Functions

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Functions
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What is a function?
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Function
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Input(s)
Output
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Purpose of function

• Make the programmer's life easier

• Functions create new conceptual units
  programmer thinks the problem in terms of
  functions

• Write code once, and call it from many places
  - this insures consistency, and reduces the cost
  of writing and debugging.

• Allows code to be parameterized by passing
  values to it

• Functions can be collected in libraries and
  reused in other programs

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Built-in Functions

• Some of the common functions are programmed
  and stored in C library

• These functions can be called in any program

• These functions are called as built-in
  functions or library functions

• E.g. 1 sin(x) - returns sine of x, x in
  radians

• E.g. 2 clrscr() clears the screen

• Built-in mathematical functions

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User-defined functions

• C allows programmers to define their own
  functions relevant to their program

• These functions are called as user- defined
  functions or library functions

• To use a user-defined we must do the following

• Provide a function prototype

• Provide a function definition

• Call the function

• Example

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Prototypes

• Every function (except main) should be
  declared near the beginning of a program

• Contains return type (or void), name, and
  parameter names (optional) and types and end
  with a semicolon ()

• Tells C compiler in advance about some
  characteristics of a function

• return type

• number and the types of parameters

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Prototypes ensure

• The program correctly handles the function
  return value

• Checks that the program uses the correct
  number of parameters

• Checks that the program uses the correct type
  of parameters

• If not, it converts the parameters to the
  correct type, if possible

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Function definition

• Comprises of two parts

• function heading served as interface with
  the rest of the program

• function body consists of instructions to be
  executed inside the function

• General form
• return_type functionName( parameter_list )
• local_variable_declarations function-impleme
  ntation
• return value //value is of type function_type

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Function definition (Cont.)

• If the function returns a value then the type
  of that value must be specified in return_type

• This could be

• integer/floating point number

• structure

• pointer

• object (discuss later)

• void function function that does not return
  a value

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Function definition (Cont.)

• Parameter list specifies the types and
  number of formal parameters passed to the
  function

• Local Variables - All variables that are
  declared in a function are local to the function

• They can not be referenced from outside the
  function (local scope).

• Created when the function is called and are
  destroyed when the function terminates

• Parameters are like local variables (local
  scope, function lifetime)

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Function definition (Cont.)

• Function_implementation - consists of C
  executable statements that implement the effect
  of the function

• Return statement - stops execution and returns
  to the calling function

• Value of the return statement should match
  with the function_type.

• Optional for void functions

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Calling the function

• Invoke simply by

• using its name including the parentheses

• within parentheses parameters to be passed (if
  any) separated by comas (,)

• ends with semicolon ()

• E.g.

• printLine () //invokes a function without
  passing arguments

• printLineOfCharacters (, 25) //invokes a
  void function with parameters

• dist distance(x1, y1) //invokes a function
  with a return value

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Naming

• Name should be clear to the reader

• Choose a name that has a clear meaning
• E.g. "f" is not a good name

• Void functions are often more readable if a
  verb name is used that describes what it does
• E.g. printAverage

• Boolean functions should usually start with
  "is" or another word that suggests a yes/no
  answer
• E.g. isOdd

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Naming (Cont.)

• Name should be clear to the reader

• Start the name with lowercase and capitalize
  the first letter of each additional word in the
  name
• E.g. printLineOfCharacters

• Value-returning functions are often named
  after the value that they return
• E.g. square

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Top-down programming

• Write a short main function that calls
  additional functions to accomplish its task

• These functions too kept short, and if
  necessary it can call more functions

• Start with the main function, and write only
  functions which display only a small message

• Get this running, then expand one of second
  level function, get this running

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One page

• Good idea to keep functions shorter than one
  page or one screen

• then it is easy understand

• If one page is not enough space, then write
  additional functions to help perform the task
  (top-down programming).