Introduction to Functions

• Functions are easy and complex
  • The basic syntax is not too difficult.
  • However, when to use functions and how to use them is sometimes challenging.
• The why is simple: To make code easier to read.
  • But it is also more complex.
• Chapter 4 and the beginning of chapter 8 provides the why of functions.
• So what should you read?
  • Chapter 8, but I am starting on 8.2
  • Chapter 4, 4.6 on, but especially 4.7
• Let's think about some functions we have seen
  • float abs(float )
  • float pow(float base, float exponent)
• Why do we build functions?
  • To make it simpler to read and understand the code.
  • To reduce repeated code.
  • To make reusable code.
  • To better organize code.
• Your last program, greater good, had severalinstances of duplicate code.
  • Validate a user's contribution was repeated three times.
  • Using functions would have reduced this.
• Functions are difficult because they are open and flexible.
• But all functions have several things in common:
  • The function must be declared before it is used.
    • In modern c++ this is done with a function prototype
    • These are declared before the main function.
  • The function must be defined
    • This is the function prototype plus the actual code for the function.
    • This is usually placed after the main function.
  • Functions are then called
    • This can be done anywhere after the function is declared.
    • In the main or in other functions.
    • Even in the function itself. (But do not do this yet)
• Function prototypes
  • In c/c++ function prototypes have
    • A return type, possibly void
    • A function name, an identifier
    • A list of 0 or more parameters
  • By EUP convention, function names
    • Start with an upper case letter.
    • Each word starts with an upper case letter.

    • ReadAWord
      OpenTheFile
      ComputeAverageIncome
                   

    • I like the function names to describe what the function does.
  • Parameters
    • Are ways to pass data into/out of a function.
    • They have a type.
    • They have a changeability
      • If we don't want to change it we pass by value.
      • If we do want to change it we pass by reference.
    • They have an identifier.
    • All data needed in a function should be passed as a parameter.
      • WE DO NOT USE GLOBAL VARIABLES.
      • WE DO NOT USE STATIC VARIABLES.
    • We will discuss this much more in the future.
  • Return value
    • This is nearly any c/c++ type
    • But is not required, it can be void
• Function declaration
  • This includes the function prototype.
  • But also includes code
  • This is just like main
    • You may have local variables
    • You will have executable code
    • You should/may have a return statement.
• Function calls
  • Include arguments
  • These must match the parameters in
    • Number of arguments/parameters
    • type of the arguments/parameters
    • variability of the arguments/parameters
      • You can pass a constant into a reference parameter.
  • Flow of control
    • When a function is called:
    • Flow of control passes to the called function.
    • The code in the function is executed
    • Then the flow of control returns calling function immediately after the function call.
• Let's write a program that computes n!

  • While the user wishes to compute a factorial
       Get a value for n between 0 and 10
       Compute n!
       Print The result.
       Ask the user if they wish to continue (Yes or No)