Computer And Programming 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 2

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]; 3 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}; 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; 5 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. #defineCOUNT 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; 7 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. 8

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 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 10

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; 11 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; 12 *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. 13 int *const p = &m; p = &n; andp++; *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 5 th element of list by using p[4] int list[100]; int *p = list; int *q; q = list; 14

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]; 15 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]; 16 p list[48], p[-2] list[49], p[-1] list[50], p[0] list[51], p[1] list[52], p[2] 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”); 17 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]; 18 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”; 19 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 20 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; 21 char *p = “dog”; printf(p); or printf(“%s”, p);