8. Flow Control

8. Flow control - 25 min read

Why should we learn about flow control?
Flow control
Sequential flow
Conditional flow
Looping flow
Sequential statements
Conditional or Decision-making statements
if statement
if else statement
if elif else statement
Looping
while loop
for loop
Infinite loops
Nested loops
Loops with else block (or) else suit
Where loops with else block flow is helpful?
break statement
continue statement
pass statement
return statement
Programs

8. Flow control

Why should we learn about flow control?

Simple answer: To understand the flow of statements execution in a program.
In any programming language, statements will be executed mainly in three ways,

Sequential.
Conditional.
Looping.

Flow control

The order of statements execution is called a flow of control.
Based on requirement the program's statements can execute in different ways like sequentially, conditionally and repeatedly etc.

1. Sequential

Statements execute from top to bottom, means one by one sequentially.
By using sequential statements, we can develop only simple programs.

2. Conditional

Based on the conditions, statements used to execute.
Conditional statements are useful to develop better and complex programs.

3. Looping

Based on the conditions, statements used to execute randomly and repeatedly.
Looping execution is useful to develop better and complex programs.

1. Sequential statements

Sequential statements execute from top to bottom, means one by one sequentially.

Program sequential execution

Name demo1.py

print(“one”)

print(“two”)

print(“three”)

Output

one

two

three

2. Conditional or Decision-making statements

if
if else
if elif else
else suite

3. Looping

while loop
for loop

4. others

break
continue
return
yield (we will discuss in function chapter)
pass

2. Conditional or Decision-making statements

2.1 if statement

syntax

if condition:

if block statements

out of if block statements

if statement contains an expression/condition.
As per the syntax colon (:) is mandatory otherwise it throws syntax error.
Condition gives the result as a bool type, means either True or False.

If the condition result is True, then if block statements will be executed
If the condition result is False, then if block statements won’t execute.

Program Executing if block statements by using if statement

Name demo2.py

num = 1

print(“num==1 value is: ”, (num==1))

if num == 1:

print(“if block statements executed”)

Output

num==1 value is: True

if block statements executed

Program: executing out of if block statements

Name demo3.py

num = 1

print(“num==2 value is: ”, (num==2))

if num == 2:

print(“if block statements”)

print(“out of if block statements”)

Output

num==2 value is: False

out of if block statements

When should we use if statement?

If you want to do either one thing or nothing at all, then you should go for if statement.

2.1 if else statement

syntax

if condition:

if block statements1

else:

else block statements2

if statement contains an expression/condition.
As per the syntax colon (:) is mandatory otherwise it throws syntax error
Condition gives the result as bool type, means either True or False

If the condition result is True, then if block statements will be executed
If the condition result is False, then else block statements will be executed.

Program Executing if block statements by using if else statement

Name demo4.py

num =1

print(“num==1 value is: ”, (num==1))

if num == 1:

print(“if block statements executed”)

else:

print(“else block statements executed”)

Output

num==1 value is: True

if block statements executed

Program printing else block statements

Name demo5.py

num =1

print(“num==2 value is: ”, (num==2))

if num == 2:

print(“if block statements executed”)

else:

print(“else block statements executed”)

Output

else block statements executed

Program User name validation checking by using if else statement

Name demo6.py

user_name =”nireekshan”

name = input(“Please enter user name: ”)

if user_name== name:

print(“Welcome to Gmail : ”, name)

else:

print(“Invalid user name, please try again”)

Output

Please enter user name: nireekshan

Welcome to Gmail: nireekshan

Please enter user name: chiru

Invalid user name, please try again

Program User name validation checking by using if else statement

Name demo7.py

user_name =”nireekshan”

user_password = “dvs”

name = input(“Please enter user name: ”)

password = input(“Please enter your password: ”)

if (user_name == name) and (user_password == password):

print(“Welcome to Gmail: ”, name)

else:

print(“Invalid user name or password, please try again”)

Output

Please enter your name: nireekshan

Please enter your password: dvs

Welcome to Gmail: nireekshan

Please enter your name: nireekshan

Please enter your password: sampurneshbabu

Invalid user name or password, please try again

Make a note

if and else blocks are BEST friends,
So, we should not break the relationship between if and else by writing other statements in between.
Conclusion of the story

In between, if and else blocks we should not write any other statements

Program In between if and else we should not write anything

Name demo7a.py

x=1

if x==1:

print("if block")

print("Hello ")

else:

print("else block")

Output

SyntaxError: invalid syntax

When should we use if else statement?

If you want to do either one thing or another thing, then you should go for if else statement.

2.2 if elif else statement

syntax

if condition1

if block statements

elif condition2

elif block1 statements

elif condition3

elif block2 statements

else

else block statements

if statement contains an expression/condition.
As per the syntax colon (:) is mandatory otherwise it throws an error
Condition gives the result as bool type means either True or False

If the condition result is True, then any matched if or elif block statements will execute.
If all if and elif conditions results are False, then else block statements will execute.

Make a note

Here, else part is an optional

Program printing corresponding value by using if, elif, else statements

Name demo8.py

print(“Please enter the values from 0 to 4”)

x=int(input(“Enter a number: “))

if x==0:

print(“You entered:”, x)

elif x==1:

print(“You entered:”, x)

elif x==2:

print(“You entered:”, x)

elif x==3:

print(“You entered:”, x)

elif x==4:

print(“You entered:”, x)

else:

print(“Beyond the range than specified”)

output

Enter a number: 1

You entered: 1

Enter a number: 100

Beyond the range

When should we use if elif else statement

If we want to choose one option from many options, then we should use if elif else statements.

Programs by using conditional statements

Program Find biggest of given 2 numbers from the command prompt

Name demo9.py

x=int(input("Enter First Number: "))

y=int(input("Enter Second Number: "))

if x>y :

print("Biggest Number is: ", x)

else :

print("Biggest Number is: ", y)

Output

Enter First Number: 10

Enter Second Number: 20

Biggest Number is: 20

Program Find biggest of given 3 numbers from the command prompt

Name demo10.py

x=int(input("Enter First Number: "))

y=int(input("Enter Second Number: "))

z=int(input("Enter Third Number: "))

if (x>y) and (x>z):

print("Biggest Number is: ",x)

elif y>z :

print("Biggest Number is: ",y)

else :

print("Biggest Number is: ",z)

Output

Enter First Number: 10

Enter Second Number: 20

Enter Second Number: 30

Biggest Number is: 30

Program To find the number belongs to which group

Name demo11.py

x=int(input("Enter number:"))

if (x>=1) and (x<=100) :

print("Entered number", x ,"is in between 1 to 100")

else :

print("Entered number", x ,"is not in between 1 to 100")

Output

Enter number: 55

Entered number 55 is in between 1 to 100

Enter number: 201

Entered number 201 is not in between 1 to 100

3. Looping

If we want to execute a group of statements in multiple times, then we should go for looping kind of execution.

while loop
for loop

3.2 while loop

If we want to execute a group of statements repeatedly until the condition reaches to False, then we should go for while loop.

Syntax

while condition:

statements

while loop contains an expression/condition.
As per the syntax colon (:) is mandatory otherwise it throws a syntax error
Condition gives the result as bool type means either True or False

If the condition result is True, then while loop executes till the condition reaches to False.
If the condition result is False, then while loop execution terminates.

while loop main parts

Initialization section.
Condition section.
Either increment or decrement by using arithmetic operator.

while loop explanation

Initialization section.

Initialization is the first part in while loop.
Here, before entering the condition section, initialization part is required.

Condition section

With the initialized value, the next step is checking the condition.
In python, while loop will check the condition at the beginning of the loop.
In the first iteration:

In the first iteration, if the condition returns True, then it will execute the statements which having inside while loop.
In the first iteration, if the condition returns False, then while loop terminates immediately.

Then the loop execution goes to increment or decrement section.

Increment or decrement

Next step is, increment or decrement section.
In python, we need to use arithmetic operator inside while loop to increment or decrements the value.
So, based on the requirement, the value will be either increment or decrement.
In the second iteration:

In the second iteration, if the condition returns True, then it will execute the statements which having inside while loop.
In the second iteration, if the condition returns False, then while loop terminates immediately.

Conclusion

Till condition is True the while loop statements will be executed.
If the condition reaches to False, then while the loop terminates the execution.

Program Printing numbers from 1 to 5 by using while loop

Name demo12.py

x=1

while x<=5:

print(x)

x+=1

print(“End”)

output

End

Program Printing even numbers from 10 to 20 by using while loop

Name demo13.py

x=10

while (x>=10) and (x<=20):

print(x)

x+=2

print(“End”)

output

End

for loop

Basically, for loop is used to get or iterate elements one by one from sequence like string, list, tuple, etc…
While iterating elements from the sequence we can perform operations on every element.

Syntax

for variable in sequence:

statements

Program printing elements from the list object

Name demo14.py

l1 = [10, 20, 30, ‘Nireekshan’]

for x in l1:

print(x)

output

Nireekshan

Program printing characters from a string

Name demo15.py

x= ”python”

for ch in x:

print(ch)

output

Program printing every item cost by adding gst from list

Name demo16.py

items_costs = [10, 20, 30,]

gst = 2

for x in items_costs:

print(x+gst)

output

Program printing elements by using range() function and for loop

Name demo17.py

for x in range(1, 5):

print(x)

output

Program sum of the items cost in list using for loop

Name demo18.py

item_costs = [10, 20, 30]

sum=0

for x in item_costs:

sum=sum + x

print(sum)

output

Infinite loops

The loop which never ends is called infinite group.

Program infinite loop

Name demo19.py

while True:

print(“Hello”)

output

Hello

Make a note

To come out of the infinite loop we need to press ctrl + c

Nested loops

It is possible to write one loop inside another loop, this is called nested loop

Program printing stars by using nested for loop

Name demo20.py

rows = range(1, 5)

for x in rows:

for star in range(1, x+1):

print('* ', end=' ')

print()

output

* *

* * *

* * * *

Loops with else block (or) else suit

In python, it is possible to use ‘else’ statement along with for loop and while loop.

for with else	while with else
for variable in sequence: statements else: statements	while condition: statements else: statements

The statements are written after ‘else’ are called as else suite.
The else suite will be always executed irrespective of the loop executed or not.

Program for with else

Name demo21.py

values = range(5)

for x in values:

print(“item is available”)

else:

print(“sorry boss, item is not available”)

output

item is available

sorry boss, item is not available

Program for with else

Name demo22.py

values = range(0)

for x in values:

print(“item is available”)

else:

print(“sorry boss, item is not available”)

output

sorry boss, item is not available

Make a note

Seems to be for loop and else suit both got executed.

Where this kind of execution will helpful?

If you are searching an element from list object, if unable to find element then for loop won’t return anything, so at that time else suit help us to tell not found the element.

Program Element searching in list

Name demo23.py

group = [1, 2, 3, 4]

search = int(input(“Enter the element in search: “))

for element in group:

if search == element:

print(“An element found in group”, element)

break

else:

print(“Element not found”)

output

Enter element to search: 4

An element found in group : 4

Enter element to search: 6

Element not found in group

break statement

The break statement can be used inside the loops to break the execution based on some condition.
Generally, break statement is used to terminate the for loop and while loop.
break statement we can use inside loops only otherwise it gives SyntaxError.

Program while loop without a break

Name demo24.py

x=1

while x<=10:

print(‘x= ’, x)

x+=1

print(“out of the loop”)

output

x= 1

x= 2

x= 3

x= 4

x= 5

x= 6

x= 7

x= 8

x= 9

x= 10

out of the loop

Program printing just 1 to 5 by using while loop and break

Name demo25.py

x=1

while x<=10:

print('x', x)

x+=1

if x == 5:

break

print("out of the loop")

output

x= 1

x= 2

x= 3

x= 4

out of the loop

continue statement

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

Program Applying discount based on condition and printing remaining

Name demo26.py

cart=[10, 20, 500, 700, 50, 60]

for item in cart:

if item>=500:

continue

print("No discount applicable: ", item)

output

No discount applicable: 10

No discount applicable: 20

No discount applicable: 50

No discount applicable: 60

pass statement

We can use pass statement when we need a statement syntactically, but we do not want to do any operation.

Program pass statement

Name demo27.py

num = [10, 20, 30,400, 500, 600]

for i in num:

if i<100:

print(“pass statement executed”)

pass

else:

print("Give festival coupon for these guys who bought: ",i)

output

pass statement executed

Give festival coupon for these guys who bought: 400

Give festival coupon for these guys who bought: 500

Give festival coupon for these guys who bought: 600

return statement

Based on the requirement a function or method can return the result.
If any function or the method is returning a value, then we need to write a return statement along with the value.

Program Function displaying information

Name demo28.py

def display():

print(“This is python programming”)

display()

output

This is python programming

Program Function displaying information and that function returns None

Name demo29.py

def display():

print(“This is python programming”)

x = display()

print(x)

output

This is python programming

None

Make a note

If any function is not returning anything then by default that function returns None data type.

Make a note

If any function or the method is returning a value, then we need to write a return statement along with the value.

Program Function returns a result

Name demo30.py

def sum(a, b):

return a+b

res=sum(10, 20)

print(res)

output

Friday, 3 May 2019

8. Flow Control

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