Pointers & Arrays

Pointers and Arrays

Pointer

Pointer is a variable that store the address of another variable

Syntax :
  <type> *ptr_name;

Two operators mostly used in pointer : * (content of) and & (address of)

Example:
Initialize an integer pointer into a data variable:
int i, *ptr;
ptr = &i;
To assign a new value to the variable pointed by the pointer:
*ptr = 5;  /* means i=5 */

Pointer to Pointer

Pointer to pointer is a variable that saves another address of a pointer

Syntax:

             <type> **ptr_ptr ;

Example:

 int i, *ptr, **ptr_ptr;

  ptr = &i;

  ptr_ptr = &ptr;

         To assign new value to i:

           *ptr = 5;  // means i=5 ;

          **ptr_ptr = 9;   // means i=9; or *ptr=9;

Array

Data saved in a certain structure to be accessed as a group or individually. Some variables saved

using the same name distinguish by their index.

Array characteristics:

–Homogenous

  All elements have similar data type

–Random Access

  Each element can be reached individually, does not have to be sequential

Accessing Arrays

Two analogous ways of accessing an element i=2;

         (A+2) or A[2]

A  is equivalent with &A[0] or a constant pointer to the first element of particular array

    To show A[2] on the monitor screen:

          printf(“%d”,A[2]) or

          printf(“%d\n”,*(A+2));

Pointer Constant & Pointer Variable

Pointer constant can only be initialized at definition time

Example:

     int Arr1[10];

          Arr1[10] = {1, 2, 3, 4, 5};  // error

          Arr1 = {1, 2, 3, 4, 5};  // error

          Arr1[10] = 12;  // error max 9

          Arr1[0] = 23;  // ok

     int Arr2[10] = {1, 2, 3, 4, 5}; //ok

One Dimensional Array

C compiler does not limit number of dimensional which can be created. Our PC memory does.

Example Array 1D:

     #include<stdio.h>

     int SIZE = 5;

     void main() {

            int i, j;

            int n[SIZE] = {15, 9, 1, 7, 5};

            for( i=0 ; i<= SIZE ; i++) {

      printf("%5d ", n[i]);

      for ( j=1; j<=n[i] ; j++) printf("%c","*");

      printf("\n");

         }

    }

Two Dimensional Array

Syntax 2D Array:

        type name_array[row][col];

Three Dimensional Array

Syntax 3D Array :

  type name_array[row][col][depth];

Example:

  int x[3][2][4] = {{{1,2,3,4}, {5,6,7,8}},

                          {{11,12,13,14}, {15,16,17,18}},

                          {{21,22,23,24}, {25,26,27,28}}

       };  

   void main() {

     int x[4][3][5] = {{{1, 2, 3}, {0, 4, 3, 4}, {1, 2}},

     {{9, 7, 5}, {5, 7, 2}, {9}},       

     {{3, 3, 5}, {2, 8, 9, 9}, {1, 2, 1}},

     {{0}, {1}, {0, 1, 9}}

    };

     printf(“%5d”, x[2][1][3]);

  } 

Array of Character

Array filled with character/s

Syntax:

  char array_name[value_dim];

Example:

char name[40];

char ss[20]={‘B’,’I’,’N’,’U’,’S’};  //20 elements

char ss[ ]= {‘B’,’I’,’N’,’U’,’S’};  // 5 elements

String

String is an array of character that ended with null character ( ‘\0’ or in ASCII = 0)

String constant or string literal is some characters written between double quote

-String constant type is pointer constant, thus can be assigned to an array of character :

–Example :

  char name[40] = ”Amir”;  //ok

  name = ”Amir”;   // error name is a constant pointer

  Name[40]= “Amir”;  //error

A Constant String can be linked at compile-time:

  ”Hello,” ” world”

  Similar to:

  ”Hello, world”

Example string initialization:

  char s[ ] = ”BiNus”;  

  Similar to :

  char s[ ] = {’B’,’i’,’N’,’u’,’s’,’\0’};

String as a data type does not known in C

Char vs String

Character in c written between single quote. Each uses one byte of computer memory

Example:

       char ch=’A’;

       char ch=65;     //Ascii

       char ch=0x41; //Ascii

String written in between double quote.

String Manipulation

In Standard Library Function (header file string.h) provides functions to manipulate string:

–strlen()

      Return a value of string length; excluded null char

–strcpy(s1,s2)

      Copy s2 to s1

–strncpy(s1,s2,n)

      Copy first n characters of s2 to s1

–strcat(s1,s2)

      Adding string s2 to the end of string s1

–strncat(s1,s2,n)

     Adding n characters of string s2 to the end of string s1

–strcmp(s1,s2)

     Comparing the value of string s1 and s2, if similar returning 0

–etc.