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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 is the pointer to address of a[0] */

x = *pa;          /* x = contents of pa(a[0]) in this case */

Arrays and Pointers

To get somewhere in the array (see Figure above) 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.