Functions

1
Functions
2
Subprograms

• A program within a program
• A module
• code written to define one well-defined subtask
• Benefits of modularity
• Easier to test, debug, and correct
• bottom up testing - independent testing of
  modules
• Readability
• Reusability
• Maintainability

3
Structured Programming

• Subprograms are an important element in
  structured programming
• Emphasis is on the flow of control between
  modules and how the modules communicate
• Easy to distribute work between multiple
  developers
• This skill can be applied to other languages you
  wish to learn
• Examples of some other structured programming
  languages C, Java, Lisp, Smalltalk, Visual Basic

4
Functions

• Subprograms in C
• 3 components of functions
• function declaration/prototype/signature
• function call
• function definition, function body with the header

5
Function Declarations

• ltreturn typegt ltfunction namegt (ltlist of
  argumentsgt)
• ltreturn typegt is any valid type
• this specifies the type of the value returned by
  the function, if any
• this is also specifies the type of the function
• ltfunction namegt is any valid identifier
• make this descriptive
• remember verbNoun suggestion from Style Guide
  e.g. computeAverage()
• ltlist of argumentsgt
• these are called parameters
• this specifies any values that are needed by the
  function
• it is ok to just specify the type and not assign
  a name in a function declaration
• float computeAverage(int, int, int)
• float computeAverage(int first, int second, int
  third)
• just like variables, functions must be declared
  before they are used

6
Function Call

• ltreturn variable gt FunctionName(ltparameter
  listgt)
• ltreturn variablegt is only appropriate if the
  function is of type other than void
• int selection GetMenuChoice()
• DisplayMenu()
• float sqrt pow(x, 0.5)

7
Function Definition

• The function heading plus its implementation
• float computeAverage(int first, int second, int
  third)
• float retVal 0
• retVal (first second third) / 3
• return retVal

8
Library Functions

• Functions that are used so frequently that they
  are provided for you by the compiler
• To call/invoke a library function
• functionName(ltparameter listgt)
• These functions are declared in the appropriate
  header files, e.g. include ltmath.hgt
• there is no need to provide any additional
  declarations for these, it would actually be an
  error
• Return values
• float answer pow(3,4) // answer now contains
  the value 81
• To find out how to use these functions
• look in the header file where it is defined for
  its function signature
• look at the manual pages or online help for that
  function
• man pow

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Library Function Examples

• Mathematics functions
• pow, sqrt
• found in math.h
• Trigonometric functions
• sin, cos, tan, acos, asin, atan
• also found in math.h
• String manipulation functions
• strcat, strcpy
• found in string.h

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User-defined Functions

• Need for this arises because library functions do
  not provide you with all the functionality you
  desire
• float cube(float x)
• return (xxx)
• float cube(float x)
• return pow(x,3)
• Your program has specific needs and functionality

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Function Components Example
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Functions without Returned Values

• Return type of void
• Does some work and doesnt need to return
  anything
• void displayResults(int numStudents, float
  average)
• Needs to return multiple values
• void getInput(int first, int second, int
  third)

13
Functions without Parameters

• Has a parameter list of void
• Does work without requiring input
• void displayHeader(void)
• Requires no input, but returns a single value
• int getSingleInput(void)

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Parameters/Arguments

• Necessary so values can be passed to and from
  functions
• Passed by value
• this is the default
• input only
• Passed by reference
• for anyone who knows C, this is something C does
  not have
• input and output

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Terms

• Formal parameter
• the symbolic name for the parameter used in the
  function definition
• Actual parameter
• the symbolic name for the parameter used in the
  function call
• int area(int length, int width) return length
  width
• int result area(x,y)
• length and width are the formal parameters
• x and y are the actual parameters
• Allocate
• to set aside a chunk of memory
• this happens every time you declare a variable
• Deallocate
• to release a chunk of memory
• this happens every time you leave a block

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Value Parameters

• The actual value of the parameter is copied and
  used in the function
• What does this mean?
• A new chunk of memory is allocated(created) for
  the size of the parameter
• The value of the parameter is copied into this
  chunk of memory
• this is called the local copy
• Any changes that you make to the local copy will
  be lost when the function is exited unless you
  return the value
• the memory is deallocated(destroyed)
• Use value parameters when you do not intend to
  modify the values of the parameters in the
  function

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Reference Parameters

• A reference to the value of the parameter is used
  by the function
• Whats a reference?
• A pointer to the chunk of memory where the value
  is located
• Whats the effect?
• Any changes that you make to the parameter will
  still be visible after the function is exited
• No copying takes place
• Memory is allocated for a pointer
• Use reference parameters when your function needs
  to modify the values
• Use if you need to return more than one value

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More on Reference Parameters

• Uses the symbol
• // Function getData
• void getData(int length, int width)
• cout ltlt Enter a value for the length
• cin gtgt length
• cout ltlt Enter a value for the width
• cin gtgt width
• Oftentimes, functions using reference parameters
  have type void

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Program Design Using Functions

• void main(void)
• getData()
• resultType result computeResults()
• displayResults(result)

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Function Overloading

• You can create multiple functions with the same
  namethis is called overloading
• you must vary either the number of arguments or
  the types of arguments
• cannot overload by permutating the return type
  only
• char append(char dest, char src)
• char append(char dest, char src)

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Default Function Arguments

• As an alternative to overloading, you can provide
  default values for function arguments
• Provide default value in the function
  declarationNOT in the function definition
• int increment(int value, int step1)
• int increment(int value, int step)
• return valuestep
• if you choose to default an argument, you must
  default all arguments following it in the
  parameter list

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Errors with Functions

• Parameter mismatches
• void printResults(float root1)
• printResults(root1, root2)
• Forgetting to return a value if a return value is
  required
• int printResults(float root1, float root2)
• cout ltlt I should be printing out results ltlt
  endl
• Passing by value when reference is necessary
• run time error
• void getData(float a, float b, float c)

23
Function Miscellany

• Interface
• how functions communicate with main and with
  other functions
• a function signature tells other functions,
  including main, how to interface with it
• You may use a function call to determine whether
  to enter a block
• if (IsLeapYear(year))
• cout ltlt Its a leap year! ltlt endl
• You may cout the results of a function
• cout ltlt pow(2,8)
• You may nest function calls within function calls
• result pow(3, pow(3,3)) // tower function