CIS 250 - functions

Objectives

• create functions to solve problems and modularize programs
• send data to functions for processing
• use data returned from functions
• create function prototypes
• discuss variable scope and lifetime
• discuss differences between passing by-value vs. by-reference
• discuss reference variables
• define the term recursion

Functions

• The basic syntax of a function is

  returnType functionName(parameterList) {
     functionBody
     [return [expression];]
  }

• if the return data type of a function is void, then you may omit the return statement, otherwise you "must" return a value of the return data type
• calling a function is easy - just type the function name followed by a list of arguments contained within parentheses (e.g. pow(2.0, 3.0))
• before an expression using a function is evaluated, the function is called and its returned value is substituted in place of the function call
• you can write your own functions (actually, you've been writing a main() function for all the C++ programs you've been writing)
• functions must either be defined before they are used, or there must be a function prototype before the function is used
• function prototypes look like function definitions, except the function body is missing, and in its place is a semicolon
• multiple values can be sent to functions through the parameter list
• only a single value (or none) may be returned by a function
• a void return data type indicates no value is being returned

Additional function topic outline

• variable scope (local vs. global)
• duplicate variable names with different scope
• variable lifetime
• static local variables: retain value between function executions
• default arguments: must be rightmost parameters/arguments
• overloading functions: same function name, but different formal parameter list
• function header, function body, function signature
• passing by-value vs. by-reference
• reference variables
• using a comma-separated list as a return value
• signifying global scope using the scope resolution operator

Recursion

Recursion is when a function calls itself either directly or indirectly. We will look at some examples in class.

• recursion exists to make the programmer's life easier
• any recursive algorithm can be rewritten using repetition structures
• recursion usually uses many computer resources such as processing cycles and memory, so iterative alternatives may be preferable if time critical or memory intensive situations
• testfunc11.cpp: recursive and non-recursive factorial functions
• testfunc17.cpp: recursive and non-recursive GCD functions
• testfunc16.cpp: recursive Towers of Hanoi solver

Demonstration programs

• testfunc01.cpp: creating and calling a simple function
• testfunc02.cpp: demo of a function with an argument
• testfunc03.cpp: demo of a function with multiple arguments and a return value
• testfunc04.cpp: demonstration of a simple useful function
• testfunc05.cpp: function to roll a die
• testfunc06.cpp: function to roll two dice
• testfunc07.cpp: demonstration of variable scope
• testfunc08.cpp: static local variables
• testfunc09.cpp: overloading functions
• testfunc11.cpp: recursive and non-recursive factorial functions
• testfunc12.cpp: passing arguments to functions by reference
• testfunc13.cpp: using default arguments to functions
• testfunc14.cpp: a useful data validation function for menus
• testutils.cpp: demonstrates usage of utils150.h file
• utils150.h version 2