Functions in C

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Functions in C

• Top Down Design with Functions

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Top-down Design

• Big picture first
• broken down into smaller pieces

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Can you tell what this is doing?

• int main ()
• rad get_a_number()
• display_input (rad)
• pie_area find_area (rad)
• pie_cost calccost (pie_area)
• draw_pie (rad, pie_cost)
• return 0

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Why use functions?

• easier for programmers to work together
• put details off while looking at big picture
• easier to reuse code
• easier testing and debugging

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Function Syntax

• Prototypes
• before function call
• usually after using namespace std
• Calls
• value returning
• void
• Definitions
• placement optional

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Two Parts of Function Definition

• int Cube(int n) heading
• body
• return n n n

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What is in a heading?
type of value returned
parameter list
name of function
int Cube (int n)
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Prototypes
A prototype looks like a heading but must end
with a semicolon. Its parameter list should
contain the types and names of each parameter
the names are optional but good to have! int
Cube(int value ) // Prototype
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Arguments / Parameters

• They are the interface between the function and
  the "outside world"
• have number, names, types
• Their number and types must match
• matching is done by position - first to first,
  second to second, etc.
• careful about using "input" and "output" in
  referring to parameters - NOT from the keyboard
  and to the screen!

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Functions

• Every C program must have a function called
  main
• Program execution always begins with function
  main
• Any other functions are subprograms that must be
  explicitly called

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Function Calls

• One function calls another by using the name of
  the called function followed by()s that enclose
  an argument list, which may be empty
• A function call temporarily transfers control
  from the calling function to the called function

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Function Semantics

• When a function is called
• Arguments and parameters matched (if any)
• Local variables and constants created
• space for return value allocated if not void
• Function code executed
• Return statement or end of code inside brace
  causes locals to be destroyed and control to
  return to calling statement
• If a value is to be returned, it is sent back

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Functions with no return value

• Syntax
• void return type
• can use return statement or no return statement
  at all
• Semantics of execution
• called in a statement by itself
• executes function code with arguments
• returns to next statement after call

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Functions with a return value

• Syntax
• return type of some builtin type
• must use return x statement where x is some
  value or variable of the given return type
• Semantics of execution
• called as part of larger statement
• executes function code with arguments
• returns a value to the calling statement

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• include ltiostreamgt
• int Cube(int) // prototype
• using namespace std
• void main()
• int yourNumber
• int myNumber
• yourNumber 14
• myNumber 9
• cout ltlt My Number ltlt myNumber
• cout ltlt its cube is ltlt Cube(myNumber) ltlt
  endl
• cout ltlt Your Number ltlt yourNumber
• cout ltlt its cube is ltlt Cube(yourNumber)
• ltlt endl
• arguments

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Return Values

• In C, a value-returning function returns one
  value of the type specified in its heading and
  prototype (called the return type)
• In contrast, a void-function does not return any
  value

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Example

• Write a void function called DisplayMessage(),
  which you can call from main(), to describe the
  pollution index value it receives as a parameter
• Your city describes a pollution index
• less than 35 as Pleasant,
• 35 through 60 as Unpleasant,
• and above 60 as Health Hazard

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• parameter
• void DisplayMessage(int index)
• if(index lt 35)
• cout ltlt Pleasant
• else if(index lt 60)
• cout ltlt Unpleasant
• else
• cout ltlt Health Hazard

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The Rest of the Program

• include ltiostreamgt
• void DisplayMessage(int polIndex) // Prototype
• using namespace std
• int main()
• int pollutionIndex
• cout ltlt Enter air pollution index
• cin gtgt pollutionIndex
• DisplayMessage( pollutionIndex ) // Call
• return 0
• argument

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Return Statement

• return // No value to return
• Is valid only in the body block of a void
  function
• Causes control to leave the function and
  immediately return to the calling block, leaving
  any subsequent statements in the function body
  unexecuted

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Value-Returning Functions

• include ltiostreamgt
• int Square(int) // Prototypes
• int Cube(int)
• using namespace std
• int main()
• cout ltlt The square of 27 is
• ltlt Square(27) ltlt endl
• cout ltlt The cube of 27 is
• ltlt Cube(27) ltlt endl
• return 0
• function calls

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Rest of Program

• int Square(int n) // Header and body
• return n n
• int Cube(int n) // Header and body
• return n n n

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Parameter List

• A parameter list is the means used for a function
  to share information with the block containing
  the call to the function.

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Classified by Location

• Arguments Parameters

Always appear in a function call within the
calling block
Always appear in the function heading, or
function prototype
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Scope

• "Where is this identifier known?"
• Parameters
• from header line of function to right closing
  brace
• Local variables
• from line of declaration inside function
  definition to right closing brace
• Global variables
• from line of declaration to end of FILE -
  includes all functions following the declaration

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Scope continued

• Local variables
• created every time the function runs
• initializations done every time they are created
• destroyed when the function returns control

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Scope continued

• Parameters
• name is known from header line until end of
  function body
• NAME does NOT have to match argument NAME
• if passed by value, gets memory allocated and
  copy of argument made
• if passed by reference, gets matched with space
  occupied by argument

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Scope continued

• Global variables
• declared outside of any function at all
• known from that point in file to end of FILE
• allocated space at start of execution of main
• destroyed when main function returns control
• may be "shadowed" by local variables with same
  name, so the global can't be accessed

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Scope continued

• Why are global variables BAD?
• cause "side effects" - allow a function to do
  something "behind your back"
• what a function can affect / change should always
  be documented in its header
• make it harder to reuse code - can't just pick up
  the code and copy it to another program
• make it harder for people to work on teams

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"Everything global"

• Do NOT be tempted to "make everything global" -
  it is a sure way to introduce bugs!!!!
• If a function header were "void PrintLine (int
  datavalue)" you would NOT expect it to change a
  variable called "totaldata", would you?

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Interface Design

• parameters are either "in", "out" or "in/out"
• value if data just coming IN to function
• reference or the return value if going OUT
• reference if coming IN and going back OUT

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A Parameter or a Local Variable

• How to decide
• ask yourself "does this information need to COME
  FROM some other function?" parameter
• "does this information need to GO TO some other
  function?" parameter or return value
• "does ONLY this function need to know about this
  data?" local

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Stubs and Drivers

• "Scaffolding"
• Stubs - top-down
• Drivers - bottom-up

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A Driver Program

• A driver program is a short main program whose
  only purpose is to test another program
• Write a driver for function GetRating()

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Questions

• Why is a function used for a task?
• To cut down on the amount of detail in your main
  program (encapsulation)
• Can one function call another function?
• Yes
• Can a function even call itself?
• Yes, that is called recursion it is very
  useful and requires special care in writing

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More Questions

• Does it make any difference what names you use
  for parameters?
• No just use them in function body
• Do parameter names and argument names have to be
  the same?
• No

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An Assertion

• An assertion is a truth-valued statement--one
  that is either true or false (not necessarily in
  C code)
• Examples
• studentCount gt 0
• sum is assigned count gt 0
• response y or n
• 0.0 lt deptSales lt 25000.0
• beta beta _at_ entry 2

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Preconditions and Postconditions

• A precondition is an assertion describing
  everything that the function requires to be true
  at the moment the function is invoked
• A postcondition describes the state at the moment
  the function finishes executing, providing the
  precondition is true
• The caller is responsible for ensuring the
  precondition, and the function code must ensure
  the postcondition

For example . . .
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Function with Postconditions

• char GetRating( char letter)
• // Precondition None
• // Postcondition User has been prompted to enter
  a letter
• // letter one of these input values
  E,G,A, or P
• cout ltlt Enter employee rating. ltlt endl
• cout ltlt Use E, G, A, or P
• cin gtgt letter
• while((letter ! E) (letter ! G)
  (letter ! A) (letter ! P))
• cout ltlt Rating invalid. Enter again
• cin gtgt letter
• return letter

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Documentation of Function

• Short comment at prototype
• Header comment longer
• purpose of function, using the names of all
  parameters and the return value if any
• comment code in body as usual
• pre and post conditions
• Sometimes put comment at closing brace that just
  has function name in it, makes it easier to match
  braces for the body