Conditional Statements

Conditional Statements#

  • Programs we have seen/written so far will execute sequentially

    • Conditional statements execute a block of code only if specified condition(s) are met

    • The if statement

      • Execute code only if a condition is met

    • The match statement

      • A new addition to Python

      • Used for pattern matching

The if Statement#

  • The if statement

    • if, elif, and else are the Python keywords used in defining an if statement

    • The if statement is a compound statement which spans multiple lines of code

      • Tabs are used to indicate the compound statements

    • The elif and else blocks are optional and any number of elif blocks can be used

    • Only the code following the first true expression of the if, elif and else code block is executed

    • Then the code continues to run after the entire code block

  • Template for an if conditional statement

if test_expr0:
    if_body
elif test_expr1:
    elif_body1
elif test_expr2:
    elif_body2

\(\qquad \qquad \vdots\)

elif test_exprn:
    elif_bodyn
else:
    else_body

  • Examples of if statement usage

x, y = 10, 10

if x<y:
    print("x is less than y")
else:
    print("x is not less than y")
print('End of code')

x is not less than y
End of code

  • Program that prints the state of water given the temperature

# `input` function does not work with the web version; commented out
temp = 30.2 #float(input("Enter the temperature in C: "))

if temp <= 0:
    print("Solid state")
elif 0 < temp <= 100:      # Equivalent to 0 < temp and temp <= 100
    print("Liquid state")
else:
    print("Gaseous state")

Liquid state

  • Program to determine whether a number is even or odd

# `input` function does not work with the web version; commented out
n = 24 #int(input("Enter an Integer: "))

# No elif block
if n%2 == 0:
    print(f"{n} is even")
else: 
    print(f"{n} is odd")

24 is even

  • An issue with this code is that if the user enters a non-number, we get an exception

  • Adding logic to ensure input is an integer

# `input` function does not work with the web version; commented out
n = "2a" #input("Enter an Integer: ")

if n.isdigit():
    n = int(n)
    if n%2 == 0:
        print(f"{n} is even")
    else: 
        print(f"{n} is odd")
else:
    print(f"{n} is not an integer")  

2a is not an integer

  • Program to greet a user

    • The title() and istitle() string methods can be used to ensure correct capitalization

# `input` function does not work with the web version; commented out
name = "Butch" #input("Enter your name: ")

# check case
if not name.istitle():
    name = name.title()

print(f'Hi {name}!')

Hi Butch!

Multiple elif Blocks vs if Blocks#

  • An if statement with multiple elif blocks

    • The code block following the first true if or elif test expression is evaluated and the rest ignored

# An if statement with multiple elif blocks
n = 5

if n < 10:
    print(f'{n} is less than 10')
elif n < 20:
    print(f'{n} is less than 20')
elif n < 30:
    print(f'{n} is less than 30')
print("end of code")

5 is less than 10
end of code

  • Modified program using multiple if statements to evaluate all the blocks

# Multiple standalone if statements
n = 5

if n < 10:
    print(f'{n} is less than 10')

if (n >= 10) and (n < 20):
    print(f'{n} is less than 20')

if (n >= 20) and (n < 30):
    print(f'{n} is less than 30')

print("end of code")

5 is less than 10
end of code

The pass Statement#

  • pass is a keyword that does nothing.

  • Useful as a placeholder in locations where Python expects code to be

n = 15

if n < 10:
    print(f"{n} is less than 10")
elif n < 20:
    pass

n = 5

if n < 10:
    print("n is less than 10")
else:
    pass   

n is less than 10

The match Statement#

  • The match statement is a new addition to Python 3.10

    • Emulates the switch-case statement in C/C++

    • match and case are the required Python keywords

  • A program to respond to a choice made by the user

    • Alternate solution possible using the if statement

# `input` function does not work with the web version; commented out
query = 'Yes'.lower() #input("Do you acept the invitation? (Yes/No/Maybe): ").lower()

match query:
    case 'yes':
        print("Thanks for accepting!")
    case 'no':
        print("Sorry you cannot make it!")
    case 'maybe':
        print("Hope you can make it!")
    case _:
        print("Invalid response.")
        

Thanks for accepting!

  • Use strip() method to strip away leading and trailing whitespace characters (default behavior) from the input

    • For non-whitespace characters, the usage is: s.strip(chars) where chars is a string made of characters that are to be stripped

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