Functions and Program Structure

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Functions and Program Structure

• Chapter 4

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Introduction

• Functions break large computing tasks into
  smaller ones
• Appropriate functions hide details of operation
  from parts of the program that dont need to know
  about them
• Thus clarifying the whole
• And easing the pain of making changes
• C has been designed to make functions efficient
  and easy to use
• C programs generally consist of many small
  functions rather than a few big ones
• A program may reside in one or more source files

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Basics of Function

• return-type function-name (argument declarations)
  
• declarations and statements
• Various parts may be absent
• A minimal function is
• dummy( )
• Which does nothing and returns nothing
• Sometimes useful as a place holder during program
  development
• Functions declaration must be used

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Basics of Function Contd.

• A program is just a set of definitions of
  variables and functions
• Communication between the functions is by
• Arguments
• Values returned by the functions, and
• Through external variables
• The functions can occur in any order in the
  source file

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Basics of Function Contd.

• The source program can be split into multiple
  files
• The return statement is the mechanism for
  returning a value from the called function to its
  caller
• return expression

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Basics of Function Contd.

• The calling function is free to ignore the
  returned value
• There need be no expression after return
• In that case, no value is returned to the caller
• Control also returns to the caller with no value
  when execution "falls off the end", i.e.,

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External Variable

• External variables are globally accessible
• They provide an alternative to function arguments
  and return values for communicating between
  functions
• If a large number of variables must be shared
  among functions, external variables are more
  convenient and efficient than long argument list

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External Variable Contd.

• However, this reasoning should be applied with
  some caution
• External variables are also useful because of
  their greater scope and lifetime
• Internal variables come into existence when the
  function is ended
• If two functions must share some data, yet
  neither calls the other, it is often most
  convenient to use external variables rather than
  passed in and out via arguments

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External Variable Where needed?
double pop () if (sp gt 0) return val
--sp else printf (Under flow) return
0.0

• define MAXVAL 100
• int sp 0
• double val MAXVAL
• void push (double f)
• if (sp lt MAXVAL)
• val sp f
• else printf (Overflow)

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Scope Rules

• The source text of the program may be kept in
  several files
• The concerns are
• Variable declaration
• Properly declared
• Only one copy
• Initialization
• The scope of a name is that part of the program
  within which the name can be used

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Scope Rules Contd.

• For local variables
• For external variables or a function (from that
  point)
• If an external variable is to be referred to
• before it is defined, or
• if it is used in different source file from where
  it is used
• then extern declaration is mandatory
• No storage allocation

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Extern Declaration

• There must be only one definition of an external
  variable among all the files
• There may also be extern declaration in the file
  containing definition
• Array size must be specified with the definition
  but are optional with an extern declaration
• Initialization of an external variable goes only
  with definition

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Header Files

• If certain functions will be pretty common and be
  useful to many programs a head
• Then those functions can be kept in a header file
  (extension is .h) and put in the library
  directory
• Think about the necessity of the functions
  defined in stdio.h

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Static Variables

• Unlike automatic variables static variables
  remain in existence between functions calls
• Static variables provide private, permanent
  storage within a single function
• The static variable is initialized only the first
  time the block is entered
• static int i 0

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Register Variables

• register in x
• A register declaration advises the compiler that
  the variable will be heavily used
• Register variables are to be placed in machine
  registers
• Result in faster programs
• The index of the innermost loop can be register
  variable
• It is not possible to take the address of a
  register variable
• It specific restrictions on number and types of
  register variables vary from machine to machine

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Block Structure

• Variables can be declared in any block
• Variables declared in this way hide any
  identically named variables in outer block
• An automatic variable declared and initialized in
  a block is initialized each time the block is
  entered
• The static variable is initialized only the first
  time the block is entered
• Automatic variables, including formal parameters
  also hide external variables and functions of the
  same name

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Initialization

• Automatic and register variables
• External and static variables
• Initializing arrays

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Recursive Function

• A function may call itself
• When a function call itself recursively, each
  invocation gets a fresh set of all the automatic
  variables
• Recursion may provide no saving in storage nor
  will be faster
• Because of stack entry
• But recursive code is more compact and often much
  easier to write
• Recursion is especially convenient for
  recursively defined data structures (tree)

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Criteria of Recursive Function

• A terminating condition
• Recursive definition

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Factorial has a recursive definiton
int factr(int n) if(n 0) return 1
//terminating condition else //recursive
definition return nfactr(n-1)

• int facti(int n)
• int i, product 1
• for(i 2 i lt n i)
• product i
• return product

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Itoa
void itoa(int n, char s) static int
i if(n/10) itoa(n/10,s) //recursive
definition si n 10 '0' si 1
0 else si n 10
'0' //terminating return

• void itoa (int n, char s )
• if ((sign n) lt 0)
• n -n / make it positive /
• i 0
• do
• s i n 10 0
• while ((n / 10) gt 0)
• if (sign lt 0) s i -
• s i \0
• reverse (s)

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• int binsearch(int x, int v , int n)
• int low, high, mid
• low 0
• high n -1
• while(low lt high)
• mid (low high)/2
• if (x lt vmid)
• high mid 1
• else if(x gt vmid)
• low mid 1
• else return mid
• return -1

int bsearch(int x, int v, int low, int
high) static mid if(low gt high) return -1
mid (low high)/2 if(x lt vmid) return
bsearch(x, v,low,mid-1) else if(x gt
vmid) return bsearch(x, v, mid 1,
high) else return mid
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Chapter 4 Remaining

• The C Preprocessor

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Thank you very much for your prolonged patience.

• The classes for the next 4 weeks will be
  conducted by Dr. Riaz Ahmed