4.2 If-else statements

Conditional statements are the backbone of decision-making in Python programs. They allow code to execute different blocks based on specific conditions, enabling dynamic and responsive program flow.

If, elif, and else statements form the core of conditional logic. By combining these with comparison and logical operators, programmers can create complex decision trees, guiding program execution through various scenarios based on input and state.

Conditional Statements

Conditional statements for program flow

• if statements enable conditional execution of code blocks based on whether a specified condition evaluates to True or False
  • Code block under an if statement executes only if the condition is True (x > 5)
  • If the condition is False, the code block is skipped and program execution continues with the next statement after the if block
• if-else statements provide an alternative code block to execute when the if condition is False
  • Code block under the if statement executes if the condition is True (temperature > 30)
  • Code block under the else statement executes if the if condition is False (temperature <= 30)
• elif (else if) statements allow for checking multiple conditions if the preceding if or elif conditions are False
  • First condition that evaluates to True will have its corresponding code block executed (grade >= 90, grade >= 80, etc.)
  • If none of the conditions are True, the code block under the else statement (if present) will be executed

Syntax of if and if-else statements

• if statements start with the if keyword, followed by a condition and a colon
  • Condition is an expression that evaluates to either True or False (count == 0, name != "John")
  • Code block under the if statement is indented (typically by 4 spaces)
• else statements follow an if statement and begin with the else keyword and a colon
  • Code block under the else statement is also indented
• elif statements can be used between an if and else statement to check additional conditions
  • elif statements begin with the elif keyword, followed by a condition and a colon
  • Code block under the elif statement is indented
• Proper indentation is crucial for defining the scope of each code block within conditional statements
• Example syntax:

if age >= 18:
    print("You are eligible to vote")
elif age >= 16:
    print("You can pre-register to vote")
else:
    print("You are not eligible to vote yet")

Implementation of conditional code blocks

• Comparison operators create conditions that evaluate to True or False
  • == (equal to), != (not equal to), < (less than), > (greater than), <= (less than or equal to), >= (greater than or equal to)
• Logical operators combine multiple conditions
  • and (both conditions must be True), or (at least one condition must be True), not (inverts the boolean value)
• Conditions can include variables, constants, and function calls that return boolean values
  • Variable: if score > 100:
  • Constant: if MAX_VALUE == 100:
  • Function call: if is_valid_email(email):
• Example implementation:

temperature = 25
if temperature > 30:
    print("It's a hot day")
    print("Drink plenty of water")
elif temperature > 20 and temperature <= 30:
    print("It's a nice day")
else:
    print("It's a cold day")
    print("Wear warm clothes")

Advanced Conditional Concepts

• Control flow in programs is determined by the sequence of conditional statements
• Branching occurs when the program execution takes different paths based on conditions
• Nested conditionals involve placing conditional statements within other conditional statements
  • Allows for more complex decision-making structures
  • Example:

  if outer_condition:
      if inner_condition:
          # Code block for both conditions true
      else:
          # Code block for outer true, inner false
  else:
      # Code block for outer false