Functions

1
Functions

• Computer Programming I
• Dr. Tim Margush
• Mathematics, Science and Technology Department
• Farquhar Center - Nova Southeastern University

2
What is a Function?

• A function is a program unit representing the
  abstraction of some task
• Standard functions are found in the libraries
  included with most compilers
• Programmers are free to define their own
  functions to perform specialized tasks
• Functions are used via a call-return mechanism

3
Characteristics of a Function

• Declared in a function heading (prototype)
• double fabs(double) //usually in a header file
• Defined in a C or C source file
• May be written in another language
• Standard functions are precompiled and linked
  with your application
• Stored in a object file or library
• Source file is often not available

4
Declaring and Using a Function

• Functions must be declared before they are used
  in a program
• Declare via a function heading (prototype)
• Declare via function definition
• Functions are invoked using the function
  identifier and the function-call operator
• x ceil(y)
• showMoney(amount .90)

5
Function Heading or Prototype

• Declares an identifier as a function of a
  particular type and establishes its signature
• The signature of a function is its name and the
  list of types of its formal parameters
• string toUpper(string)
• double cubeRoot(double)
• float newPrice(float oldPrice, float discount)
• void showHex(double)
• void showHex(long)

6
Function Definition

• A function is defined by repeating the function
  heading and...
• Naming each formal parameter
• Providing a block to be executed when the
  function is called
• Function definitions can be in the same source
  file, or in a separately compiled file

7
Sample Declaration, Definition, and Function Call

• long round(double) //rounds to nearest integer
• ...
• long balance round(amount) //inside a
  function body
• ...
• long round(double x)//rounds to nearest integer
• double rounded_x floor(x0.5)
• return long(rounded_x) //cast to long

8
Standard Mathematical Functions

• Located in the cmath library
• include ltcmathgt //Math header file
• This brings all of the function headings into
  your file as part of the source program
• This declares (but does not define) the functions
• You must know the number and types of arguments
  required, and the return type of every function
  you use
• Consult help files, or browse the header file

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cmath Header File

• // cmath standard header
• if _MSC_VER gt 1000
• pragma once
• endif
• ifndef _CMATH_
• define _CMATH_
• ifdef _STD_USING
• undef _STD_USING
• include ltmath.hgt
• define _STD_USING
• else
• include ltmath.hgt
• endif / _STD_USING /
• endif / _CMATH_ /

• /
• math.h - definitions and declarations for math
  library
• _CRTIMP double __cdecl pow(double, double)
• _CRTIMP double __cdecl sin(double)
• _CRTIMP double __cdecl sinh(double)
• _CRTIMP double __cdecl tan(double)
• _CRTIMP double __cdecl tanh(double)
• _CRTIMP double __cdecl sqrt(double)

10
Pre/Post-conditions

• What must be true when a function call occurs
• What arguments are expected
• Number, types, and values
• What is true when the function call is complete
  (assuming preconditions were met)
• Changes to the program state
• Return value

11
Internal Documentation

• Each function should be documented
• Explain the purpose of function
• Give an overview of what it does
• Describe preconditions
• Explain what arguments are expected, etc
• Describe postconditions
• Tell what is changed by the execution of the
  function

12
External Documentation

• Gives more detailed information about a function
• MSDN Library
• Program specifications
• User guide

• Sqrt Calculates the square root. double sqrt(
  double x )
• Routine Required Header Compatibilitysqrt
  ltmath.hgt ANSI, Win 95, Win
  NT
• Return Value
• The sqrt function returns the square-root of x.
  If x is negative, sqrt returns an indefinite
  (same as a quiet NaN). You can modify error
  handling with _matherr.
• Parameter x Nonnegative floating-point value
• Example
• ...

13
Return Types

• Functions that represent a value must be defined
  to have the proper return type
• int ageNextYear(int) //return arg1
• Functions that represent an action (but not a
  value) should have a return type of void
• void clearScreen(long) //clear screen to color
  specified in arg

14
Void or Not Void

• Functions that return values are usually used in
  expressions
• cout ltlt "New age " ltlt ageNextYear(myAge)
• double dist sqrt(pow(a,2)pow(b,2))
• Functions that do not return values are usually
  statements of their own
• clearScreen(0x3C3FC2)

15
Common Errors

• Forgetting the function call operator
• clearScreen //does nothing
• Forgetting about the return value
• sqrt(4) //computes 2.0 but does nothing with it
• Forgetting an argument
• clearScreen() //will not compile
• Using the wrong type of argument
• sqrt("Tim") //will not compile

16
Argument - Parameter Correspondence

• Function calls must include the required number
  of arguments (in parenthesis)
• Arguments must match the number and types of the
  parameters
• Parameters are declared in the function
  definition (in the heading)
• Parameters are variables/objects that are
  initialized according to the arguments specified
  in the function call

17
Function Example

• include ltiostreamgt
• include ltstringgt
• using namespace std
• void display(string, int)
• int main()
• const string me "Tim"
• const int age 46
• display(me, age)
• display("Phil", age9)
• display("Dave", age11)
• return 0

• void display(string Name,
• int anAge)
• cout ltlt Name ltlt " is "
• cout ltlt anAge ltlt " years."
• cout ltlt endl
• return
• Tim is 46 years.
• Phil is 55 years.
• Dave is 57 years.

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Functions With No Parameters

• Use the void specifier in place of the parameter
  list
• int myAge(void) rather than int myAge()
• Be sure to include the function call operator
  when calling such a function
• cout ltlt myAge() not cout ltlt myAge