Write if-elif-else Statements

One Decision, Two Paths

Programs often need to choose between two actions. Alternative execution handles this choice: Python evaluates a condition, then runs one code block when the condition is true and another code block when the condition is false. The supplied material establishes this two-branch if-else form as the foundation for understanding alternative execution.

Following the Execution Path

Tracing means following the decision in order. First, evaluate the condition. If it produces True, follow the if path and execute the indented code under if. Python skips the else block. If the condition produces False, skip the if block and execute the indented code under else. Exactly one of these two blocks runs: never both and never neither.

Writing the Branch Structure

if condition:
    first_block
else:
    second_block

The structure has four essential parts. Write if, followed by a condition and a colon. Put the first block on the indented lines below it. Then write else followed by a colon, and put the second block on the indented lines below else. The condition is a boolean expression, so it results in either True or False.

Even and Odd Trace

The source material identifies an even-or-odd example as a useful way to trace alternative execution. The following concrete Python version is a generated teaching example that applies the documented if-else rules.

number = 8 if number % 2 == 0: result = "even" else: result = "odd" print(result)

even

Mistakes in Branching

• Leaving out the colon after the condition.

  The documented syntax requires a colon after the condition.

  Fix: Write if condition: before the indented first block.

• Leaving out the colon after else.

  The documented syntax requires a colon after else.

  Fix: Write else: before the indented second block.

• Treating the two blocks as if both will execute.

  Python executes exactly one block: the if block when the condition is true, or the else block when it is false.

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

• Ignoring indentation when identifying a block.

  The source requires proper indentation for each block.

  Fix: Place each block's code on indented lines beneath its if or else header.

Branch Selection Practice

value = 3

if value > 5:
    result = "large"
else:
    result = "not large"

Key Takeaways

1. Alternative execution chooses between two code blocks according to whether a condition is True or False.
2. An if-else statement requires a colon after the condition and after else.
3. Each branch is written as a properly indented block.
4. Python evaluates the condition first, then executes exactly one branch.
5. To trace a statement, determine the condition's result and follow only the corresponding execution path.