Computer And Programming

Array and Pointer

Array and Pointer

• Array provides a means to allocating and accessing memory.

• Pointers, on the other hand, provides a way to accessing memory that has been allocated by other means, often as an array

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One Dimensional Array

• A (one-dimensional) array is a series of objects, all of which have the same type.

• An array is a block of memory made up of memory locations of a fixed size.

– declares a as an array of five int objects

– declares x as an array of 100 long double objects

int a[5];

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long double x[100];

Array Initialization

• We can initialize an array when it is declared by listing the initial values for its elements, as follows:

• Array with five elements has subscript ranging from 0-4. • The values assigned to the array a can be illustrated in

figure below:

int a[5] = {75, 25, 100, -45, 60};

4

75

25

100

-45

60

a[0]

a[1]

a[2]

a[3]

a[4]

Accessing Array

• Subscripted array can be used like variable names, such as x and y. For example:

– assigns 5 to element 2 (the third element) of a,

– sets n to the difference of elements i and j, and

– increments the value of element 2*i-j. – (see postfix and prefix operations)

a[2] = 5;

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n = a[i] – a[j];

a[2*i – j]++; or ++a[2*i – j];

Declaring Array with Constant

• Constants represented by macros are frequently used in declaring and manipulating arrays.

• For example, if COUNT is defined by

– then

– declares c as an array of 1000 int objects.

#define COUNT 1000

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int c[COUNT];

Array and Loops

• An example of setting all elements to 0 is given below

• An equivalent implementation is also given as:

for(i = 0; i < COUNT; i++) c[i] = 0;

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int i = 0;while(i < COUNT) c[i++] = 0;

Pointers

• A pointer is the address of an object of a particular type.

• An address only locates the first byte of an object.

• We need the type because it can determine the size of the object and the meaning of the bits stored in it.

• Typical pointer types are pointer-to-int, pointer-to-long, pointer-to-double, and so on.

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Indirection Pointer

• If p is a variable with type pointer-to-int, the value of p is the address of an int object.

• Thus, *p is an lvalue of an object that is pointed by p.

9

75

25

100

-45

60

p

*p

75

p

P is a pointer to an object. The value of p is the address of the object. Thus, the value of *p is the value of the object pointed by pointer p.

int *p; // Declare p as a pointer-to-int type

*p = 20; // assigns 20 to the *p. And

printf(“%d”, *p); // prints the value 20, and

n = *p + 10; // assigns 35 to n,

*p += 7; , which is equivalent to *p = *p +7;// adds 7 to the *p, then stores the result to *p,

whereas (*p)++; // increments the value of the target object.

Example of Using a Pointer

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• We can use the address-of operator to compute the address of any object designated by an lvalue

• If we declare

– then &n has type pointer-to-int and &x has type pointer-to-double.

• If we now declare

– then p is initialized to point to n.

• Thus, *p and n name the same object– They are said to be aliases

Using the Address-of Operator

int n = 100;double x = 3.5;

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int *p = &n;

Using const in Pointer Declaration

• In pointer, const can be used in two difference places.

• If const is placed at the beginning of the declaration, the target object is declared constant.

• In this case, value of target object *p cannot be changed.

• Thus the following statements are invalid.

• if m and n are integer, then following statements can be applied to p.

const int *p;

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*p = 100; and (*p)--;

*p = 100; and (*p)--;

Using const in Pointer Declaration (cont)

• If const precedes the pointer variable, then the value of p (address of which p points to) now becomes constant

• The following statements are not allowed.

• However, these statements can be applied to p.

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int *const p = &m;

p = &n; and p++;

*p = 25; *p += 3; (*p)++; (*p)--;

Pointers and Arrays

• We can use the name of an array to initialize a pointer to the address of the first element of the array.

• Both p and q point to the first element of list.

• C allows the subscript operator to be applied to pointer as well as array names. – It is legitimate to use p[0] to refer to the first element of list.– It is also possible to refer to the 5th element of list by using p[4]

int list[100]; int *p = list; int *q; q = list;

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Pointers and Arrays (cont)

• So, we can use

• It can also be written as

sum = 0;for(i = 0; i < 100; i++)

sum += list[i];

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sum = 0;for(i = 0; i < 100; i++)

sum += p[i];

A pointer need not always point to the first element of an array.We can set p to point to element 50 of list : 

p = &list[50];

Pointers and Arrays Example

• A pointer need not always point to the first element of an array.

• We can set p to point to element 50 of list :

p = &list[50];

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p

list[48], p[-2]

list[49], p[-1]

list[50], p[0]

list[51], p[1]

list[52], p[2]

75

25

100

-45

60

list

(Keep in mind, p must point to a valid location of the target object.)

String

• Strings are stored in memory as arrays of char values.

• The null character ('\0') is used to indicate the end of string

• We can check it by trying this statement

– Computer will print

printf(“abcd\0efgh”);

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abcd

String Declaration

• We can declare an array of characters to hold a string

– This will make a string containing up to 3 characters– The number of charatered used will be one less than the

declaration because it needs tell the termination of string (null)

• String initilization could be as follow:

– It can also be written as

char s[4];

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char s[4] = “dog”;

char s[4] = { ‘d’, ‘o’, ‘g’, ‘\0’ };

String Declaration (cont)

• We can also leave it to compiler to determine its size

char s[] = “dog”;

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s[0]

s[1]

s[3]

'd'

'o'

'g'

'\0'

s[2]

s

String Declaration (cont)

• If we declaration statement as

– it will result in

• We can print it using

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char s[10] = “dog”;

s[0] s[1] s[2] s[3] s[4] s[5] s[6] s[7] s[8]

s[9]

'd' 'o' 'g' '\0'

Unused

printf(s);

or

printfs(“%s”, s);

String Pointer

• We can assign pointer-to-char to array of character(string).

• The initialization can be as follow:

• We can then print it likewise:

char *p = s;

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char *p = “dog”;

printf(p);

or

printf(“%s”, p);