Programming Language C-Tutorial4

Pointers and Arrays


Arrays and pointers are closely linked in C. To use arrays effectively it's a good idea to know how to use pointers with them. Let's have a look at an example:

#include

#define MAX 10

int main()
{
int a[MAX], b[MAX], i;
for(i = 0; i < MAX; i++) {
a[i] = i;
b = a;
}
return(0);
}


Compile this program and run it. It won't run, why? Because you can't assign a to b. You have to use something like this:

for(i = 0; i < MAX; i++) {
a[i] = i;
b[i] = a[i];
}


The line #define MAX 10 is a simple constant. We haven't seen this before so i should mention it. All it is in a CONSTANT value we can use throughout the program. MAX is a variable that holds the value of 10, only we cannot modify it, it's constant. Constants are normally typed in upper-case to make it obvious to anyone reading the program that it is a constant.

a and b are unusual in C in that they're not techinically arrays themselves. They are permanent pointers. What does that mean? Well, a POINTS to the first item in its array, and b POINTS to the first item in it's array, they are pointers, only they cannot be changed to point to something else. They hold the addresses of a[0] and b[0]. Therefore a = b; is illegal.

#include

#define MAX 10

int main()
{
int a[MAX], b[MAX], i, *p;
for(i = 0; i < MAX; i++) {
a[i] = i;
p = a;
}
return(0);
}


The code above is legal. p = a; works because a is techically a pointer, so assigning a to p makes p contain the address of the first element of of the array a[]. So now *p points to a.

p = a;
/* IS THE SAME AS */
p = &a[0];


Remember, a is a pointer, it points to address of the first element in the array, and always points there. &a[0]; is the address of the same item in the array, so they are the same.

Now that we have assigned the address of a[]'s first element to p, we can use the pointer to move around the array, consider the below:

#include

#define MAX 5

int main()
{
int a[MAX], i, *p;
a[0] = 5;
a[2] = 7;
p = a;
printf("pointer is pointing to the value %dn", *p);
p = p + 2;
printf("pointer is now pointing to the value %d", *p);
return 0;
}


This will output:

pointer is pointing to the value 5
pointer is now pointing to the value 7

Do you see? We can use the pointer to jump through the array elements. If we increment p then the pointer will jump the appropriate number of bytes to the next array item. C takes care of the issue of element size, so adding one will jump to the next element, adding two will jump to the element after the next element... etc.