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

Top images from around the web for Conditional statements for program flow

• 

  3. Conditionals — Computer Science 20 Saskatchewan View original

  Is this image relevant?

• 

  Python项目代码太多if-else？ 这样优化才优雅！ - 天善智能：专注于商业智能BI和数据分析、大数据领域的垂直社区平台 View original

  Is this image relevant?

• 

  Python If else syntax - Stack Overflow View original

  Is this image relevant?

• 

  3. Conditionals — Computer Science 20 Saskatchewan View original

  Is this image relevant?

• 

  Python项目代码太多if-else？ 这样优化才优雅！ - 天善智能：专注于商业智能BI和数据分析、大数据领域的垂直社区平台 View original

  Is this image relevant?

1 of 3

Top images from around the web for Conditional statements for program flow

• 

  3. Conditionals — Computer Science 20 Saskatchewan View original

  Is this image relevant?

• 

  Python项目代码太多if-else？ 这样优化才优雅！ - 天善智能：专注于商业智能BI和数据分析、大数据领域的垂直社区平台 View original

  Is this image relevant?

• 

  Python If else syntax - Stack Overflow View original

  Is this image relevant?

• 

  3. Conditionals — Computer Science 20 Saskatchewan View original

  Is this image relevant?

• 

  Python项目代码太多if-else？ 这样优化才优雅！ - 天善智能：专注于商业智能BI和数据分析、大数据领域的垂直社区平台 View original

  Is this image relevant?

1 of 3

• if

  statements enable conditional execution of code blocks based on whether a specified condition evaluates to

  [True](https://www.fiveableKeyTerm:TRUE)

  or

  [False](https://www.fiveableKeyTerm: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](https://www.fiveableKeyTerm: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