This document provides an overview of Python's if-else conditional statements and loops. It discusses the syntax of if statements, elif (else if), else, conditional expressions, nested if statements, and while loops. Examples are given to demonstrate how to use basic conditional logic and loops to control program flow in Python. Key topics covered include using logical operators like ==, !=, <, > to define conditions, the importance of indentation, and how else clauses work with if and while statements.

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PYTHON BASICS
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DEPARTMENT OF COMPUTER SCIENCE
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Programming in Python

2. Python If ... Else
• Python supports the usual logical conditions from mathematics:
• Equals: a == b
• Not Equals: a != b
• Less than: a < b
• Less than or equal to: a <= b
• Greater than: a > b
• Greater than or equal to: a >= b
• These conditions can be used in several ways, most commonly in "if statements" and loops.
• An "if statement" is written by using the if keyword.
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3. • Example
• a = 33
b = 200
if b > a:
print("b is greater than a")
• Indentation
• If statement, without indentation (will raise an error):
• a = 33
b = 200
if b > a:
print("b is greater than a") # you will get an error
print("b is greater than a")
^
IndentationError: expected an indented block
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4. Elif
• The elif keyword is pythons way of saying "if the previous conditions were not true, then try this condition".
• Example
• a = 33
• b = 33
• if b > a:
• print("b is greater than a")
• elif a == b:
• print("a and b are equal")
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5. Else
• The else keyword catches anything which isn't caught by the preceding conditions.
• a = 200
• b = 33
• if b > a:
• print("b is greater than a")
• elif a == b:
• print("a and b are equal")
• else:
• print("a is greater than b")
• C:UsersMy Name>python demo_if_else.py
a is greater than b
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6. Continued….
• You can also have else without elif:
• a = 200
• b = 33
• if b > a:
• print("b is greater than a")
• else:
• print("b is not greater than a")
• C:UsersMy Name>python demo_if_else2.py
b is not greater than a
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7. Conditional expression or one line if-else
• If we have only one statement in both if and else and we want to execute the if else in one line then we use this syntax.
• One line if else statement, with 3 conditions:
• a = 330
• b = 330
• print("A") if a > b else print("=") if a == b else print("B")
• Output:
• C:UsersMy Name>python demo_if_else_short2.py
=
• The and keyword is a logical operator, and is used to combine conditional statements:
• a = 200
• b = 33
• c = 500
• if a > b and c > a:
• print("Both conditions are True")
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8. Nested If
• Example
• x = 41
• if x > 10:
• print("Above ten,")
• if x > 20:
• print("and also above 20!")
• else:
• print("but not above 20.")
• Output
• C:UsersMy Name>python demo_if_nested.py
Above ten,
and also above 20!
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9. Python Loops
• A loop statement allows us to execute a statement or group of statements multiple times.
• While Loop: In python, while loop is used to execute a block of statements repeatedly until a given condition is satisfied.
• And when the condition becomes false, the line immediately after the loop in program is executed.
Syntax :
while expression:
statement(s)
Note: Python uses indentation as its method of grouping statements.
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10. While Loop Example
Example:
# Python program to illustrate
# while loop
count = 0
while (count < 3):
count = count + 1
print("Hello World")
Output:
Hello World
Hello World
Hello World
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11. Using else statement with while loops:
• The else clause is only executed when your while condition becomes false.
• If you break out of the loop, or if an exception is raised, it won’t be executed.
Example:
count = 0
while (count < 3):
count = count + 1
print("Hello World")
else:
print(“Out of the loop")
Output:
Hello World
Hello World
Hello World
Out of the loop
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12. Single statement while block
• Just like the if block, if the while block consists of a single statement the we can declare the entire loop in a single line
Example:
count = 0
while (count == 0): print("Hello World")
Note: It is suggested not to use this type of loops as it is a never ending infinite loop where the condition is always true and
you have to forcefully terminate it.
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13. THANK YOU
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