CSCI 235

Scope ​

Chapter 6

Void Functions ​

  1. Value-returning functions have a return type.
    • Return a value of a specific data type using the return statement
  2. Void functions do not have a return type.
    • Do not use a return statement to return a value.

Not all functions return a value.

Scope of an Identifier ​

The Scope of an identifier is where in the program the identifier is accessible.

Types of Scopes:

  • Local identifier: identifiers declared within a function (or block)
  • Global identifier: identifiers declared outside of every function definition
  • C++ does not allow nested functions, which is the definition of one function inside of another function's body.

Rules when an identifier is accessed:

  • Must be declared before being used.

  • Global identifiers are accessible by a function or block if:

    • Declared before the function definition
    • All local identifiers (variables and parameters) have different names than the global identifier.

  • Nested block

    • Identifiers are accessible from their declaration to the end of the block where it is declared.
    • Within nested blocks if no identifier with the same name exists

  • The scope of a function name is like the scope of an identifier declared outside any block.

    • i.e., function name scope = global variable scope

  • By using the scope-resolution operator, ::,

    • A global variable declared before the definition of a function (or block) can be accessed by the function (or block).
    • Even if the function (or block) has an identifier with the same name as the global variable.

Global Variables Are Problematic ​

  • Do NOT use global variables unless they are constants!
  • Using global variables causes side effects.
    • A function that uses global variables is not independent
    • If more than one function uses the same global variable,
      • Debugging problems with the global variable is more difficult.
      • Problems caused in one area of the program may appear to be from another area.
  • Global constants have no side effects because they never change values.

Example: Creating an Animation ​

This video is an example of putting the things we know together for something fun. You also learn more about timing.

Example: Reversing the Digits in a Number ​

This example uses a user-defined function to reverse the digits of a number. As you follow the program, notice how the function fits into the rest of the program:

  • The function prototype appears before main, so the compiler knows its name, return type, and parameters before it is called. The prototype and definition must have matching signatures.
  • The call reverseDigits(input) passes input as an argument. The parameter num receives that value inside the function; it is a local variable, so changes to it do not change input in main.
  • The long long return type tells the caller what kind of value the function sends back. main stores that returned value in reversed.
  • The function does its work with local variables, then uses return to send the result back. In the loop, % 10 gets the last digit and / 10 removes it as the reversed number is built.