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LECTURE 9

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Local Variables

Variables declared in a function: Are localto that function, they cannot be used

from outside the function

Have the function as their scope

Variables declared in the main part of aprogram: Are local to the main part of the program, they

cannot be used from outside the main part Have the main part as their scope

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Global Constants

Global Named Constant Available to more than one function as well as the

main part of the program eclared outside any function body

eclared outside the main function body eclared before any function that uses it

!"ample: const double #$ % &'1(1)*+ double volumedouble-+

int main- ./0 #$ is available to the main function and to function

volume

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Global Variables Global Variable

2arely used when more than one function must use acommon variable eclared 3ust li4e a global constant e"cept const

is not used Generally ma4e programs more difficult to

understand and maintain

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Storage Classes Attributes of variable includes

name, type, si5e and value

!ach identifier in a program also has other attributes

including Storage class

How long variable e"ists in memory

Scope 6here variable can be referenced in program

Linkage 7or multiple sourcefile program see Ch' 8-, which files can use it

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Storage Classes C99 provides five storage class specifiers

Auto

Register

Extern

Mutable Static

An identifiers storage class specifier helps determine its ;torage class, scope and lin4age

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Automatic Storage Classes =ey words autoand registerare used to declare variables of

automatic storage class

Variable created when program enters its bloc4

Variable e"it while the bloc4 is active

Variable destroyed when program leaves bloc4

>nly local variables of functions can be automatic

Automatic by default

4eyword autoe"plicitly declares automatic auto double x, y; /*indicates x and y are local

variables of automatic storage class, exist only in

the body of function in which definition appear*/

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Automatic storage class

registerkeyword Hint to place variable in highspeed register

Good for oftenused items loop counters-

>ften unnecessary, compiler optimi5es

;pecify either registeror auto, not both

register int counter = 1; /*integer variable

counter be placed in one of the computers

registers, counter is initialized to 1*/

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Static storage class =ey words externand staticare used to declare identifiers for

variables and functions of static storage class

Variables e"ist for entire program 7or functions, name e"ists for entire program

?ay not be accessible, scope rules still apply more later-

statickeyword @ocal variables in function declared with 4eyword static

=eeps value between function calls

>nly 4nown in own function

;tatic local variables retain their values when function is e"ited

static int count = 1; /* declare local variable count to be

static and initialized to 1

@ocal variable $nitiali5ed to 0by default

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Static storage class

externkeyword !"ternal identifier

efault for global variablesfunctions names

Global: variable declaration outside of a function bloc4

=nown in any function that comes after it

Global variables retain value through out the e"ecution ofprogram

Generally the use of global variable should be avoided e"ceptin certain situations with uniBue performance reBuirement

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Scope Rules

Scope #ortion of program where identifier can be

used

;copes for an identifier are 7unction scope

7ile scope

loc4 scope

7unction prototype scope

Class scope discuss later, chapter 8-

namespace scope discuss later, chapter D1-

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Scope Rules

7ile scope efined outside a function, 4nown in all functions

Global variables, function definitions and prototypes allhave file scope

7unction scope Can only be referenced inside function in which they

appear, can not be referenced outside function body

>nly labels are the identifiers with function scope labels are identifiers with a colon e'g' case: as used in switch-

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Scope Rules

loc4 scope $dentifiers declared inside the bloc4

egins at declaration, ends at right brace } of thebloc4 Can only be referenced in this range

@ocal variables, function parameters

$n case of nested bloc4s and identifiers in inner andouter bloc4 have same name $dentifier in the outer bloc4 is hidden until the inner bloc4

terminates

staticvariables still have bloc4 scope ;torage duration does not effect the scope of bloc4

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Scope Rules

7unctionprototype scope

>nly identifiers with this scope are those used in#arameter list of prototype

Names in function prototype optional, only type reBuired Compiler ignores the name if used in parameter list of

functionprototype

$n a single prototype, name can be used once

$dentifier used in function prototype can be reusedelsewhere in the program

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A scoping example program

Ne"t we are going to discuss a program that willdemonstrates the scoping issues with

Global variables

Automatic local variables

;tatic local variables

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A scoping example program

Program description

Globalvariable xis declared and initiali5ed to 1, hidden in anybloc4 or function with a variable named xdeclared in it

$n main local variable xis declared and initiali5ed to andprinted to show that global xis hidden in main E%) is printed to show that global " is hidden in main

New bloc4 defined in main with localvariable xinitiali5ed to !,printed to show it hides xin outer bloc4 of main, "%Fautomatically destroyed when bloc4 is e"ited #rinted to show it hides " in outer bloc4 of main

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A scoping example program 7unctionuseLocaldefines automaticvariable x " #' calling

useLocal prints x$increment and then again print x !ach time function is called auto variable is recreated and

initiali5ed to #

%seStaticLocaldeclare staticvariable x " 0local variabledeclared static retain their value even when out of scope'calling %seStaticLocalprints x$increment and then againprint x

useGlobaldoes not declare any variable so it uses the globalx' Calling &unctionprints globalvariable, multiply it by 10,printed again before e"iting function ne"t time it is called, global variable has its modified value i'e' 10

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A scoping example program

7inally the program prints the local variable " in main again toshow

None of the function calls modified the value of x

ecause the function referred to variables in other scopes

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C++ code1 7ig' &'1D: fig&1D'cpp2 A scoping e"ample'3 IincludeJiostreamK4

5 usingstd::cout+6 usingstd::endl+7

8 voiduse@ocal void-+ function prototype9 voiduse;tatic@ocal void-+ function prototype10 voiduseGlobal void-+ function prototype

1112 int" % 1+ global variable13

14 intmain-15 .16 int" % )+ local variable to main17

18 cout JJ Llocal " in mainMs outer scope is LJJ " JJ endl+19

20 . start new scope21

22 int" % F+23

24 cout JJ Llocal " in mainMs inner scope is LJJ " JJ endl+25

26 0 end new scope27

28 cout JJLlocal " in mainMs outer scope is LJJ " JJ endl+29

Declared outside of function;

global variable with file

scope.Local variable with functionscope.

Create a new block, giving x

block scope. When the block

ends, this xis destroyed.

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C++ code30 use@ocal-+ use@ocal has local "

31 use;tatic@ocal-+ use;tatic@ocal has static local "

32 useGlobal-+ useGlobal uses global "

33 use@ocal-+ use@ocal reinitiali5es its local "

34 use;tatic@ocal-+ static local " retains its prior value

35 useGlobal-+ global " also retains its value

36

37 cout JJ Lnlocal " in main is LJJ " JJ endl+

38

39 return+ indicates successful termination40

41 0 end main

42

43 use@ocal reinitiali5es local variable " during each call

44 voiduse@ocal void-

45 .

46 int" % D)+ initiali5ed each time use@ocal is called

47

48cout JJ endl JJ Llocal " is L JJ "

49 JJ L on entering use@ocalLJJ endl+

50 99"+

51 cout JJ Llocal " is LJJ "

52 JJ L on e"iting use@ocalLJJ endl+

53

54 0 end function use@ocal

55

Automatic variable local

variable of function!. "his isdestroyed when the function

e#its, and reinitiali$ed when

the function begins.

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C++ code56 use;tatic@ocal initiali5es static local variable " only the57 first time the function is called+ value of " is saved58 between calls to this function59 voiduse;tatic@ocal void-

60 .61 initiali5ed only first time use;tatic@ocal is called62 static int" % )+63

64 cout JJ endl JJ Llocal static " is L JJ "65 JJ L on entering use;tatic@ocalLJJ endl+

66 99"+67 cout JJ Llocal static " is LJJ "68 JJ L on e"iting use;tatic@ocalLJJ endl+69

70 0 end function use;tatic@ocal

71

72 useGlobal modifies global variable " during each call73 voiduseGlobal void-74 .75 cout JJ endl JJ Lglobal " is L JJ "

76 JJ L on entering useGlobalL JJ endl+77 " O% 1+

78 cout JJ Lglobal " is LJJ "79 JJ L on e"iting useGlobalL JJ endl+80

81 0 end function useGlobal

%tatic local variable of

function; it is initiali$ed only

once, and retains its value

between function calls.

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Output

Compare this output to the description of the programon slides D and D1