Control Statements:
Those statements will control the flow of program are known as control
statements.
It is of two types
a) Decision Making
b) Looping
a) Decision Making Statements:
There comes situations in real life when we need to make some decisions and
based on these decisions, we decide what should we do next. Similar situations
arise in programming also where we need to make some decisions and based on
these decisions we will execute the next block of code.
Decision-making statements in programming languages decide the
direction(Control Flow) of the flow of program execution.
Types of Control Flow in Python
In Python programming language, the type of control flow statements is as
follows:
1. The if statement
2. The if-else statement
3. The nested-if statement
4. The if-elif-else ladder
1) Python if statement
The if statement is the most simple decision-making statement. It is used to
decide whether a certain statement or block of statements will be executed or
not.
Syntax:
if condition:
# Statements to execute if
# condition is true
Here, the condition after evaluation will be either true or false. if the statement
accepts boolean values – if the value is true then it will execute the block of
statements below it otherwise not.
As we know, python uses indentation to identify a block. So the block under an
if statement will be identified as shown in the below example:
if condition:
statement1
statement2
# Here if the condition is true, if block
# will consider only statement1 to be inside
# its block.
Flowchart of Python if statement
Flowchart of Python if statement
Example of Python if Statement
As the condition present in the if statement is false. So, the block below the if
statement is executed.
Python3
# python program to illustrate If statement
i = 10
if (i > 15):
print("10 is less than 15")
print("I am Not in if")
Output:
I am Not in if
2) Python If-Else Statement
The if statement alone tells us that if a condition is true it will execute a block of
statements and if the condition is false it won’t. But if we want to do something
else if the condition is false, we can use the else statement with if statement to
execute a block of code when the if condition is false.
Syntax:
if (condition):
# Executes this block if
# condition is true
else:
# Executes this block if
# condition is false
Flowchart of Python if-else statement
Flowchart of Python is-else statement
Example of Python if-else statement
The block of code following the else statement is executed as the condition
present in the if statement is false after calling the statement which is not in the
block(without spaces).
Python3
# python program to illustrate If else statement
#!/usr/bin/python
i = 20
if (i < 15):
print("i is smaller than 15")
print("i'm in if Block")
else:
print("i is greater than 15")
print("i'm in else Block")
print("i'm not in if and not in else Block")
Output:
i is greater than 15
i'm in else Block
i'm not in if and not in else Block
Example of Python if else statement in a list comprehension
In this example, we are using an if statement in a list comprehension with the
condition that if the element of the list is odd then its digit sum will be stored else
not.
Python3
# Explicit function
def digitSum(n):
dsum = 0
for ele in str(n):
dsum += int(ele)
return dsum
# Initializing list
List = [367, 111, 562, 945, 6726, 873]
# Using the function on odd elements of the list
newList = [digitSum(i) for i in List if i & 1]
# Displaying new list
print(newList)
Output :
[16, 3, 18, 18]
3) Nested-If Statement in Python
A nested if is an if statement that is the target of another if statement. Nested if
statements mean an if statement inside another if statement. Yes, Python allows
us to nest if statements within if statements. i.e., we can place an if statement
inside another if statement.
Syntax:
if (condition1):
# Executes when condition1 is true
if (condition2):
# Executes when condition2 is true
# if Block is end here
# if Block is end here
Flowchart of Python Nested if Statement
Flowchart of Python Nested if statement
Example of Python Nested if statement
In this example, we are showing nested if conditions in the code, All the If
conditions will be executed one by one.
Python3
# python program to illustrate nested If statement
i = 10
if (i == 10):
# First if statement
if (i < 15):
print("i is smaller than 15")
# Nested - if statement
# Will only be executed if statement above
# it is true
if (i < 12):
print("i is smaller than 12 too")
else:
print("i is greater than 15")
Output:
i is smaller than 15
i is smaller than 12 too
4) Python if-elif-else Ladder
Here, a user can decide among multiple options. The if statements are
executed from the top down. As soon as one of the conditions controlling the if
is true, the statement associated with that if is executed, and the rest of the
ladder is bypassed. If none of the conditions is true, then the final else
statement will be executed.
Syntax:
if (condition):
statement
elif (condition):
statement
.
.
else:
statement
Flowchart of Python if-elif-else ladder
Flowchart of if-elif-else ladder
Example of Python if-elif-else ladder
In the example, we are showing single if condition and multiple elif conditions,
and single else condition.
Python3
# Python program to illustrate if-elif-else ladder
#!/usr/bin/python
i = 20
if (i == 10):
print("i is 10")
elif (i == 15):
print("i is 15")
elif (i == 20):
print("i is 20")
else:
print("i is not present")
Output:
i is 20
Short Hand if statement
Whenever there is only a single statement to be executed inside the if block
then shorthand if can be used. The statement can be put on the same line as
the if statement.
Syntax:
if condition: statement
Example of Python if shorthand
In the given example, we have a condition that if the number is less than 15,
then further code will be executed.
Python3
# Python program to illustrate short hand if
i = 10
if i < 15: print("i is less than 15")
Output:
i is less than 15
Short Hand if-else statement
This can be used to write the if-else statements in a single line where only one
statement is needed in both the if and else blocks.
Syntax:
statement_when_True if condition else statement_when_False
Example of Python if else shorthand
In the given example, we are printing True if the number is 15 else it will
print False.
Python3
# Python program to illustrate short hand if-else
i = 10
print(True) if i < 15 else print(False)
Output:
True
