Pointers and Arrays

Pointers and Arrays

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) */

                        0   1 …………                     9

 

                    a

                        pa    ++pa                                pa+i

Fig. 9.3 Arrays and Pointers

To get somewhere in the array (Fig. 9.3) using a pointer we could do:

pa + i º 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] º a + i.

We also express pointer addressing like this:

pa[i] º *(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) º 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[] ºchar *s.

strlen() is a standard library function (Chapter 18) 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.