‟ will cause output to appear without
changing line and give one tab space between next output.

Program to print table of any number
num = int(input("Enter any number "))
print(num,'x',i,'=',num*i)
Program to find the sum of all number divisible by 7 between 1 to 100
sum=0
if i % 7 == 0:
sum+=i
print("total of number divisible by 7 between 1 to 100 is ",sum)

Lab work
1. WAP to enter any number and find its
factorial
2. WAP to print the following fibonacci series
0, 1, 1, 2, 3, 5, 8,……..n terms
3. WAP to enter 10 number and find the sum
and average.
4. WAP to enter Lower_Limit, Upper_Limit
and find the sum of all odds and evens
number between the range separately

while loop
 While loop in python is conditional loop
which repeat the instruction as long as
condition remains true.
 It is entry-controlled loop i.e. it first
check the condition and if it is true then
allows to enter in loop.
 while loop contains various loop
elements: initialization, test condition,
body of loop and update statement

while loop elements
1. Initialization : it is used to give starting
value in a loop variable from where to
start the loop
2. Test condition : it is the condition or last
value up to which loop will be executed.
3. Body of loop : it specifies the
action/statement to repeat in the loop
4. Update statement : it is the increase or
decrease in loop variable to reach the test
condition.

Example of simple while loop
i=1
while i<=10:
print(i)
i+=1

Example of simple while loop
i=1
while i<=10:
print(i)
i+=1
initialization
Test condition
Body of loop
U
p
d
a
t
e

Let us see the flow of loop
i=1
while i<=10:
print(i)
i+=1

i=1
while i<=10:
print(i)
i+=1
i = 1
1<=10?
True

i=1
while i<=10:
print(i)
i+=1
i = 1
1<=10?
True
OUTPUT
----------
1

i=1
while i<=10:
print(i)
i+=1
i=2
OUTPUT
----------
1

i=1
while i<=10:
print(i)
i+=1
i = 2
2<=10?
True
OUTPUT
----------
1

i=1
while i<=10:
print(i)
i+=1
i = 2
2<=10?
True
OUTPUT
----------
1
2

i=1
while i<=10:
print(i)
i+=1
i=3
OUTPUT
----------
1
2

i=1
while i<=10:
print(i)
i+=1
i = 3
3<=10?
True
OUTPUT
----------
1
2

i=1
while i<=10:
print(i)
i+=1
i = 3
3<=10?
True
OUTPUT
----------
1
2
3

i=1
while i<=10:
print(i)
i+=1
i=4
OUTPUT
----------
1
2
3
In this way loop will execute for
the values 4 to 10, let us see from
the value 9

i=1
while i<=10:
print(i)
i+=1
i=9
OUTPUT
----------
1
2
3
4
5
6
7
8
In this way loop will execute for
the values 4 to 10, let us see from
the value 9

i=1
while i<=10:
print(i)
i+=1
i = 9
9<=10?
True
OUTPUT
----------
1
2
3
4
5
6
7
8

i=1
while i<=10:
print(i)
i+=1
i = 9
9<=10?
True
OUTPUT
----------
1
2
3
4
5
6
7
8
9

i=1
while i<=10:
print(i)
i+=1
i=10
OUTPUT
----------
1
2
3
4
5
6
7
8
9

i=1
while i<=10:
print(i)
i+=1
i= 10
10<=10?
True
OUTPUT
----------
1
2
3
4
5
6
7
8
9

i=1
while i<=10:
print(i)
i+=1
i = 10
10<=10?
True
OUTPUT
----------
1
2
3
4
5
6
7
8
9
10

i=1
while i<=10:
print(i)
i+=1
i=11
OUTPUT
----------
1
2
3
4
5
6
7
8
9
10

i=1
while i<=10:
print(i)
i+=1
i= 11
11<=10?
False
OUTPUT
----------
1
2
3
4
5
6
7
8
9
10
Now the value of i
is 11 and condition
will return False so
loop will terminate

Programs of while loop
 Convert all „for‟ loop program with while loop
 WAP to enter any number and find its reverse
 WAP to enter any number and a digit and count
how many times digit is in the number
 WAP to enter any number and check it is
armstrong or not
 WAP to enter any number and check it is
palindrome or not
 WAP to enter numbers as long as user wishes to
enter and find the sum highest and lowest
number entered by user

WAP to enter any number and find its reverse
num = int(input("Enter any number "))
rev = 0
num2=num
while num>0:
rem = num % 10
rev = rev * 10 + rem
num //=10
print("Reverse of ",num2," is ",rev)

WAP to enter numbers as long as user wishes to enter and
find the sum highest and lowest number entered by user
choice="y"
i=1
while choice=="y":
num = int(input("Enter any number (>0)"))
if i==1:
low = num
high = num
i+=1
else:
if (high<num):
high=num
if (num<low):
low = num
choice=input("Enter continue or not (y/n)")
print("Highest value was ",high)
print("Lowest value was ",low)

Jump Statements – break & continue
break statement in python is used to
terminate the containing loop for any
given condition. Program resumes from the
statement immediately after the loop
Continue statement in python is used to
skip the statements below continue
statement inside loop and forces the
loop to continue with next value.

Example – break
if i % 6 == 0:
break
print(i)
print(“Loop Over”)
The above code produces output
1
2
3
4
5
Loop Over
when the value of i reaches to 6 condition will becomes True
and loop will end and message “Loop Over” will be printed

Example – continue
if i % 6 == 0:
continue
print(i, end = „ „)
print(“Loop Over”)
The above code produces output
1 2 3 4 5 7 8 9 10 11 13 14 15 16 17 19
Loop Over
when the value of i becomes divisible from 6,
condition will becomes True and loop will skip all the
statement below continue and continues in loop with
next value of iteration.

Loop .. else .. Statement
 Loop in python provides else clause with
loop also which will execute when the
loop terminates normally i.e. when the
test condition fails in while loop or when
last value is executed in for loop but not
when break terminates the loop

Syntax of Python loop along with
„else‟ clause
for with “else” while with “else”
for <var> in <seq>:
statement 1
statement 2
else:
statement(s)
while <test condition>:
statement 1
statement 2
else:
statement(s)

Example (“else” with while)
i=1
while i<=10:
print(i)
i+=1
else:
print("Loop Over")
Output
1
2
3
4
5
6
7
8
9
10
Loop Over

Example (“else” with for)
names=["allahabad","lucknow","varanasi","kanpur","agra","ghaziabad"
,"mathura","meerut"]
city = input("Enter city to search: ")
for c in names:
if c == city:
print(“City Found")
break
else:
print("Not found")
Output
Enter city to search : varanasi
City Found
Enter city to search : unnao
Not found

Example- program to enter numbers repeatedly and print
their sum. The program ends when the user says no more
to enter(normal termination) or program aborts when the
number entered is less than zero
count = sum = 0
ans = 'y'
while ans=='y':
num = int(input("Enter number :"))
if num < 0:
print("You entered number below zero,
Aborting...")
break
sum += num
count+=1
ans=input("Continue(y/n)")
else:
print("You entered ", count, " numbers so far")
print("Sum of entered number is :",sum)

Example- program to enter numbers repeatedly and print
their sum. The program ends when the user says no more
to enter(normal termination) or program aborts when the
number entered is less than zero - OUTPUT
Enter number :3
Continue(y/n)y
Enter number :6
Continue(y/n)y
Enter number :8
Continue(y/n)y
Enter number :-10
You entered number below
zero,Aborting...
Sum of entered number is : 17
Enter number :10
Continue(y/n)y
Enter number :20
Continue(y/n)y
Enter number :50
Continue(y/n)n
You entered 3 numbers so far
Sum of entered number is : 80

Example- program to input number and tests if it is a
prime number or not
num = int(input("Enter any number :"))
lim = num//2 + 1
for i in range(2,lim):
rem = num % i
if rem == 0:
print(num," is not a prime
number ")
break
else:
print(num," is a prime number ")

Enter any number :23
23 is a prime number
Enter any number :36
23 is not a prime number

Generating Random numbers
 Python allows us to generate random
number between any range
 To generate random number we need to
import random library
 Syntax: (to generate random number)
◦ random.randint(x,y)
◦ it will generate random number between x to
y
◦ For e.g. : random.randint(1,10)
◦ Refer to code page no. 156

Infinite loop and „break‟
 We can also execute infinite loop purposely
by specifying an expression which always
remainsTrue.
 However in loop we can mention any
condition to exit or terminate loop
a=2
while True:
print(a)
a*=2
if a>100:
break

Nested Loop (loop within loop)
 A loop may contain another loop in its
body. This form of loop is called nested
loop.
 In Nested loop, the inner loop will
execute for each value of outer loop.
 For e.g. if the outer loop is of 4 times and
inner loop is of 5 times then statement
will execute 4 x 5 = 20 times

Syntax of Nested Loop
Nested Loop – FOR Nested Loop – WHILE
for i in sequence:
for j in sequence:
Statement1
Statement2
while condition:
while condition:
Statement1
Statement2

Example – to print table of 2 to 10
4
to
8

Example – Program to print the following patterns

Write a program to print the following
pyramid pattern

LOOPS TOPIC FOR CLASS XI PYTHON SYLLABUS

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