Senem KUMOVA METİN CS115 2008-2009 FALL 1 POINTERS && ARRAYS CHAPTER 6

Senem KUMOVA METİN CS115 2008-2009 FALL

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POINTERS &&

ARRAYS

CHAPTER 6


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POINTERS

POINTER is a programming language data type whose value refers directly to

(or "points to") another value stored elsewhere in the computer memory using its address


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POINTERS int x= 5 is a variable and

5 is the value of itprintf(“value of x : %d”, x);

&x is the location or address of the xprintf(“address of x in memory : %d”, &x);

Pointers are declared in programs and then used to take addresses !!!

int x=5; // some stored valueint * p; // a pointer that can hold the address of an integerp=&x;

x= 5&x

….

….

…..

MEMORY


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Pointers: & operator

55 0x123456780x12345678xxint int xx = = 55;;

pp

int *pint *p;;

pp = & = &xx;;

// adress of x // adress of x is // stored in pis // stored in p

0x123456780x12345678

p is a pointer: it contains the p is a pointer: it contains the address of a piece of dataaddress of a piece of data

MemoryMemory

AddressAddress

&x&x


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POINTERSint * p; // A pointer that can hold an

address // of an integer integer pointer

int a =0;p= &a;

double * q // A pointer that can hold an address // of a double double pointer

double b=1.2;q= &b;

char * t // A char pointerchar c=‘D’;t = &c;


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POINTERSmain(){ int * p; // integer pointer

int a =0;p= &a; // a equals to *p

printf(“%d \n”, a);printf(“%d \n”, *p);

*p=6;

printf(“%d \n”, a);printf(“%d \n”, *p);

a++;

printf(“%d \n”, a);printf(“%d \n”, *p); }

a=0

p=&a

*p


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POINTERSint a=1, b=2, *p;

p=&a;

&a &b &p

a=1 b=2 ??will point somewhere in memory

&p

1 2 p=&a

&a &b


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POINTERSb=*p;

// equivalent to b=a

&a &b &p

a=1 b=1

( the value of a )

p=&a

THE VALUE POINTED BY A POINTER CAN BE CHANGED BY

* name_pointer

*p


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Pointers : EXAMPLE 1

#include <stdio.h>void main(void){

int i = 7, j=3 , *k, *m; k = &i; // k ??? *k ??? j=*k; // k ??? *k ???

m=&i; // i ??? *i ???(*m) ++; // m ??? *m ???printf(“ %d %d %d %d”, i, j, *k, *m);

}


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Pointers : EXAMPLE 2

#include <stdio.h>void main(void){

char a = ‘D’; char *p = &a;

*p=*p+2;printf(“%c %c”, a, *p);

a=a+1;printf(“%c %c”, a, *p); }


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CALL BY VALUE/* Whenever variables are passed as arguments to a function, their

values are copied to the function parameters */

void main(){ int a=20;

incr1 (a);printf(“%d: “, a);

incr2 (a);printf(“%d: “, a);}

void incr1(int x) { x++; }

void incr2(int a) { a++; }

R E M E M B E R !


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CALL BY REFERENCE/* Pointers are passed as arguments to a function, their addresses are assigned to

the function parameters defined as pointers:*/

void main()

{ int a=20;

incr (&a);

printf(“%d: “, a); }

void incr(int * x)

{ (*x)++; }


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CALL BY REFERENCE : Example 1

void addi(int *a, int b);

void main(){ int a=20, int b=30;

addi (&a , b)printf(“%d %d :“, a, b);}

void addi(int *a, int b) { int tmp = *a +b;

*a= tmp;b= b++;}

OUTPUT??


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void main(){ int a=20; int b=30;

swap1 (a, b);swap2 (&a, &b);printf(“%d %d: “, a, b);}

void swap1(int x, int y) { int tmp;

tmp=x;x=y;y=tmp; }

void swap2(int *x, int *y) { int tmp;

tmp = *x; // get value pointed by x.*x = *y; // assign value pointed by y to x*y = tmp; }

CALL BY REFERENCE Example 2 : SWAP


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What is an Array?

int grade0, grade1, grade2;

int grade [3];

Arrays make it possible to represent large number of homogenous values

grade0 grade1 grade2

grade[0] grade[1] grade[2]


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One Dimensional Arrays type name_of_array[size]

EXAMPLE

int grade[5];

data

datagrade[1]

datagrade[2]

data

data

grade[0]

grade[3]

grade[4]

MEMORY

first element in array

fift ( the last) element in array

starts from 0

ends at size - 1


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One Dimensional Arraysint grade[5], x=9, y;

grade[0]= 0; grade[3]= 4; grade[2]= -1;

grade[1]=grade[3]+grade[2];

grade[4]= x;

x= grade[2];y= grade[grade[3]];

03grade[1]-1grade[2]49

grade[0]

grade[3]grade[4]

MEMORY

x ?

y ?


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INITIALIZATION float f1[]= { 1.0, 1.1, 1.2, 1.3 }; /* declaration

without the size */

float f2[4]= { 1.0, 1.1, 1.2, 1.3 }; /* declaration with size */

char z[] =“abc”; // char arrays char z[] = {‘a’, ‘b’, ‘c’, ‘\0’}

int a[80]={0};


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EXAMPLE1:/* array1.c*/main(){

int x[10], k;

for(k=0; k<10; k++){

x[k]=0;printf(“x[%d] = %d\n”, k, x[k]); //what will be the output ???

}}


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int x[10], k;

for(k=0; k<10; k++){

x[k]=k;printf(“x[%d] = %d\n”, k, x[k]); //what will be the output ???

}}


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int x[10], k, sum=0;

for(k=0; k<10; k++)scanf(“%d”, &x[k]);

for(k=0; k<10; k++){ printf(“%d”, x[k]); sum=sum + x[k]; }

printf(“sum is %d”, sum);}


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Relationship between Pointers and Arrays

/*An array name by itself is an adress or a pointer value */

int a[5] , *p;p=&a[0];

// Equals to

p=a;


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Relationship between Pointers and Arrays

&a[0] equals to a then a[0] equals to *a&a[1] equals to a+1 then a[1] equals to *(a+1)&a[2] equals to a+2 then a[2] equals to *(a+2)………………………………………………………………………………&a[i] equals to a+i then a[i] equals to *(a+i)

EXAMPLE:int a [5]={1,2,3,4,5};int * p;printf(“%d”,a[0]); printf(“%d”,*a);printf(“%d”,a[2]); printf(“%d”,*(a+2));p=&a[4];P=a+4;


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Relationship between Pointers and Arrays : Example1

main(){ int x[10], k;

for(k=0; k<10; k++){ x[k]=k;

printf(“%d\n”, x[k]); }

}


for(k=0; k<10; k++){ *(x+k)=k;

printf(“%d\n”, *(x+k)); }

}


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Relationship between Pointers and Arrays : Example2


for(k=0; k<10; k++){

*(x+k)=0;printf(“%d\n”,

*x+k); }

}


for(k=0; k<10; k++){

*(x+k)=0;printf(“%d\n”,

*(x+k)); }

}


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Pointer ArithmeticEXAMPLEdouble a[2],*p,*q;

p=a; // p=&a[0]q=p+1; // q=&a[0]+1= &a[1]

printf(“%d\n”, q-p); /* difference in terms of array elements */

printf(“%d\n”, (int) q-(int) p); /* difference in memory locations */


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Arrays as Function Argumentsmain(){ int x[9], r;

r=sum(x,9); //sum(&x[0],9)

}double sum( int a[], int n){ int i;

double result =0.0;for (i=0; i<n; i++)

result+=a[i];return result;

}

main(){ int x[9], r;

r=sum(x,9); // sum(&x[0],9)

}double sum( int * a, int n){ int i;

double result =0.0;for (i=0; i<n; i++)

result+=*(a+i);return result;

}


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Dynamic Memory Allocation

Calloc : contiguous memory allocation Malloc : memory allocation


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CALLOC calloc(n, el_size) an array of n elements, each element

having el_size bytesint main(){ int *a; //will be used as an array

int n; // size of array....a=calloc(n, sizeof(int)); /* get space for a , and

initialize each bit to zero */free(a); /* each space opened dynamically must be returned */

}


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MALLOC/* malloc does not initialize memory */int main(){ int *a; //will be used as an array

int n; // size of array....a=malloc(n*sizeof(int)); /* get space for a */....free(a);

}


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DYNAMIC ARRAY : Example

main(){

int * x, k,s;printf(“give array size”);scanf(“%d”,&s);

x=malloc(s*sizeof(int));

for(k=0; k<s; k++) x[k]=k;for(k=0; k<s; k++) printf(“%d ”,x[k]);

}


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NEXT WEEK

MULTIDIMENSIONAL ARRAYS

SORTING ALGORITHMS : BUBBLE SORT MERGE SORT

STRINGS && STRING_HANDLING FUNCTIONS

ARRAYS OF POINTERS

ARGUMENTS TO MAIN

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Senem KUMOVA METİN CS115 2008-2009 FALL 1 POINTERS && ARRAYS CHAPTER 6