Python: Looping

Looping

• Looping is a technique that allows to execute a block of statements repeatedly.

• Definition : Repeated execution of a set of statements is called loop or iteration.

• The programming constructs used for iteration are while, for, break, continue and so on.

• Let us discuss the iteration techniques with the help of illustrative examples.

1. while Loop 

2. for Loop 

3. The break 

4. The continue 

5. The pass

 

1. while Loop

The while statement is popularly used for representing iteration.

Syntax

while test_condition:

body of while

Flowchart for while statement is as given below

The flow of execution is specified as follows ‒

1. Using the condition determine if the given expression is true or false.

2. If the expression is false then exit the while statement.

3. If the expression is true then execute the body of the while and go back to step 1 in which again the condition is checked.

For example

while i<= 10:

    i=i+1

The body of a while contains the statement which will change the value of the variable used in the test condition. Hence finally after performing a definite number of iterations, the test condition gets false and the control exits the while loop.

If the condition never gets false, then the while body executes for infinite times. Then in this case, such while loop is called infinite loop.

There is another version of while statement in which else is used.

Syntax

while test_condition:

        body of while

else:

        statement

Example

while i<=10:

          i=i+1

else:

           print("Invalid value of i")

Example:1

Write a python program for computing the sum of n natural numbers and finding out the average of it.

Solution:

The python script is as given

sum_avg.py

Output

Click on New‒>Python 3(ipykernel) inorder to get the prompt window

Issue the import command as follows ‒

In [1]: import sum_avg

Enter the value of n:

5

sum is: 55

average is: 11.0

Example:2

Write a python program to display square of n numbers using while loop.

Solution:                                         

print("Enter the value of n")

n int(input())

i=1

print("The sqaure table is as given below...")

while i<=n:

      print(i,i+1)

       i=i+1

Output

In [1]: import test

Enter the value of n

7

The sqaure table is as given below...ordered

1 1

2 4

3 9

4 16

5 25

6 36

7 49

Example:3

Write a python program for displaying even or odd numbers between 1 to n.

Solution :

print("Enter the value of n")

n = int(input())

i=1

j=i

while j<=n:

    if i%2== 0:

          print(i," is even")

    else:

         print(i," is odd")

j=j+1

i=j

Output

In [1]: import test

Enter the value of n

10

1 is odd

2 is even

3 is odd

4 is even

5 is odd

6 is even

7 is odd

8 is even

9 is odd

10 is even

Example:4

Write a python program to display fibonacci numbers for the sequence of length.

Solution :

print("Enter the value of n")

n=int(input())

a=0

b=1

i=0

print("Fibonacci Sequence is...")

while i<n;

   print(a)

    c=a+b

    a=b

    b=c

    i=i+1

Output

Enter the value of n

10

Fibonacci Sequence is...

0

1

1

2

3

5

8

13

21

34

2. for Loop

The for loop is another popular way of using iteration. The syntax of for loop is ‒

Syntax

for variable in sequence:

Body of for loop

The variable takes the value of the item inside the sequence on each iteration.

Loop continues until we reach the last item in the sequence. The body of for loop is separated from the rest of the code using indentation.

Example

for val in numbers:

val=val+1

Similarly, we can have for loop with

else statement

Syntax

for variable in sequence:

      Body of for loop

else:

       Statement

Example

for i in myList:

       print(i)

else:

       print("The number is not present in your list")

Example:5

Write a python program to find the sum of 1 to 10 numbers.

Solution :

print("Enter the value of n")

n = int(input())

sum = 0

for i in range(1,n+1):

      sum =  sum+i

print("The sum of ",n,"numbers is ",sum)

Output

Enter the value of n

5

The sum of 5 numbers is

15

Example:6

Write a python program to display the multiplication table.

Solution:

print("Enter number for its multiplication table")

n=int(input())

for i in range(1,11):

         print(n,"X",i,i*n)

Output

Enter number for its multiplication table

10

10 X 1 10

10 X 2 20

10 X 3 30

10 X 4 40

10 X 5 50

10 X 6 60

10 X 7 70

10 X 8 80

10 X 9 90

10 X 10 100

Example:7

Write a program to print 1+1/2+1/3+1/4+......... +1/N series.

Solution :

print("Enter value of N")

n=int(input())

sum=0

for i in range(1,n+1):

      sum=sum+1/i

print("The Sum is ",sum)

Output

Enter value of N

5

The Sum is 2.283333333333333

Example:8

Write a python program to check whether a given number is prime or not.

Solution :

# User can enter the value thru keyboard

print("Enter the number");

num = int(input())

# prime numbers are greater than 1

if num > 1:

# check for factors

for i in range(2,num):

     if (num % i) = = 0:

          print(num,"is not a prime number")

          break

    else:

          print(num,"is a prime number")

# if input number is less than

# or equal to 1, it is not prime

else:

     print(num,"is not a prime number")

Output

Enter the number

3

3 is a prime number

Example:9

Write a python program to find the prime numbers between given intervals.

Solution :

lower = int(input("Enter lower range: "))

upper = int(input("Enter upper range: "))

print("Prime numbers between", lower, "and",upper, "are:")

for num in range(lower,upper + 1):

prime numbers are greater than 1

if num > 1:

for i in range(2,num):

if (num % i) = = 0:

break

else:

print(num)

Output

Enter lower range: 1

Enter upper range: 10

Prime numbers between 1 and 10 are:

2

3

5

7

Example:10

Write a python program to display the star pattern as

*

**

***

****

*****

…

…

Users should enter the value of N

Solution :

print("Enter value of n")

n=int(input())

for i in range(0,n):

for j in range(0,i+1):

print('* ',end=""")

print("")

Output

Enter value of n

10

*

* *

* * *

* * * *

* * * * *

* * * * * *

* * * * * * *

* * * * * * * *

* * * * * * * * *

* * * * * * * * * *

Example:11

Write a python program to display the number pattern as follows

1 2 3 4 5

2 3 4 5

3 4 5

4 5

5

Solution:

for i in range (1,6):

for k in range(1,i):

print(end="")

for j in range(i,6):

print(" ",j,end="")

print();

Output

1 2 3 4 5

2 3 4 5

3 4 5

4 5

5

Example:12

Write a python program to print the pattern as given below

A

A B

A B C

A B C D

A B C D E

Solution :

for i in range(1, 6):

     for j in range(65, 65+i):

         a = chr(j)

          print(a," ",end=""")

   print()

Output

A

A B

A B C

A B C D

A B C D E

Example:13

Write a python program to print the pattern for alphabets

A A A A A

B B B B

C C C

D D

E

Solution :

num=65 #ASCII Value for A

for i in range(0,5):

for j in range(i,5):

ch=chr(num+i) # Converting ASCII to character

print(ch," ",end=""")

print();

Output

A A A A A

B B B B

C C C

D D

E

 

3. The break

The break statement is used to transfer the control to the end of the loop.

When break statement is applied then loop gets terminates and the control goes to the next line pointing after loop body.

Syntax

break

Flowchart

For example

for i in range(1,11):

     if i==5:

     print("Element {} Found!!!".format(i))

     break

print(i)

Output

1

2

3

4

Element 5 Found!!!

4. The continue

The continue statement is used to skip some statements inside the loop. The continue statement is used with decision making statements such as if...else.

The continue statement forces to execute the next iteration of the loop to execute.

Flowchart

Syntax

     continue

Example

In while loop the continue passes the program control to conditional test.Following example illustrates the idea of Condition

i=0

     while i<10:

          i=i+1

     if i%2==0:

          continue

print(i)

Output

1

3

5

7

9

5. The pass

The pass statement is used when we do not want to execute any statement. Thus the desired statements can be bypassed.

Syntax

Pass

Example

for i in range(1,5):

     if i==3:

          pass

          print("Reached at pass statement")

print("The current number is ",i)

Output

The current number is 1

The current number is 2

Reached at pass statement

The current number is 3

The current number is 4

 Example:14

Write a python program to print sum of cubes of the values of n variables.

Solution :

sum = 0

n = int(input("Enter some number: "))

for i in range(1, n+1):

     sum = sum+(i*i*i)

print("Sum of cubes of first",n,"numbers is = ",sum)

Output

Enter some number: 5

Sum of cubes of first 5 numbers is = 225

>>> 

Example:15

Find the syntax error in the code given: while True print('Hello world').

Solution:

There must be colon after true and the print statement must be indented. The correct code is as follows ‒

while True:

     print('Hello world')

 

Review Question

1. Mention the different types of iterative structure allowed in python. Explain the use of continue and break statements with an example.