if-else Statements

Choosing a Path

Programs often need to respond differently depending on a condition. An if-else statement provides two alternative execution paths: one block runs when the condition is true, and the other block runs when the condition is false. Learning to trace those paths lets you predict what a program will do before it runs and helps you locate unexpected behavior.

First Trace

temperature = 25 if temperature > 20: print("Warm") else: print("Cool")

Python Structure

An if-else statement contains a condition and two indented code blocks. Write if, then the condition, followed by a colon. The statements belonging to the if branch must be indented. Write else followed by a colon, then indent the statements belonging to the else branch. The colon marks the start of each block, while indentation shows which statements belong to that block.

if condition:
    statement_for_true_case
else:
    statement_for_false_case

Alternative Execution

Use if-else when both outcomes need an explicit response. The if block describes what should happen when the condition is true, and the else block describes the alternative when it is false. Because exactly one block executes, an if-else statement gives the program a complete two-way choice for that condition.

Second Trace

number = 7 if number % 2 == 0: print("Even") else: print("Odd")

Debugging Branches

Debugging an if-else statement means tracing the program from the condition onward. First determine whether the condition is True or False. Then check that the selected block is the one you expected, that its statements are indented as part of the block, and that the other block is skipped. The point where the actual path differs from the expected path identifies where to investigate.

• Leaving out the colon after the condition.

  The syntax requires a colon after the condition.

  Fix: if score > 50:

• Leaving out the colon after else.

  The else branch also requires a colon before its indented block.

  Fix: else:

• Failing to indent the statements in a branch.

  Indentation identifies the code belonging to the if block.

  Fix: if score > 50: print("Pass")

• Expecting both branches to execute.

  An if-else statement executes exactly one block: the if block for True or the else block for False.

  Fix: Evaluate the condition first, then follow only the corresponding branch.

• Tracing the branch before evaluating the condition.

  Python evaluates the condition before selecting an execution path.

  Fix: Write down the condition's boolean result first, then identify the selected block.

Branch Practice

value = 3

if value > 5:
    print("Large")
else:
    print("Small")

Key Takeaways

1. An if-else statement evaluates a condition before selecting an execution path.
2. A True condition runs the indented if block and skips the else block.
3. A False condition skips the if block and runs the indented else block.
4. A colon is required after the condition and after else, and indentation identifies each block.
5. To debug an unexpected result, evaluate the condition first and trace the selected branch step by step.