Explain Pointers and Arrays with example

Pointers and arrays are very closely linked in C.
Hint: think of array elements arranged in consecutive memory locations.
Consider the following:

   int a[10], x;
         int *pa;

         pa = &a[0];  /* pa pointer to address of a[0] */

         x = *pa;
         /* x = contents of pa (a[0] in this case) */
  
Fig.  Arrays and Pointers
To get somewhere in the array as in above fig. using a pointer we could do:
   pa + i  a[i]
WARNING: There is no bound checking of arrays and pointers so you can easily go beyond array memory and overwrite other things.
C however is much more subtle in its link between arrays and pointers.
For example we can just type
   pa = a;
instead of
   pa = &a[0]
and
   a[i] can be written as *(a + i).
i.e. &a[i]  a + i.
We also express pointer addressing like this:
   pa[i]  *(pa + i).
However pointers and arrays are different:
•    A pointer is a variable. We can do
pa = a and pa++.
•    An Array is not a variable. a = pa and a++ ARE ILLEGAL.
This stuff is very important. Make sure you understand it. We will see a lot more of this.
We can now understand how arrays are passed to functions.
When an array is passed to a function what is actually passed is its initial elements location in memory.
So:
   strlen(s)  strlen(&s[0])
This is why we declare the function:
     int strlen(char s[]);
An equivalent declaration is : int strlen(char *s);
since char s[]  char *s.
strlen() is a standard library function that returns the length of a string. Let's look at how we may write a function:

   int strlen(char *s)
           { char *p = s;

                 while (*p != `\0’);
                   p++;
                 return p-s;
           }
Now lets write a function to copy a string to another string. strcpy() is a standard library function that does this.

   void strcpy(char *s, char *t)
           {  while ( (*s++ = *t++) != `\0’);}

This uses pointers and assignment by value.
Very Neat!!
NOTE: Uses of Null statements with while.