Created: 2023-01-15 12:33
Status: #concept
Subject: Programming
Tags: C Pointer Memory Address Operator

Pointer Arithmetic

Since pointers point to objects, we can easily shift between contiguous C Arrays of them using Pointer Arithmetic.

How do we set a Pointer p to an Array’s a FIRST element?

We can do p = a or p = &a[0] because using a alone "decays" to a pointer to the Memory Address of the first element of an C Array.

Array First Element Pointer Decay Illustration.png

What does Adding/Subtracting an Integer j to a Pointer p do?

If we add an integer j to a pointer p, it shifts the position FORWARD by j places.

Example

p = &a[i]
p + j = &a[i+j]
p - j = &a[i-j]

Pointer Addition Illustration.png

What does Subtracting a Pointer q from another Pointer p evaluate to?

It returns the positional difference (in C Array elements) between the two pointers.

WE CANNOT ADD POINTERS TOGETHER.

Example

p = &a[i]
q = &a[j]
p - q = i - j

Pointer Subtraction.png

#include <stdio.h>

int main(void)
{
	int arr[] = {1,2,4,4,5}, *p, *q;
	
	p = &arr[0];
	q = &arr[2];
			
	printf("%d %d %d\n", p-q, p, q);
}
	// -2 608172000 608172008
	// separated by 4 bytes per integer

Can we use Comparison Operators on Pointers?

Yes, we can use <, <=, >, >= and ==, != on pointers.

It’s only meaningful when the two pointers we’re comparing are pointing to element from the same C Array.

It compares the positions in the array in which the pointers are pointed too.

Boolean Operators on Pointers Illustration.png

Using Pointer Arithmetic in Array Traversal

Idiom

We have to initialize the pointer variable to the pointer of the first position of the array because we cannot modify const C Array pointers

p = &a[0]; p < &a[N]; p++

It has to loop until the max element/boundary of the array. It has to increment the pointer variable by 1, shifting its position by 1 index.

Pointer Arithmetic in Array Traversal Illustration.png

References

C Programming, A Modern Approach, 2nd Edition, Chapter 12.1