Chapter 4 Subprograms: Functions for Problem Solving

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Chapter 4 Subprograms Functions for Problem
Solving
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Modular Programming

• Each section of the code has a specific purpose
• Each specific purpose may be reusable by other
  programs
• To do so, we create functions to be shared
• Similarly, within a program, we would like to
  employ the function concepts

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Declarations of Variables and Constants

• Local variables (constants) are variables used
  within only one function.
• Local variables should be declared in the
  function used
• Global variables (constants) are variables that
  can be used by all functions in a program
• Global variables should be declared after the
  prepecessor statements (include)

4
Functions

• All programs have a main function
• Functions should be grouped together and placed
  either before or after the main function
• My preference is after the main function
• However, since the main function will refer to
  other functions, the other function must be
  declared as a global variable

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Declaration of Functions

• ltreturn typegt ltfunction namegt(ltlist of
  parametersgt)
• Return type is the data type that will be
  returned to the calling function
• Valid entries are int, float, char, bool, or void
  (no return value)
• Function name is any valid identifier
• combination of letters, numbers, and underscores
  but start with a letter
• list of parameters are values to be passed into
  the function
• Consist of data type and identifier
• If more than one, separate by comma
• Example int cube(int x)

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Example

• float example_function(int x)
• BODY
• Return some_value

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• include ltiostream.hgt
• include ltiomanip.hgt
• //Main Function
• void main()
• //Declare Variables
• float length,width,area
• //Input Section
• coutltlt"Enter The Length and Width "
• cingtgtlengthgtgtwidth
• //Calculation Section
• arealengthwidth
• //Print Headings

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• include ltiostream.hgt
• include ltiomanip.hgt
• float length, width, area
• float get_record()
• //Main Function
• void main()
• get_record()
• //Calculation Section
• arealengthwidth
• //Print Headings
• coutltltsetw(12)ltlt"Length"ltltsetw(12)ltlt"Width
  "ltltsetw(12)ltlt"Area"ltltendl
• //Print Detail Lines
• coutltltsetprecision(2)ltltsetiosflags(iosfi
  xediosshowpoint)

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• //A Program to Cube an Integer
• includeltiostreamgt
• int cube(int)
• void main()
• int in_integer
• int cubed_integer
• coutltlt"Enter in Integer and hit Enter"ltltendl
• cingtgtin_integer
• cubed_integercube(in_integer)
• coutltltendlltlt"The cube of "ltltin_integerltlt" is
  "ltltcubed_integer
• int cube(int x)

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• //A Program to Cube an Integer
• includeltiostreamgt
• int cube(int)
• void main()
• int in_integer
• coutltlt"Enter in Integer and hit Enter"ltltendl
• cingtgtin_integer
• coutltltendlltlt"The cube of "ltltin_integerltlt" is
  "ltltcube(in_integer)
• int cube(int x)
• return xxx

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• include ltiostream.hgt
• include ltiomanip.hgt
• float length, width, area
• float get_record()
• float calculate_area(float x,float y)
• void print_heading()
• void print_detail()
• void main()
• get_record()
• calculate_area(length,width)
• print_heading()
• print_detail()
• float get_record()

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• float calculate_area(float x, float y)
• arealengthwidth
• return area
• void print_heading()
• coutltltsetw(12)ltlt"Length"ltltsetw(12)ltlt"Width"ltlts
  etw(12)ltlt"Area"ltltendl
• void print_detail()
• coutltltsetprecision(2)ltltsetiosflags(iosfixed
  iosshowpoint)
• coutltltsetw(12)ltltlengthltltsetw(12)ltltwidthltltsetw
  (12)ltltarealtltendl

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Declaration of Functions

• ltreturn typegt ltfunction namegt(ltlist of
  parametersgt)
• Return type is the data type that will be
  returned to the calling function
• Valid entries are int, float, char, bool, or void
  (no return value)
• Function name is any valid identifier
• combination of letters, numbers, and underscores
  but start with a letter
• list of parameters are values to be passed into
  the function
• Consist of data type and identifier
• If more than one, separate by comma
• Example int cube(int x)

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Reference Parameters

• A method to return values to the calling
  function.
• Lets examine the program for calculating the
  area of a rectangle.

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Area of a Rectangle

• includeltiostream.hgtincludeltiomanih.hgt
• void main()float length, width, area
• coutltltEnter the length\ncingtgtlengthcoutltlt\n
  Enter the width\n
• arealengthwidth
• coutltltsetiosflags(iosshowpointiosfixed)ltltsetp
  recision(2)coutltltlength ltltlengthltltendlcoutltlt
  width ltltwidthltltendlcoutltltarea ltltarealtltendl

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Area of a Rectangle

• get_data
• calc_area
• out_results

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Reference Parameters

• The syntax is
• data_type function_name (data_type
  parameter_name1,...)
• Examplevoid get_data(float length, float
  width)

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Area of a Rectangle

• includeltiostream.hgtincludeltiomanih.hgt
• void main()
• float length, width, area void
  get_data(float length, float width) float
  calc_area(float length, float width) void
  out_results(float length, float width,
  float area)
• get_data(length, width)
  calc_area(length, width)
  out_results(length, width, area)

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get_data

• void get_data(float length, float width)
  coutltltEnter the length\n cingtgtlength
  coutltlt\nEnter the width\n cingtgtwidth

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calc_area

• float calc_area(float length, float width)
  arealengthwidth return area

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out_results

• void out_results(float length, float width,
  float area)coutltltsetios
  flags(iosshowpointiosfixed)
  ltltsetprecision(2)coutltltlength
  ltltlengthltltendlcoutltltwidth ltltwidthltltendlcout
  ltltarea ltltarealtltendl

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4.6 Programmer-Defined Libraries
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Library Header File

• Need
• library header file
• library implementation file

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Header file

• extension is .h
• ifndef file_identifier
• define file_identifier
• function declaration(s)
• endif
• The ifndef...endifasks if the file identifier
  has been defined. If it has, the library file has
  already preprocessed the current inclusion and
  jumps to endif.
• Header file contains the name of the functions.

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Library Implementation File

• extension is .cxx
• include file_identifier
• file_identifier is the header file and must be in
  quotations.
• This file actually defines the function.

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Example - a switch program

• The header file
• my_input.h
• ifndef my_inputdefine my_inputvoid
  do_switch(int x)endif

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Example - a switch program

• The function file
• my_input.cxx
• include my_input.h void do_switch(int
  x)switch(x)case 1 coutltltThis is the first
  number\n breakcase 2 coutltltThis
  is the second number\n
  breakdefault coutltltThis number is out of
  range\n

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Example - a switch program

• include my_input.hvoid main ()int
  in_number0while(in_number!-999)coutltltInput
  an integer or -999 to quit\ncingtgtin_numberdo
  _switch(in_number)