Lecture 7 User Defined Functions

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Lecture 7User Defined Functions

• ENGR17 Engineering Programming
• Section 1
• Fall 2001

9/28/01
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Outline

• Creating and invoking void functions
• Passing information by value
• Passing information by reference
• Creating a function that returns a value
• Passing a string to a function

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Programmer-Defined Function

• Can be either
• Value-returning function
• Void function
• Called in exactly the same way as a built-in
  function
• Void Function
• A function that does not return a value
• Could be used to display information
• Could be used to output to a file
• And other uses

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Example Program

• Sample OutputEnter a number 4-----------------
  ------------------------------------------------S
  quare and Square Root CalculationsSquare
  16Square Root 2Press any key to continue_

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Void Function Definition

• Defines the functions task
• Is entered in programmer-defined section of
  program
• Uses same naming rules as naming variables
• Function header begins with keyword void
• Enclose statements that comprise the function in
  a set of braces
• Formal parameter
• Each item of information received by a function

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Function Definition
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Calling a Void Function

• Include a function definition and function
  call(s) in a program
• Enter function prototype above the main function
• Function prototype and function definitions
  header must begin with keyword void
• Use the functions name and actual arguments, if
  any, as a separate statement in the program

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Calling a Void Function
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Calling a Void Function
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Using Function Prototypes

• Statements that tell C the following
• The functions name
• The data type of its return value
• The data type of each of its formal parameters

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3 Parts of a Function

• include ltiostream.hgt
• include ltmath.hgt
• // function prototypes
• void displayLine()
• void main ()
• //declare and initialize variables
• float numberInput (float) 0.0
• displayLine()
• void displayLine()

1. Prototype
2. Call
3. Definition
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Passing Information to a Function

• Include one or more actual arguments in the
  function calls argumentlist
• Typically, information passed to a function is
  one of the following
• A literal constant
• The contents of a variable
• The address of a variable in memory
• Used when the function needs to process
  information

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Passing Information to a Function

• Allow a function to receive information passed to
  it by including formal parameter(s) in function
  headers parameterlist
• Data type and name of formal parameters must be
  explicitly stated in function header
• Data type of formal parameters must match data
  type of actual arguments
• Order in which formal parameters are listed in
  function header must match order of actual
  arguments listed in the function

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Passing Information to a Function
Argument List
Parameter List
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Passing Information to a Function
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The Scope of a Variable

• Indicates which portions of the program can use
  the variable
• Is determined by where the variable is declared
  in the program
• Local
• Declared in a statement block known only to
  function or statement block in which they are
  declared
• Global (avoid using)
• Declared outside of any function in the program
• Allow unintentional errors
• Lifetime of a Variable
• Indicates how long the variable remains in the
  computers memory

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Passing Variables by Value

• C passes only the contents of the variable to
  the receiving function
• Receiving function does not have access to the
  variable in memory, so it cannot change the
  variables contents
• Used when the receiving function needs to know,
  but not change, the value stored inside the
  variable
• Unless specified otherwise, all variables in C
  are passed by value

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Passing Variables by Reference

• C passes the variables address in memory
• Contents of the variable can be changed by the
  receiving function
• Used when the receiving function needs to change
  the contents of the variable
• Must include an ampersand, called the address-of
  operator, before the corresponding formal
  parameters name in the receiving functions
  parameterlist

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Pass Variables by Value and Reference

• To pass the three variables, modify the
  following
• Function prototype
• Function call
• Function definition

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Pass Variables by Value and Reference
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Pass Variables by Value and Reference
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Pass Variables by Value and Reference
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Pass Variables by Value and Reference
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Summary of Void Functions

• Creating and invoking void functions
• Passing information by value
• Passing information by reference
• Variable Scope

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Value-returning Functions

• Similar to creating and invoking a void function
• Enter same three items of information
• A function prototype
• One or more function calls
• A function definition

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Value-returning Functions

• Function prototype and function definitions
  header must begin with a data type
• Function definition must end with a return
  statement that returns a value to the caller
• A call to a value-returning function does not
  typically appear as a statement by itself, but as
  a part of another statement
• Can return only one value

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Value-returning Functions
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Value-returning vs. Void Functions
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continued
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Value-returning Functions
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Passing a String to a Function

• Strings are always passed by reference
• Thus, strings can be modified in the function
• Examplevoid main () char first50
  Jon char last50 Smith char name50
  make_string(first, last, name)void
  make_string(char first , char last , char
  name ) strcpy(name, last) strcat(name, ,
  ) strcat(name, first)

Dont need string lengths in header
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Summary of Functions

• Creating and invoking void functions
• Passing information by value
• Passing information by reference
• Variable Scope
• Creating a function that returns a value
• Passing a string to a function

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Questions

• What comes before main() when you use functions?
• Function prototype
• What is within main() when you use a function?
• Function call
• What is after main() when you use a function?
• Function definition
• What is the name for the information within the
  () of a function call?
• Argument list
• What is the name for the information within the
  () of a function header in function definition?
• Parameter list

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Questions

• Give the function header for a void function
  foo with no arguments
• void foo(void)
• Give the function header for a function foo
  that returns and integer but has no arguments
• int foo(void)
• Give the function header for a function foo
  that returns and integer and is passed the
  integer a by value
• int foo(int a)
• Give the function header for a void function
  foo that is passed the float a by reference
  and the char b by value
• void foo(float a, char b)

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Example

• Rewrite Lab01 using functions
• Design a program to compute the area and
  perimeter of a rectangle. The user will input the
  height and width (in meters) of the rectangle and
  the program will print the area and perimeter to
  the screen.
• Use a value returning function with pass by value
  to calculate the area
• Use a void function with pass by reference and
  pass by value to calculate the perimeter

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Code (1)

• include ltiostream.hgt
• float calc_area(float height, float width)
• void calc_perimeter(float perimeter, float
  height, float width)
• int main ()
• float height
• float width
• float area
• float perimeter
• // get height and width from user
• cout ltlt "Enter height "
• cin gtgt height
• cout ltlt "Enter width "
• cin gtgt width
• // calculate area
• area calc_area(height,width)

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Code (2)

• float calc_area(float height, float width)
• return height width
• void calc_perimeter(float perimeter, float
  height, float width)
• perimeter 2height 2width