Functions in C

1
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

4
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

5
Suppose you had some code that looked like this

• side2 pow(a, 2) pow (b, 2)
• side4 pow(c, 2) pow (side2, 2)
• side7 pow(b, 2) pow (side4, 2)
• side3 pow(5.3, 2) pow (side2, 2)
• cout ltlt pow (19.2, 2) pow(angle3, 2)
  angle9
• why not abstract that expression and only have to
  write it once?

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Function definition example

• //definition
• float myfun (float one, float two) // assumes
  //cmath has been included
• return pow(one, 2) pow (two, 2)
• could you name the parameters?

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Function call examples

• // the code before becomes
• side2 myfun (a, b)
• side4 myfun (c, side2)
• side7 myfun (b, side4)
• side3 myfun (5.3, side2)
• cout ltlt myfun (19.2, angle3) angle9
• could you name the arguments?

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Function prototype example

• float myfun (float one, float two)
• // assumes cmath has been included

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Value-returning Function Syntax

• The Definition
• has header
• "return type" "name" ( parameter list )
• has body - must be between braces
• body must have return x where x is a constant
  or variable or expression
• location - anywhere in file but NOT nested in
  another function's body!

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Value-returning Function Syntax

• The function call is an expression using
  "name ( argument list)"
• since the call is an expression, it must be part
  of a larger statement
• output
• assignment statement (RHS)
• if statement
• while statement
• location of a call - wherever needed in code

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Value-returning Function Syntax

• The Prototype
• just like header except ends with semicolon
  "float funA (int a, float b)"
• location near top of file
• prototypes go OUTSIDE of any function
  definitions! (not inside a function)
• parameter names are optional but a good idea!
  "int myfun (int, int, int)" is mysterious!

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Value-returning Function Semantics

• A function is a control structure so how does it
  change the order of execution?
• Assume execution is happening at the statement
  below
• x myfun (5.0, angle3) 17.2
• steps that happen are described on next two
  slides

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Value-returning Function Semantics x myfun
(5.0, angle3) 17.2

• 1. The right hand side of assignment statement
  must be evaluated
• 2. In order to do that, function call must be
  evaluated before the multiplication
• 3. Execution of this statement is paused
• 4. Arguments are copied into other memory
  locations for parameters
• 5.0 to one, angle3 to two
• 5. Any local variables declared are given space

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Value-returning Function Semantics x myfun
(5.0, angle3) 17.2

• 6. Execution continues with the body of the
  function definition. The calculation takes place
  (including calls to pow) until one value results
• 7. The return value is prepared by placing in
  special memory location for ret value
• 8. Local variables and the copies made for the
  parameters are destroyed in memory

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Value-returning Function Semantics x myfun
(5.0, angle3) 17.2

• 9. Execution picks up at the statement that was
  paused in step 3 and finishes the assignment
  statement

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Some important points about function call
semantics

• Arguments and parameters are matched up by the
  compiler
• as to type - corresponding args and parms must
  match or be able to be converted
• as to quantity - must have same number of args
  and parms
• if these matchings don't happen correctly, you
  get a syntax error

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Some important points about function call
semantics

• Note that NAMES of arguments and parameters do
  NOT have to match!
• Just because a function is defined in a program,
  does not mean that it WILL always be executed -
  if it is not called by some statement it will not
  be done
• Arguments are in function calls, Parameters are
  in function definitions or prototypes

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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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Arguments / Parameters

• They are the interface between the function and
  the "outside world"
• 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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What is "an overloaded function"?

• You can have more than one function with the same
  name as long as the parameter list is different
  for each
• The compiler figures out which one you mean by
  the arguments you send
• If it can't distinguish which one you mean, then
  you get an error
• Usually an error message with this phrase in it
  means that you got the argument list wrong. Check
  types and number of arguments to see if they
  match parms

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Value-Returning Functions

• include ltiostreamgt
• int Square(int n) // Prototypes
• int Cube(int n)
• 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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22
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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Void function semantics

• Simpler than value-returning but similar
• 1. call is from a stand-alone statement
• 2. calling function paused at this point
• 3. arguments are copied to parameters
• 4. matching process takes place
• 5. control transfers to code of function
  definition

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Void function semantics (contd)

• 6. if any local variables declared, they get
  space
• 7. code of function body executed
• 8. when end of body or a return statement
  encountered, prepare to return
• Destroy locals and copies of arguments
• 9. Return control to statement AFTER call

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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 point of declaration in file to end of
  FILE
• allocated space at start of execution of main
• destroyed when main function returns control to
  OS
• 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 in 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? with a
  global variable it can!

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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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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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Documentation of Functions

• 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