Iterations FOR LOOP AND WHILE LOOP .pptx

Problem Solving and Programming in
Python
Iterative / Looping
Statements
Vijaya Lakshmi A
Assistant
Professor
Department of CSE

Session Objectives
2 v 1.2
• Learning iterative statements in
python

Session Outcomes
3 v 1.2
• At the end of this session, participants will be
able to
– Apply looping statements in Python for solving
problems

Agenda
4 v 1.2
• Iterative / repeated execution
– While loop
– For loop
• Range
• Nested Loops
• Break
• Continue
• Pass

Control Flow
Control flow — determines order in which statements are executed
Three fundamental methods of control flow are:
Sequential Statement
Conditional Statement
Iterative Statement or Repeated Execution
5 v 1.2

Iterative Statement / Repeated
Execution
6 v 1.2
• To automate the repetitive tasks, iterative statements are used.
• Iterative statements are decision control statements that are
used to repeat the execution of list of statements.
• Two types of iterative statements.
• While loop
• Used when the number of iterations are
unknown.
• Keep on iterating until the condition is false
• For loop
• Used when the no n
ut r
ml F
bl o w
er of iteratD
ioe c
ne m
sb
are known.
• Iterate a predefined number of times

While Loop
7 v 1.2
• The while loop is a loop control statement in
Python and frequently used in programming for
repeated execution of statement(s) in a loop.
• It executes a sequence of statements repeatedly
as
long as a condition remains true.
• The syntax for while loop is given as follows:
w h il
e
t es t −c on d i t i on :
# l o o p body
s t a t em en t b l
oc k

while statement
8 v 1.2
• while statement has header and body
• while followed by conditional (boolean) expression with
colon(:) in the header
• Returns a value True or False
• The body should be intended.
• The loop body will be executed when the condition is
True
• When the condition is false, the loop body will be
skipped
and the first statement after the while loop will be
executed.
• The condition of the while loop has to be updated after
each iteration, if not the condition never becomes false
and enters into infinite loop.

While loop
• Inside the loop some condition
decides when to terminate the
loop, called as Loop control
variable.
• At each iteration it can be
changed by a constant value.
• The value of loop control
variable determines the end of
the loop.
i , sum , av g = 0 , 0 , 0 . 0
w h il
e
( i < 10 ) :
sum= sum+ i
avg= sum / 10
p r i n t ( sum , av
g )
2022
9 v 1.2
9 / 38

While loop - Example1
10 v 1.2
To print numbers from 1 to 5 using while loop
count
w h il
e
=0
count < = 5:
p r i n t ( ” count= ” ,
count ) co u nt = co u nt + 1
OUT PUT :
c o u nt = 0
c o u nt = 1
c o u nt =
2 c o u nt =
3 c o u nt =
4 c o u nt =
5

11 v 1.2
c oun
ter
zero
To find the sum of 10 numbers
count= 0 # I n i t i a l i z e the
sum= 0 # i n i t i a l i z e sum to
w h i l e count < = 1 0 : #t e s t c o n d i t i o
n
i f t r u
e
count= count+ 1 # i n c r eas e the
sum= sum + count # add sum + c oun t
v alu e
o f
c oun t
by 1
p r i n t ( ”Sum of F i r s t 10 Numbers =
” ,sum )

the number"
)) t h e
number " ) )
n u m = i n t ( i npu t ( "E nt
er end = i n t ( i npu t ( "E nt
er
count odd= 0
ev en= c oun t ev en+
1
odd=count odd+ 1
count even= 0
w h il e ( num < = e n d ) :
p r i n t ( " w h il e
")
i f ( num% 2 = = 0):
c ou n t
e l s e :
count
num=num+1
p r i n t (num)
p r i n t ( ” ev
en
p r i n t ( ” ev
en
12 v 1.2
numbe rC
so n
”t
, cF l o
ow
unt
eve nD
numbers” , count odd )
)
c

While loop - GCD
13 v 1.2
Euclidean algorithm or Euclid’s algorithm
• G C D of two numbers is the greatest common divisor of
two
positive integers.
• A much more efficient method is the Euclidean algorithm.
I n p u t : T
wo
p o s i t i v e i n t eg er s , a and
b .
The g r ea t es t common d i v i s o r , g ,
b .
Output :
of a
and
I n t er n a
l
computation :
If b!=0
a, b = b, a%b
Repeat the above steps

While loop - GCD
14 v 1.2
• To compute gcd(48,18)
• Divide 48 by 18 to get a remainder of 12.
• Then divide 18 by 12 to get a remainder of 6.
• Then divide 12 by 6 to get a remainder of 0, which means
that 6
is the gcd.
• Note that we ignored the quotient in each step except to
notice when the remainder reached 0, signalling that we
had arrived at the answer. Formally the algorithm can be
described as:

While loop - GCD
15 v 1.2
Greatest Common Divisor by Using the Euclidian Algorithm
m= i n t ( i npu t ( ” g i v e v a l u e f o r
a ” ) )
n= i n t ( i npu t ( ” g i v e v a l u e f o r
b” ) )
w h i l
e
n != 0 :
m, n =
n ,m%n
g c d va l = m
p r i n t ( ” T he GCD VALUE i s : ” , g c d va l )

For Loop
• The Python for loop iterates through a sequence of objects, i.e.
it iterates through each value in a sequence, where the sequence
of object holds multiple items of data stored one after another.
• The for loop is used to repeat any computations a fixed number
of times.
• The for.... in statement is used to
iterate over an ordered collection (sequence) of items
Syntax for loop :
fo r l o o
p
16 v 1.2
c o n t r o l v a r
in
tsr tl aF l otwe ments
( Dse )e
sequence :

For Loop
• When a for loop is used, a range of sequence is specified
• For loop is executed for each item in the sequence
• With completion of every iteration the loop control var gets
updated with the next item in the sequence
• After executing for all items in the sequence the flow of
control
jumps to the immediate statement followed by the for loop
17 v 1.2

For Loop
y , z = 1 , 1
fo r i i n [ 1 , 2 , 3 , 4 ]
:
= y
∗i
= z+ 1
y
z
p r i n t ( y ,
z )
O utput
24 5
mb
18 v 1.2

Example
19 v 1.2
f r u i t s =
f o r
f r u i t
p r i n t
[ ’ banana ’ , ’ apple ’ , ’ mango ’ ]
in f r u i t s :
( ’ C u r r e nt f r u i t : ’ , f r u i t )
Output
Current f r u i t : banana
Current f r u i t : apple
Current f r u i t : mango

20
Range
• range() is a in-built function used to iterate over a sequence
of numbers
• The general form of the range function is:
range( begin, end, step)
The ‘begin’ is the first beginning number in the sequence at which
the list starts.
The ‘end’ is the limit, i.e. the last number in the sequence.
The ‘step’ is the difference between each number in the sequence.
range( 0 , n) generates sequence {0,1,...,n-1}
• By default, every number in the range is incremented by 1 but
we can specify a different increment using step
f o r i in range ( 1 , 5 ) :
p r in t ( i , end=” ”)
1 2 3 4
f o r i in range ( 1 , 2 0 , 2 ) :
p r in t ( i , end=” ”)
1 3 5 7 9 11 1v31.2 15
17 19

For Example-1
21 v 1.2
• To print the square of the
numbers:
f o r i i
n
r a ng e ( 1 , 6 )
:
sq u a r e= i ∗i
p r in t ( " Square of “ , i ,“ i s : " , square )
• To print the even numbers and print the
sum
sum=0
p r i n t ( ” E ven numbers from 0 t o
10 a r e a s f o r i in range
( 0 , 1 1 , 1 ) :
f o l l o w s
” )
i f i % 2= = 0:
p r i n t ( i )
sum= sum+ i
p r i n t ( ” Sum of E ven numbers from 0 t o 10 i s = ” ,
sum )

Nested
Looping
22 v 1.2
• Loops within the loops or when one loop is inserted completely
within another loop, then it is called nested loop.
num= i n t ( i npu t ( ” en t er t h e number ” ) )
f o r i in range ( 6 ) :
p r i n t ( ” i : ” , i )
r a ng e ( i + 1 ) :
( ” j : ” , j , end = ’
’ )
f o r
j
i n p r i
n t
p r i n t ( )
Tr a c i n g FOR
LOOP: en t er t h e
number 4
i : 0 i : 3
j : 0 j : 0 j : 1 j : 2 j : 3
i : 1 i : 4
j : 0 j : 1 j : 0 j :
ec
1 j :
2022
2 j :
38
3 j : 4
i : 2 i : 5
j : 0 j : 1 j : 2 j : 0 j : 1 j : 2 j : 3 j : 4 j : 5

Nested Looping
23 v 1.2
f o
r
i i n r a ng e ( 5 , −1 ,
−1) :
f o r j i n r
a ng e ( 0 , i + 1 ) : p r i n t
( i , end = ’ ’ )
p r i n t ( )
Tra c i ng FOR LOOP:
loop =1: i =5 , j =(0 , 6 ) p r in t 5 5 5 5 5 5
loop =2: i =4 , j =(0 , 5 ) p r in t 4 4 4 4 4
loop =3: i =3 , j =(0 , 4 ) p r in t 3 3 3 3
loop =4: i =2 , j =(0 , 3 ) p r in t 2 2 2
loop =5: i =1 , j =(0 , 2 ) p r in t 1 1
loop =6: i =0 , j =(0 C, 1n t )r p r in t 0

Example - Pattern
24 v 1.2
f o r i i n r a ng e ( 5 )
:
p r i n t ( )
f o r j i
n
r a ng e ( 5 ) :
p r i n t ( ’ ∗’ , end = ’
’ )
Output :
∗
∗
∗
∗
∗
∗
∗
∗
∗
∗
∗
∗
∗
∗
∗
∗
∗
∗
∗
∗
∗
∗
∗
∗
∗

Example for list of values in For loop
25 v 1.2
For loop generate sequence explicitly using list of values or implicitly
using the range
# To f in d f a c t o r s of a number
num=int ( input ("e nte r the number : " ) )
print (" the f a c t o r s are ")
for i in range ( 1 ,num+1 ):
if (num%i ==0):
print ( i )
Output :
1
3
5
15

Break
The break statement is used to terminate the execution of the
nearest enclosing loop in which it appears.
Used with for loop and while loop.
When compiler encounters a break statement the control passes to
the statement that follows the loop in which the break statement
appears.
26 v 1.2

Break
n= 0
w h i l
e
n < = 1 0 :
n=n+ 1
i f n = = 5 :
b r eak
p r i n t ( n )
O utput
1
2
3
4 b er 12, 2022
27 v 1.2
27 / 38

Break - Prime Number
28 v 1.2
num= 23
f o r
i
i n r a ng e ( 2 , num )
: i f num% i = = 0:
p r i n t ( ” c om p osi t e ”
)
break
e l s e :
p r i n t ( ” pr i m e ” )

Continue
The continue statement is used to skip the rest of the code inside a
loop for the current iteration only.
Loop does not terminate but continues on with the next iteration.
29 v 1.2

Continue
for val in ” s t r i n g ” :
i f val == ” i
” : c o nt i n u e
p r i n t ( val ,
end=’ ’ ’ ’ )
Output
s t r n g
fo r ii n r ang e ( 1 , 1 1 ) :
i f ( i = = 5 ) :
c on t i nu e
p r i n t ( i , end= ” ” )
Output
1 2 3 4 6 7 8 9 10
er 12, 2022
30 v 1.2
30 / 38

Pass statement
Pass is a null statement.
Nothing happens when pass is executed.
Acts as a place holder
fo r l e t t e r i n ” h el l o ” :
pass
i f ( l e t t e r= = ’ o
’ ) :
break
p r i n t ( l et t er , end= ” ” )
Output :
h e
l
l
31 v 1.2

Summar
y
32 v 1.2
• Iterative / repeated execution
– While loop
– For loop
• Range
• Nested Loops
• Break
• Continue
• Pass

Check your
understanding
33 v 1.2
Use while loops to solve the following
• Finding the odd numbers in a given range
• Exponent of a number.
• Reversing the given number
• Counting Number of digits in a given number
• Write a program to find sum of the digits of the given number
• Write a program to print the sum of the numbers from 1 to 20 (1
and 20 are included) that are divisible by 5 using the while loop.
• Check whether a number is palindrome or not.
• Convert a number to binary.
• Check a number is Armstrong (sum of cubes of
individual digits of a number is same as the
number)

Check your
understanding
34 v 1.2
Use for loops to solve the following
• To print the Fibonacci series up to 8.
• What is the output of the code?
for x in ”banana”:
print(x)
• Reversing of a number.

Check your understanding- For loop
1 Sum of the following series:
1 1 − 1/ 2 + 1/ 3 − 1/ 4 + ...1/n
2 1 + 1/22 + 1/32 + 1/42 + ...1/n2
1 + 22 + 32 + 42 + ...n2
Print the following patterns:
i ) i i ) i i i ) iv )
∗ ∗∗∗∗∗ 1 1
∗
∗ ∗
∗
∗
∗ 12 121
∗
∗
∗ ∗
∗
∗ 123 12321
∗
∗
∗
∗ ∗
∗ 1234 1234321
∗∗∗∗∗ ∗ 12345 315 /23
83454321
35 v 1.2

Iterations FOR LOOP AND WHILE LOOP .pptx

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Iterations FOR LOOP AND WHILE LOOP .pptx