Overview
This lecture covers the structure and usage of if statements in Python, emphasizing the role of indentation, code blocks, and code readability.
Basic If Statements
- An if statement is a conditional that runs a block of code only if a specified condition is true.
- The generic structure is:
if condition:, followed by an indented block.
- The condition is evaluated directly; do not write
if x < 10 == True, just use if x < 10.
Execution Flow and Flowcharts
- If the condition is true, the indented code block runs; if false, the block is skipped.
- After the if block (regardless of condition), the program continues with the next unindented line.
- Flowcharts follow true/false paths based on the condition, always leading to the program end.
Indentation and Code Blocks
- Indentation (4 spaces in Python) defines which lines belong to the if block.
- Multiple lines can be grouped into a block using the same indentation.
- Once indentation returns to the previous level, the block ends.
- Indents can be nested for deeper structures, like if statements inside if statements.
Nested Blocks
- You can nest blocks by placing an if (or loop) inside another block, increasing indentation.
- Skipping an outer if block due to a false condition also skips all nested blocks inside it.
- Indentation levels signal block beginnings and endings.
Style and Readability
- Avoid blank lines directly after the start of an if block.
- Vertical spaces within blocks are okay to separate code logically.
- Clean and consistent indentation makes code readable and easy to follow.
- Using editors (like Spyder) can help visualize and collapse code blocks.
Practical Examples
- Blocks can contain any number of lines, including nested if statements.
- Ending multiple blocks at once is possible by reducing indentation back to the prior level.
- Excessive nesting is allowed, but deep nesting can make code hard to read and maintain.
Key Terms & Definitions
- If Statement — A programming construct that runs code conditionally based on whether an expression is true.
- Condition — An expression evaluated as true or false to control if block execution.
- Block of Code — One or more lines grouped by indentation that are executed together.
- Indentation — Spaces at the start of a line used in Python to define code blocks.
Action Items / Next Steps
- Practice writing if statements with single and multiple-line blocks.
- Experiment with nested if statements and observe how indentation affects execution.
- Review code style guidelines for indentation and block separation.