Flow Control

Flow control describes the order in which statements will be executed at runtime.

 

I. Conditional Statements

1) if

if condition : statement

or

if condition :

statement-1

statement-2

statement-3

If condition is true then statements will be executed.

Eg:

1) name=input("Enter Name:")

2) if name=="Sohan" :

3) print("Hello Sohan Good Morning")

4) print("How are you!!!")

 

2) if-else:

if condition :

Action-1

else :

Action-2

if condition is true then Action-1 will be executed otherwise Action-2 will be executed.

Eg:

1) name=input("Enter Name:")

2) if name=="Sohan" :

3) print("Hello Sohan Good Morning")

4) else:

5) print("Hello Guest Good Moring")

6) print("How are you!!!")

7) 

 

3) if-elif-else:

Syntax:

if condition1:

Action-1

elif condition2:

Action-2

elif condition3:

Action-3

elif condition4:

Action-4

...

else:

Default Action

Based condition the corresponding action will be executed.

Eg:

1) brand=input("Enter Your Favourite Brand:")

2) if brand=="RC" :

3) print("It is childrens brand")

4) elif brand=="KF":

5) print("It is not that much kick")

6) elif brand=="FO":

7) print("Buy one get Free One")

8) else :

9) print("Other Brands are not recommended")

10) 

 

Note:

1. else part is always optional

Hence the following are various possible syntaxes.

1. if

2. if - else

3. if-elif-else

4.if-elif

2. There is no switch statement in Python

Q. Write a program to find biggest of given 2 numbers from the commad prompt?

1) n1=int(input("Enter First Number:"))

2) n2=int(input("Enter Second Number:"))

3) if n1>n2:

4) print("Biggest Number is:",n1)

5) else :

6) print("Biggest Number is:",n2)

7) 

 

 

Q. Write a program to find biggest of given 3 numbers from the commad prompt?

1) n1=int(input("Enter First Number:"))

2) n2=int(input("Enter Second Number:"))

3) n3=int(input("Enter Third Number:"))

4) if n1>n2 and n1>n3:

5) print("Biggest Number is:",n1)

6) elif n2>n3:

7) print("Biggest Number is:",n2)

8) else :

9) print("Biggest Number is:",n3)

10) 

 

II. Iterative Statements

If we want to execute a group of statements multiple times then we should go for

Iterative statements.

Python supports 2 types of iterative statements.

1. for loop

2. while loop

1) for loop:

If we want to execute some action for every element present in some sequence(it may be

string or collection)then we should go for for loop.

Syntax:

for x in sequence :

body

where sequence can be string or any collection.

Body will be executed for every element present in the sequence.

Eg 1: To print characters present in the given string

1) s="Sunny Leone" 

2) for x in s :

3) print(x)

4) 

 

Eg 2: To print characters present in string index wise:

1) s=input("Enter some String: ")

2) i=0

3) for x in s :

4) print("The character present at ",i,"index is :",x)

5) i=i+1

6) 

 

Eg 3: To print Hello 10 times

1) for x in range(10) :

2) print("Hello")

Eg 4: To display numbers from 0 to 10

1) for x in range(11) :

2)

print(x) 

Eg 5: To display odd numbers from 0 to 20

1) for x in range(21) :

2) if (x%2!=0):

3) print(x)

Eg 6: To display numbers from 10 to 1 in descending order

1) for x in range(10,0,-1) :

2) print(x)

 

2) while loop:

If we want to execute a group of statements iteratively until some condition false,then we

should go for while loop.

Syntax:

while condition :

body

Eg: To print numbers from 1 to 10 by using while loop

1) x=1

2) while x <=10:

3) print(x)

4) x=x+1

Eg: To display the sum of first n numbers

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

2) sum=0

3) i=1

4) while i<=n:

5) sum=sum+i

6) i=i+1

7)

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

 

Infinite Loops:

1) i=0;

2) while True :

3) i=i+1;

4) print("Hello",i)

Nested Loops:

Sometimes we can take a loop inside another loop,which are also known as nested loops.

Eg:

1) for i in range(4):

2) for j in range(4):

3) print("i=",i," j=",j)

4) 

 

III. Transfer Statements

1) break:

We can use break statement inside loops to break loop execution based on some

condition.

Eg:

1) for i in range(10):

2) if i==7:

3) print("processing is enough..plz break")

4) break 

5) print(i)

6) 

 

Eg:

1) cart=[10,20,600,60,70]

2) for item in cart:

3) if item>500:

4) print("To place this order insurence must be required")

5) break 

6) print(item)

7) 

 

2) continue:

We can use continue statement to skip current iteration and continue next iteration.

Eg 1: To print odd numbers in the range 0 to 9

1) for i in range(10):

2) if i%2==0:

3) continue 

4) print(i)

5) 

 

 

Eg 2:

1) numbers=[10,20,0,5,0,30]

2) for n in numbers:

3) if n==0:

4) print("Hey how we can divide with zero..just skipping")

5) continue 

6) print("100/{} = {}".format(n,100/n))

7) 

 

loops with else block:

Inside loop execution,if break statement not executed ,then only else part will be

executed.

else means loop without break

Eg:

1) cart=[10,20,30,40,50]

2) for item in cart:

3) if item>=500:

4) print("We cannot process this order")

5) break 

6) print(item)

7) else:

8) print("Congrats ...all items processed successfully")

9) 

 

Q. What is the difference between for loop and while loop in Python?

We can use loops to repeat code execution

Repeat code for every item in sequence ==>for loop

Repeat code as long as condition is true ==>while loop

Q. How to exit from the loop?

by using break statement

 

Q. How to skip some iterations inside loop?

by using continue statement.

 

Q. When else part will be executed wrt loops?

If loop executed without break

 

 

3) pass statement:

pass is a keyword in Python.

In our programming syntactically if block is required which won't do anything then we can

define that empty block with pass keyword.

pass

|- It is an empty statement

|- It is null statement

|- It won't do anything

Eg:

if True:

SyntaxError: unexpected EOF while parsing

if True: pass

==>valid

def m1():

SyntaxError: unexpected EOF while parsing

 

 

def m1(): pass

use case of pass:

Sometimes in the parent class we have to declare a function with empty body and child

class responsible to provide proper implementation. Such type of empty body we can

define by using pass keyword. (It is something like abstract method in java)

Eg:

def m1(): pass

Eg:

1) for i in range(100):

2) if i%9==0:

3) print(i)

4) else:pass 

5) 

 

del statement:

del is a keyword in Python.

After using a variable, it is highly recommended to delete that variable if it is no longer

required,so that the corresponding object is eligible for Garbage Collection.

We can delete variable by using del keyword.

Eg:

1) x=10

2) print(x)

3)

del x 

 

After deleting a variable we cannot access that variable otherwise we will get NameError.

Eg:

1) x=10

2) del x

3) print(x)

NameError: name 'x' is not defined.

Note:

We can delete variables which are pointing to immutable objects.But we cannot delete

the elements present inside immutable object.

Eg:

1) s="Sohan" 

2) print(s)

3) del s==>valid

4) del s[0] ==>TypeError: 'str' object doesn't support item deletion

Difference between del and None:

In the case del, the variable will be removed and we cannot access that variable(unbind

operation)

1) s="Sohan" 

2) del s

3) print(s) ==>NameError: name 's' is not defined.

But in the case of None assignment the variable won't be removed but the corresponding

object is eligible for Garbage Collection(re bind operation). Hence after assigning with

None value,we can access that variable.

1) s="Sohan" 

2) s=None

3) print(s) # None