Page 70 - DCAP201_FUNDAMENTALS_OF_DATA_STRUCTURES

Unit 5: Pointers

Consider the following: Notes
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) */

Figure 5.3: Arrays and Pointers

0 1 …………. 9
a

pa ++pa pa + 1

Source: http://www.cs.cf.ac.uk/Dave/C/node10.html
To get somewhere in the array (Figure 5.3) using a pointer we could do:
pa + i ≡ a[i]
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.
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[]);

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