Chapter 7 Key Takeaways: Procedures and Functions

Use this page for quick review and spaced repetition. Revisit it 1 day, 3 days, 1 week, and 2 weeks after completing the chapter.

Core Concepts

• Procedures are named blocks of code that perform a specific task. They are declared before the main begin...end block and called by name.

• Functions are like procedures but return a value. A function call is an expression and can appear anywhere a value of the appropriate type is expected.

• Procedural decomposition (top-down design) is the process of breaking a complex problem into small, named, self-contained subprograms. The main body reads like an outline; each procedure is a self-contained chapter.

Parameter Modes

Function Return Values

• Classic style: Assign to the function name: FunctionName := value;
• Modern style (preferred): Assign to Result: Result := value;
• Using Result avoids confusion with recursive calls.

Scope and Nesting

• Local variables are declared inside a procedure and exist only while it executes. They prevent naming conflicts between procedures.
• Nested subprograms are declared inside other subprograms. They can access the enclosing scope's variables. Limit nesting to one or two levels.

Forward Declarations

• When two procedures call each other (mutual reference), use a forward declaration to break the circular dependency.
• Syntax: Write the procedure heading followed by the keyword forward;, then provide the full definition later.

Best Practices

1. Default to const for read-only parameters, especially strings and records.
2. Use var only when the procedure must modify the caller's data.
3. Prefer functions for computations — they should be pure (no side effects).
4. One task per subprogram. If you cannot name it with a single verb phrase, it is doing too much.
5. Name procedures with verbs (ReadInput, PrintReport), functions with nouns/adjectives (Average, IsValid).
6. Declare variables locally — avoid global variables when a local will do.
7. Keep the main body short — 5 to 15 lines of procedure/function calls that read like an outline.

Common Errors to Avoid

• Using end. (period) instead of end; (semicolon) after a procedure — this ends the entire program.
• Forgetting var when a procedure needs to modify the caller's variable.
• Passing a literal (like 42) to a var parameter — the compiler requires a variable.
• Forgetting to assign a return value in a function (no Result := statement).
• Declaring a procedure after code that calls it without using a forward declaration.

PennyWise Progress

• Version 0.7: PennyWise decomposed into ShowMenu, AddExpense, DisplayExpenses, CalculateTotal, and DisplaySummary. The main body is a simple menu loop. Each procedure uses appropriate parameter modes: var for data modification, const for read-only access.

Key Vocabulary