Designing Programs with Functions

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Chapter 5

• Designing Programs with Functions

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Outline

• Predefined C functions - Library
• writing value-returning functions
• program design with value-returning functions
• void functions and program design
• Function calling other functions
• Function stubs
• SKIP section 5.7
• section 5.8

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Introduction to Functions

• A function is a named block of code
• Top-down (outside-in) design
• to divide a complicated problem into smaller
  subtasks and write a separate block of code fore
  each subtask
• as we think of how to build a program, think of
  blocks of code as functions to be implemented in
  detail later
• Code re-use
• Write the code once use it as many times as you
  need to
• Information hiding
• Without caring about the implementation details
• Protect data in a function from other functions
  mishandling

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Predefined functions - cmath Library

• cmath - Library of Mathematical Functions
• To use them, we must first include ltcmathgt
• Example Getting the square root of variable X
• Y sqrt (X)
• function call a statement or expression that
  transfers control to a function which performs
  its subtask.
• Evaluate expression in parentheses (the argument)
• Pass argument (in this case, Xs value) to
  function sqrt
• Store the result (return value) of sqrt into
  variable Y
• Function pow (power) 2 parameters
• We dont have an exponentiation operator in C
• cout ltlt pow (2.0, 8.0)
• Lots of other cmath functions (page 94 of text)

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Predefined functions - cmath Library
y sqrt(9) is a statement
sqrt (9) is an expression
int x, y x 2 y sqrt(9) cout ltlt y ltlt
endl y sqrt(10x 5) cout ltlt y ltlt endl

• 3
• 5
• 1.41421

function-name (argument1, argument2, . . .)
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Value returning Functions
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Writing Value-Returning Functions

• To write our own functions, we need 1) a
  declaration
• 2) a definition

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Writing Value-Returning Functions

• Function Declaration (prototype)

semicolon

• MUST be placed BEFORE main function
• specifies what the function accomplishes
• 1) parameters (with data type) used to receive
  data
• 2) the data type of the return value

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Writing Value-Returning Functions

• Function Definition (implementation)

NO semicolon

• specifies how the function performs its subtask
• 1) Header same as declaration without the
  semicolon
• 3) Body actual code that carries out the task,
  and returns the value to the point where the
  function was called
• It must have at least one return statement
• Of the form return value
• Causes control to return to caller, passing the
  value back to it

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Parameters vs. Arguments
parameters are variables that belong to the
function (in declaration and definition header),
used to receive data from the calling statement.
parameters
arguments appear in the function call, hold the
data to be passed to a function
arguments
parameters
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Parameter and Argument Matching

• A function call creates a memory cell for each
  parameter and then copies the value of the
  matching argument into that cell
• Arguments and parameters are matched from left to
  right based on their positions, i.e., the value
  of the first argument is passed (assigned) to the
  first parameter.

IMPORTANT!
The names of the arguments parameters do not
matter for this purpose Theyd better be of
compatible data types
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Value parameters passing by value

• What were doing here is passing by value
• Argument is evaluated
• Then a copy of that value is passed to the
  corresponding parameter of the called function
• If a function changes the value of one of its
  parameters, that change will stay local to the
  function
• it will NOT affect the arguments value back at
  the point of the call

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Scope of Variables
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Local Variables

• Variables declared within a function
• accessible only by statements within the function

Parameters are actually local variables
pre-filled with copied values from arguments
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Local variables
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Scope of variables

• The scope of a variable is the portion of the
  program in which statements can use them
• Scope for local variables and value parameters is
  limited to the body of that function
• Scope of (local) variables defined in main is
  limited to the body of main
• Global variables and constants are defined before
  main and can be used in all functions
• Read Section 9.4

include ltiostreamgt const double RATE
5.00 double function (int iHours)int main(
) double salary int hours salary
function(hours) double function(int
iHours) doulbe dSalary dSalary iHours
RATE return dSalary
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Program design with value-returning functions
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Program design with value-returning functions

• Two rules when designing a program
• Simplify the body of main in a way it calls
  functions to perform subtasks
• Modularize the program so that each piece of
  program corresponding to difference subtasks
  handled by separate functions
• Each section of assignment 2 can be organized as
• int main ( )
• double result
• // input subtask
• result function ( ) // calculation
  subtask
• // output subtask
• double function ( )

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Program design with value-returning functions

• int main ( )
• const int R_RATE 100, N_RATE 200
• int iResTuition, iNResTuition,
• int iNumCred
• char chResStatus
• // input subtask
• // calculation subtask
• if (chResStatus R)
• iResTuition CalTuition(iNumCred, R_RATE)
• else
• iNResTuition CalTuition(iNumCred, N_RATE)
  
• // output subtask
• return 0

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void Functions
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void (Non-Value Returning) Functions

• A non-value returning function has return type
  void
• void print_value(int d, int q)
• used for calculation and output the results
• It can (optional) have return statements, but not
  with a value attached
• Of the form return
• Causes control to return to caller, but without a
  value

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void (Non-Value Returning) Functions
declare a void function
call a void function
define a void function
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Parameterless Functions

• It could be a void function like
• void output( ) // declaration
• output( ) // call
• It could be a value-returning function like
• int calculate( ) // declaration
• result calculate( ) // call

Youve defined one before int main ()
return 0 main Is Just Another Function
called by the operating system Most programs
have lots of functions where to start? OS
starts executing a program by calling its main
function
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Functions Calling Other Functions
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Functions calling other functions

• Month Day Year Display
• 5 6 1998 05/06/1998
• 12 2 2000 12/06/2000

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Function Stubs

• A Tool For Use During Development
• Stub a function that has the correct interface,
  but doesnt do all the things its supposed to
• So you can easily write something to develop
  test other functions that call this one
• The stub functions return a phony value for
  testing

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Function Stubs
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• double payment (double A, double r, int m)
• double num, den
• num r A / 12
• den 1 pow (1 r/12, -m)
• return num/den

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• void decide_load (double payment, double
  ann_income)
• if ( ann_income / 12 gt payment)
• cout ltlt LOAN APPROVED! ltlt endl
• cout ltlt The monthly payment is ltlt payment
  ltlt endl
• else
• cout ltlt LOADN DENIED ltlt endl
• cout ltlt Have a nice day. ltlt endl

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Saving and Reusing Functions

• Modifying a library
• Creating your own library
• Using header (.h) files to contain all the
  function declarations (prototypes) for a library
• Compiled code for librarys functions gets
  included into the executable program by the
  linker

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Using Other Library Functions

• In library cctype
• Character classification manipulation
• isalpha (Char) isdigit (Char)
• toupper (Char) tolower (Char)
• In library stdlib
• Pseudo-random number generation
• Pseudo because computers are deterministic
• Function declaration int rand ()
• Generates a pseudorandom number between 0
  INT_MAX