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ESC101NFundamentals of Computing

Arnab Bhattacharya

[email protected]

Indian Institute of Technology, Kanpurhttp://www.iitk.ac.in/esc101/

1st

semester, 2010-11

Tue, Wed, Fri 0800-0900 at L7

Arnab Bhattacharya ([email protected]) ESC101N 2010 I 1 / 18
http://localhost/var/www/apps/conversion/tmp/scratch_3/[email protected]://www.iitk.ac.in/esc101/http://www.iitk.ac.in/esc101/http://localhost/var/www/apps/conversion/tmp/scratch_3/[email protected]://find/


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Sizes of variables

The size of a variable can be found out using the sizeof function

p r i n t f ( % d \ n , s i z e o f ( int ) ) ;

In this machinechar: 1 byteint: 4 bytesfloat: 4 bytes

double: 8 bytesWhy are these required?

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Sizes of variables

The size of a variable can be found out using the sizeof function

p r i n t f ( % d \ n , s i z e o f ( int ) ) ;

In this machinechar: 1 byteint: 4 bytesfloat: 4 bytes

double: 8 bytesWhy are these required?

To know the limitsType Minimum limit Maximum limit

char -128 127int -2147483648 2147483647

Type Maximum precision Minimum limit Maximum limit

float 1.175494E-38 -3.403823E38 3.403823E38double 2.225074E-308 -1.798693E308 1.798693E308

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Sizes of variables

# i n c l u d e < s t d i o . h >

in t m a i n ( ){char c ;p r in t f (" S iz e o f c h ar a ct e r = % d b yt es \ n " , sizeof ( c ) ) ;

int i ;p r in t f (" S iz e o f i nt = % d b yt e s \n " , sizeof ( i ) ) ;

float f ;p r in t f (" S iz e o f f lo at = % d b yt es \ n " , sizeof ( f ) ) ;

double d ;p r in t f (" S iz e o f d o ub le = % d b y te s \ n" , sizeof ( d ) ) ;

}

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Limits of variables

# i n c l u d e < s t d i o . h ># i n c l u d e < l i m i t s . h > / / f or l i mi ts o n i n te g er t yp es

# i n c l u d e < float .h >/ / f or l i m it s o n f l oa t in g - p o i nt t y pe s

void m a i n ( ){

p r in t f (" c ha r s t or es v al u es f ro m % d t o % d \ n" , C HA R_ MI N , C H AR _ MA X ) ;p r i n t f ( " \ n " ) ;p r in t f (" i nt s to r es v a lu e s f ro m % d t o % d \ n" , I NT _M IN , I N T_ M AX ) ;p r i n t f ( " \ n " ) ;p r in t f (" S m al l es t n on - z e r o v al ue o f t yp e f lo at i s % e \ n" , F L T_ M IN ) ;p r in t f (" L a rg es t v al ue o f t yp e f lo at i s % e \ n" , F L T_ MA X ) ;p r in t f (" S m al l es t v al u e o f t yp e f lo at i s % e \ n" , - F L T_ M AX ) ;p r in t f (" S m al l es t n on - z e r o a d di t io n v al ue o f t yp e f lo at i s % e \ n" , F L T_ E PS I LO N ) ;p r i n t f ( " \ n " ) ;p r in t f (" S m al l es t n on - z e r o v al ue o f t yp e d o ub le i s % e \ n" , D B L_ MI N ) ;p r in t f (" L a rg es t v al ue o f t yp e d o ub l e i s % e \ n" , D B L_ M AX ) ;p r in t f (" S m al l es t v al u e o f t yp e d ou b le i s % e \ n" , - D B L_ MA X ) ;p r in t f (" S m al l es t n on - z e r o a d di t io n v al ue o f t yp e d o ub l e i s % e \ n" , D B L_ E PS I LO N ) ;p r i n t f ( " \ n " ) ;

}

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Memory model

Memory is a list of bytes, each having a particular index called address

c

10

i

7 ...4

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Memory model


c

10

i

7 ...4

All variables are stored in memory

Depending on the size, contiguous bytes are occupied

The address of a variable is the first byte where it is stored

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Memory model


c

10

i

7 ...4

All variables are stored in memory

Depending on the size, contiguous bytes are occupied

The address of a variable is the first byte where it is stored

int i ;

char c ;

Address of i is 1

It occupies bytes 1 through 4

Address of c is 7

It only occupies byte 7

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Pointer

A pointer is a variable that stores the address of another variable

p...i

10 ... 49

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Pointer


p...i

10 ... 49

The type of a pointer depends on the type of the variable it points to

A pointer is denoted by the symbol *int i ; // i is a v a r i a b l e of type int

int * p ; // p is a p o i nt e r to int

The address of a variable is denoted by the symbol &

p = & i ; // p is now 1 ( in fi g ur e )

p stores the address of i, i.e., &i

*p denotes the content pointed by p, i.e., i

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Pointer


p...i

10 ... 49

The type of a pointer depends on the type of the variable it points to

A pointer is denoted by the symbol *int i ; // i is a v a r i a b l e of type int

int * p ; // p is a p o i nt e r to int

The address of a variable is denoted by the symbol &

p = & i ; // p is now 1 ( in fi g ur e )

p stores the address of i, i.e., &i

*p denotes the content pointed by p, i.e., i

The value of a pointer can be printed using %u

The size of a pointer can be found usingsizeof

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Pointers

# i n c l u d e < s t d i o . h >

in t m a i n ( ){

int i ;int * p ;

p r in t f (" S iz e o f p o in te r p i s % d \ n" , sizeof ( p ) ) ;

i = 5;

p = & i ;

p r in t f (" % d \t % d \t % u \t % u \n " , i , ( * p) , & i , p ) ;

* p = 7 ;p r in t f (" % d \t % d \t % u \t % u \n " , i , ( * p) , & i , p ) ;

}

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Pointer operations

Integers can be added to or subtracted from a pointerDifferent from simple integer arithmetic

Adds (subtracts) in units of the size of the variable pointed toint * p ; // s up po se p is now 1234

p + + ; // p is now 1234 + s i ze of ( int ) = 1238 and not 1235



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Pointer operations




A pointer cannot be added to another pointer

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Pointer operations





Pointers cannot be multiplied or divided with integers or other

pointers



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Pointer operations






pointersA pointer can be subtracted from another pointer

Useful in arrays


P i i
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Pointer operations







Useful in arrays

Pointers can be compared

A pointer can be assigned the address of a variable or another pointer


P i i


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Pointer operations

Integers can be added to or subtracted from a pointerDifferent from simple integer arithmeticAdds (subtracts) in units of the size of the variable pointed to

int * p ; // s up po se p is now 1234





Useful in arrays


A pointer can be assigned the address of a variable or another pointerA pointer can be assigned a constant integerIt is best to avoid such assignments


P i i


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Pointer operations

Integers can be added to or subtracted from a pointerDifferent from simple integer arithmeticAdds (subtracts) in units of the size of the variable pointed to

int * p ; // s up po se p is now 1234





Useful in arrays


A pointer can be assigned the address of a variable or another pointerA pointer can be assigned a constant integerIt is best to avoid such assignments

A special value NULL can be assigned to a pointerIt signifies that the pointer points to nothingIt is equivalent to 0


P i t ti I
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Pointer operations I

# i n c l u d e < s t d i o . h >

in t m a i n ( ){

int i , j , *p , * q;

p r i nt f ( " A s s i g ni n g c o n st a n t t o a p o i nt e r \ n " ) ;p = 3 05 64 37 8; / / w ar ni ng , m ay r es ul t i n s e gm e nt a ti o n f au ltp r i nt f ( " % u \ n " , p ) ;/ / p ri nt f ( "% d \ n ", ( * p) ) ; / / m ay r es u lt i n s e gm e nt a ti o n f au lt

p r in t f (" A s si g ni n g N UL L t o a p o in t er \ n " ) ;p = NULL ;

p r i nt f ( " % u \ n " , p ) ;/ / p r i nt f ( " % d \ n " , ( * p ) ) ; / / e r ro r

p r i nt f ( " P r i n t in g a p o i nt e r \ n " ) ;p = & i ;p r i nt f ( " % u \ n " , p ) ;

p r i nt f ( " I n c r e m en t i n g a p o i nt e r \ n " ) ;p + + ;

p r in t f (" % u \t % u \n " , p , ( p - 3 ) );

p r i nt f ( " P r i n t in g a p o i nt e r \ n " ) ;q = & j ;p r i nt f ( " % u \ n " , q ) ;

// p = p * 2; // error

// q = q / p ; // error


P i t ti II
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Pointer operations II

p r i nt f ( " C o m p a ri n g t w o p o i nt e r s \ n " ) ;p ri nt f (" % d\ t %d \ n" , ( p < q ) , ( p > q )) ;

p r i nt f ( " A s s i g ni n g a d d re s s t o p o i nt e r s \ n " ) ;p = & i ;* p = 3 ;q = & j ;* q = 9 ;p r in t f (" % u \t % u \t % d \t % d \t % d \t % d \n " , p , q , ( * p) , ( * q) , i , j ) ;

p r i nt f ( " A s s i g ni n g a p o i nt e r t o a n o th e r p o i nt e r \ n " ) ;

p = q ;p r in t f (" % u \t % u \t % d \t % d \n " , p , q , ( * p) , ( * q) ) ;

* p = 4 ;p r in t f (" % u \t % u \t % d \t % d \n " , p , q , ( * p) , ( * q) ) ;

p r i nt f ( " S u b t r a ct i n g a p o i nt e r f r om a n o th e r p o i nt e r \ n " ) ;p = q + 3;p ri nt f (" % u\ t %u \ t% d \t %d \ n" , p , q , ( p - q ) , ( q - p ) );

p r in t f (" I n c re m en t in g t he c on t en t o f a p o in te r \ n ") ;p r in t f (" % d \t % d \t % d \n " , ( * p) , ( *( p + 1 ) ) , ( * p + 1 ) );

}


Pointers and arrays
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Pointers and arrays

Array names are essentially pointers

int a [ 8 ] ;

Array elements are stored contiguously in memory

a is a pointer to the first element of the array, i.e., a[0]

*a is equivalent to a[0]

(a + i) is a pointer to a[i]

*(a + i) is equivalent to a[i]Remember: (a + i) is actually a + i * sizeof(int)


Pointers and arrays
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Pointers and arrays


int a [ 8 ] ;





*(a + i) is equivalent to a[i]Remember: (a + i) is actually a + i * sizeof(int)When a[i] is used, compiler actually computes the address of (a +i) and accesses the variable in that address

No error checking on array boundaries is done

Therefore, a[-2], a[12], etc. become legal as they are computed topossibly valid memory addresses


Pointers and arrays
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Pointers and arrays


int a [ 8 ] ;





*(a + i) is equivalent to a[i]Remember: (a + i) is actually a + i * sizeof(int)When a[i] is used, compiler actually computes the address of (a +i) and accesses the variable in that address

No error checking on array boundaries is done

Therefore, a[-2], a[12], etc. become legal as they are computed topossibly valid memory addresses

Pointers can be assigned array names

int * p ;

p = a ;


Passing pointers to functions
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Pointers can be passed to functions just like any other variableFunction header must indicate that the parameters are pointers

int s w a p ( int * p a , int * p b ){

int t = * p a ; * p a = * p b ; * p b = t ;

}

Important: If content of a pointer is changed inside the function, the

change is permanent, i.e., visible even outside the functionReason: When a pointer is passed as an argument, a copy of thepointer is available inside the functionHowever, the copy still points to the same addressHence, changing the content of this address changes the content of

the original addressSimilar to the situation when pointers p and q are same, i.e., p = q ;and *p is changed; *q changes automatically


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Pointers can be passed to functions just like any other variableFunction header must indicate that the parameters are pointers

int s w a p ( int * p a , int * p b ){

int t = * p a ; * p a = * p b ; * p b = t ;

}

Important: If content of a pointer is changed inside the function, the

change is permanent, i.e., visible even outside the functionReason: When a pointer is passed as an argument, a copy of thepointer is available inside the functionHowever, the copy still points to the same addressHence, changing the content of this address changes the content of

the original addressSimilar to the situation when pointers p and q are same, i.e., p = q ;and *p is changed; *q changes automaticallyOther rules of parameter passing are followedChange in pointer itself is temporary


Swapping two integers
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Swapping two integers

# i n c l u d e < s t d i o . h >

void s w a p ( in t * p a , in t * p b ){

int t = * p a ;* pa = * pb ;* pb = t ;

}

in t m a i n ( ){

int x = 3 , y = 4 ;int * a , * b ;

p ri nt f (" x = % d, y = % d\ n" , x , y );

/ / s wa p ( &x , & y ) ; / / e q ui v al e nt t o t he n ex t t hr ee l in es

a = & x ;b = & y ;s wa p ( a , b ) ;

p ri nt f (" x = % d, y = % d\ n" , x , y );}

pax

10 495

y ...

...

a pb... ...

...... 5325 29

b


scanf and printf
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scanf and printf

scanf requires passing a pointer

Reason: scanf requires to change the content of the variable to what

has been typed using the keyboardTherefore, the address of the variable is passed

This is also why scanf requires & with the variable

s c an f ( % d , & i ) ;


scanf and printf
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scanf and printf





s c an f ( % d , & i ) ;

printf does not require passing a pointerReason: printf does not require to change the content of thevariable; it just needs to print it

Therefore, only the variable is passed

This is also why printf does not require & with the variable

p r in t f ( % d , i ) ;


scanf and printf


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scanf and printf





s c an f ( % d , & i ) ;

printf does not require passing a pointerReason: printf does not require to change the content of thevariable; it just needs to print it

Therefore, only the variable is passed

This is also why printf does not require & with the variable

p r in t f ( % d , i ) ;

General rule: If a variable needs to be changed in a function, pass apointer that holds the address of the variable and change the contentsof the pointer


Passing arrays as pointers
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Since array names are pointers, arrays can be passed where pointersare required

To access the successive elements of the array, the pointer can beincremented


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g y p

# i n c l u d e < s t d i o . h >

void r e a d _ a r r a y ( in t *p , in t s i z e ){

int i = 0;while ( i < s ize ){

s c an f ( " % d " , p ) ;p + + ;i + + ;

}}

void p r i n t _ a r r a y ( int *p , int s i z e )

{int i = 0;while ( i < s ize ){

p r i nt f ( " % d \ t " , * p ) ;p + + ;i + + ;

}p r i n t f ( " \ n " ) ;

}

in t m a i n ( ){

int a [] = {3 , -2 , 7 , 19 };p r i n t_ a r r ay ( a , 4 ) ;r e a d_ a r r ay ( a , 4 ) ;p r i n t_ a r r ay ( a , 4 ) ;

}


Passing pointers as arrays
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g p y

Pointers can also be passed where arrays are required

The pointer must point to the correct array addressPointer may point to the middle of the array, and operations maystart from there


Passing pointers as arrays
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g p y

# i n c l u d e < s t d i o . h >

void r e a d _ p o i n t e r ( in t a [ ] , in t s i z e ){


s c an f ( " % d " , & a [ i ] ) ;i + + ;

}}

void p r i n t _ p o i n t e r ( in t a [ ] , int s i z e ){


p r i nt f ( " % d \ t " , a [ i ] ) ;i + + ;

}p r i n t f ( " \ n " ) ;

}

in t m a i n ( ){

int b [] = {3 , -2 , 7 , 19 };int * p = b ;p r i n t_ p o i nt e r ( p , 4 ) ;p r in t _p o in t er ( p + 1 , 3 ) ;r e a d _p o i n te r ( p , 4 ) ;p r i n t_ p o i nt e r ( p , 4 ) ;

}

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