[Python] Comprehensive Guide to Using "if"

 

 

Using "if" in Python

 

 

 

 

Table of Contents

 

* Code is basically composed of <input # output> format.

 

Basic Form (1): if

if condition:
    statement

• If the condition is True (or 1), the statement is executed.
• If the condition is False (or 0), the statement is not executed.

if True:
    print('if statement executed')

# Output: if statement executed

if False:
    print('if statement executed')

# Output:

 

Basic Form (2): if else

if condition:
    statement
else:
    statement

• If the condition is True (or 1), the statement in the "if" block is executed.
• If the condition is False (or 0), the statement in the "else" block is executed.

if 1:
    print('if statement executed')
else:
    print('else statement executed')

# Output: if statement executed

if 0:
    print('if statement executed')
else:
    print('else statement executed')

# Output: else statement executed

 

Basic Form (3): if elif else

if condition:
    statement
elif condition:
    statement
...
elif condition:
    statement
else:
    statement

• If a condition in "if" or "elif" is True (or 1), the corresponding statement is executed.
• If all conditions are False (or 0), the statement in the "else" block is executed.
• Starting from the top, only the first "if" or "elif" block with a True (or 1) condition is executed.

a = 0
if a == 0:
    print('if statement executed')
elif a == 1:
    print('elif statement executed')
else:
    print('else statement executed')

# Output: if statement executed

a = 1
if a == 0:
    print('if statement executed')
elif a == 1:
    print('elif statement executed')
else:
    print('else statement executed')

# Output: elif statement executed

a = 2
if a == 0:
    print('if statement executed')
elif a == 1:
    print('elif statement executed')
else:
    print('else statement executed')

# Output: else statement executed

a = 3
if a == 0:
    print('if statement executed')
elif a == 1:
    print('first elif statement executed')
elif a == 2:
    print('second elif statement executed')
elif a == 3:
    print('third elif statement executed')
else:
    print('else statement executed')

# Output: third elif statement executed

 

Types of Conditions: Existence of Elements in an Object

• Here, an object typically refers to a string, list, tuple, or dictionary.
• If there is at least one element in the object, it evaluates to True.
• If there are no elements, it evaluates to False.

if 'abc':
    print('if statement executed')
else:
    print('else statement executed')

# Output: if statement executed

if '':
    print('if statement executed')
else:
    print('else statement executed')

# Output: else statement executed

if [1, 2, 3]:
    print('if statement executed')
else:
    print('else statement executed')

# Output: if statement executed

if []:
    print('if statement executed')
else:
    print('else statement executed')

# Output: else statement executed

 

Types of Conditions: in, not in

• Here, an object typically refers to a string, list, tuple, or dictionary.
• element in object: Evaluates to True if the specific element exists within the object, otherwise evaluates to False.
• element not in object: Evaluates to True if the specific element does not exist within the object, otherwise evaluates to False.

if 'a' in 'abc':
    print('if statement executed')
else:
    print('else statement executed')

# Output: if statement executed

if 'k' in 'abc':
    print('if statement executed')
else:
    print('else statement executed')

# Output: else statement executed

if 6 not in [1, 2, 3]:
    print('if statement executed')
else:
    print('else statement executed')

# Output: if statement executed

if 1 not in [1, 2, 3]:
    print('if statement executed')
else:
    print('else statement executed')

# Output: else statement executed

 

Types of Conditions: Comparison Operators (==, !=, >, >=, <, <=)

• Here, values typically refer to numbers or strings.
• value1 == value2: True if the two values are equal, False if they are not.
• value1 != value2: True if the two values are different, False if they are the same.
• value1 > value2: True if value1 is greater than value2, False if it is less than or equal to value2.
• value1 >= value2: True if value1 is greater than or equal to value2, False if it is less than value2.
• value1 < value2: True if value1 is less than value2, False if it is greater than or equal to value2.
• value1 <= value2: True if value1 is less than or equal to value2, False if it is greater than value2.
• object1 is object2: True if object1 and object2 are the same, False if they are different.
• object1 is not object2: True if object1 and object2 are different, False if they are the same.
• Value Comparison: == and != check if two values are equal or not (e.g., do the characters match? Is the numerical value the same?). These operators are recommended for simple value comparisons.
• Object Comparison: is and is not check if the objects themselves are the same, considering data type, memory address, etc. These operators are generally used for equality comparison of objects created through classes.
• Example: 3 == 3.0 returns True because the values are equal, but 3 is 3.0 returns False because the data type and memory address are different.

if 'abc' == 'abc':
    print('if statement executed')
else:
    print('else statement executed')

# Output: if statement executed

if 'abc' != 'abc':
    print('if statement executed')
else:
    print('else statement executed')

# Output: else statement executed

if 3 >= 2:
    print('if statement executed')
else:
    print('else statement executed')

# Output: if statement executed

if 3 < 2:
    print('if statement executed')
else:
    print('else statement executed')

# Output: else statement executed

if 3 is 3.0:
    print('if statement executed')
else:
    print('else statement executed')

# Output: else statement executed

 

Types of Conditions: Logical Operators (and, or, not)

• condition1 and condition2: True if both condition1 and condition2 are True, otherwise False.
• condition1 or condition2: True if either condition1 or condition2 is True, otherwise False.
• not condition: True if the condition is False, False if the condition is True.
• and and or can be used to connect multiple conditions.
• Example: condition1 and condition2 or condition3 and condition4 or condition5 and condition6 ...

if 1 == 1 and 'kkk' not in 'abc':
    print('if statement executed')
else:
    print('else statement executed')

# Output: if statement executed

if 5 == 1 and 'kkk' not in 'abc':
    print('if statement executed')
else:
    print('else statement executed')

# Output: else statement executed

if 5 == 1 or 'kkk' not in 'abc':
    print('if statement executed')
else:
    print('else statement executed')

# Output: if statement executed

if not 5 == 1:
    print('if statement executed')
else:
    print('else statement executed')

# Output: if statement executed

if 1 == 1 and 2 == 2 and 3 == 3:
    print('if statement executed')
else:
    print('else statement executed')

# Output: if statement executed

 

Conditional Expression in if Statement

(statement if True) if (condition) else (statement if False)

if 'a' in 'abc':
    print('if statement executed')
else:
    print('else statement executed')

# Output: if statement executed

print('if statement executed') if 'a' in 'abc' else print('else statement executed')

# Output: if statement executed

 

To Leave the Statement Empty: pass

• When pass is used in place of a statement, nothing happens even if the condition is met.

if 1 == 1:
    pass
else:
    print('else statement executed')

# Output: