Functions and Function Prototypes

1
Functions and Function Prototypes

• Lesson 11

2
Agenda

• Able to use C built-in functions
• The cmath library
• The cstdlib library
• The cstring library
• Able to design and implement customised functions
• Able to create function prototypes

3
Functions

• Functions allow you to group commonly used code
  into a compact unit that can be used repeatedly.
• You have already encountered one function,
  main(), which is a special function called at the
  beginning of the program after all static and
  global variables have been initialized.
• Functions are the basics of modular programming
  and allow you to manage your program better.

4
Functions

• Before creating the function, use sometime to
  think about the following issues
• Name of the function
• Description of what the function does
• Description of parameter(s) to the function
• Description of the return value of the function

5
C Standard Libraries

• C comes with large number of built-in functions
  organised into sets of libraries.
• You can use them without knowing how they are
  written as long as you understand what they do
  and how to communicate with them.
• For example, there are useful mathematical
  functions in the math library. To use these
  functions, you have to
• Include the header file ltcmathgt
• double sqrt(double x)
• Example
• cout ltlt sqrt(900.0)
• sqrt (square root) function The preceding
  statement would print 30
• All functions in math library return a double

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6
C Standard Libraries
6
7
Using C Standard Library

• Related C standard functions are grouped
  together and organized into libraries.
• To use them, you have to include appropriate
  header files.
• How do you know what functions are available,
  what they do, how you can use them?
• References http//www.cplusplus.com/

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8
Using C Standard Library

• Many functions do come from (old) C libraries. In
  fact, most of functions are from old C libraries.
• On the references, you may see include ltmath.hgt.
• This is C style. Although its still supported,
  you should follow the new C standard way
  specifying the standard library header files,
  which is include ltcmathgt.
• All C headers are suffixed with c in C.
  ltstdlib.hgt becomes ltcstdlibgt, ltctype.hgt becomes
  ltcctypegt, and so on.

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9
Using C Standard Library

• Browse on any references of the standard library
  and make yourself familiar with kinds of
  functions available.
• As users of the functions, you do not have to
  know how they are internally written.
• You need to know
• what name is the function called,
• what a function does (description),
• what parameters it requires, and
• what it returns.
• This is called Information Hiding or Abstraction.

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10
Functions

• Declaring a Function
• ReturnType FunctionName(param1,param2,,paramN)
• //Do Something
• return ReturnTypeVal//Return will exit the
  function
• Calling a Function
• value FunctionName(param1,param2,,paramN)
• Note The argument and the parameter do not need
  to have the same name!

11
Function Creation Example 1

• In our program, we want to calculate the cube of
  a number often (nnn).
• To prevent typing mistake, you want to create a
  function that will calculate the cube for you.

12
Function Creation Example 1

• Thinking Process
• Name cubeOf
• Description The function will take an input and
  return the power 3 of that number.
• Parameters integer num
• Returns a large number (Cube returns huge
  number!)

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Function Creation Example 1

• Writing the Function
• long cubeOf(int num)
• return (num num num)

14
Function Creation Example 1

• Using cubeOf function
• include ltiostreamgt
• using namespace std
• long cubeOf(int no)
• return (nonono)
• int main()
• long output cubeOf(3) // Using cubeOf
• cout ltlt output
• return 0

15
Function Creation Example 2

• In our program, we have to print the following
  menu many times
• 1 - Buy Item
• 2 - Sell Item
• 3 - View Item
• 4 Exit Item

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Function Creation Example 2

• Thinking Process
• Name printMenu
• Description Print the menu
• Parameters none
• Returns none

17
Function Creation Example 2

• Writing the Function
• void printMenu()
• cout ltlt "1 Buy Item" ltlt endl
• cout ltlt "2 Sell Item" ltlt endl
• cout ltlt "3 View Item" ltlt endl
• cout ltlt "4 Exit Item" ltlt endl
• // Notice no return!

18
Function Creation Example 2

• Using the Function
• include ltiostreamgt
• using namespace std
• void printMenu()
• cout ltlt "1 Buy Item" ltlt endl
• cout ltlt "2 Sell Item" ltlt endl
• cout ltlt "3 View Item" ltlt endl
• cout ltlt "4 Exit Item" ltlt endl
• int main()
• printMenu()
• return 0

19
Function Prototypes

• In order to use your function, tell C/C the
  function exists before using it.
• In our previous example notice the function comes
  before where you call it
• include ltiostreamgt
• using namespace std
• long cubeOf(int num)
• return (num num num)
• int main()
• long output cubeOf(3)
• cout ltlt output
• return 0

20
Function Prototypes

• However it would not be working in the following
  case, when the function is declared in the order
  after you use the function
• include ltiostreamgt
• using namespace std
• int main(void)
• long output cubeOf(3) // ERROR!
• cout ltlt output
• return 0
• long cubeOf(int num)
• return (num num num)

21
Function Prototypes

• To solve that, you can create function prototypes
  to tell your C/C program that a function exists
  and is declared later in your program
• include ltiostreamgt
• using namespace std
• long cubeOf(int no) // Function Prototype
• int main(void)
• long output cubeOf(3) // Now Working
• stdcout ltlt output
• return 0
• long cubeOf(int no)
• return (nonono)