Stage 11: Pointer Arithmetic in C++

Pointer Arithmetic: Navigating Through Memory

Pointer arithmetic is a mechanism that allows you to perform addition and subtraction on memory addresses. It is the foundation of how arrays work in C++. The key rule to remember for your exam is that these operations are scaled by the size of the data type the pointer points to.

1. Incrementation and Decrementation (ptr++, ptr--)

When you increment a pointer by 1 (ptr++), you are not moving by one byte in memory. You move by as many bytes as the data type occupies (i.e., by sizeof(type)).

If an int takes 4 bytes, ptr++ increases the address by 4.
If a double takes 8 bytes, ptr++ increases the address by 8.

cpp

int arr[] = {10, 20, 30};
int *p = arr; // points to arr[0]

p++; // p now points to arr[1]
std::cout << *p; // Prints 20

2. Adding and Subtracting Integers

You can move a pointer by any number of elements by adding or subtracting an integer.

$New\\_Address = Old\\_Address + (n \\cdot sizeof(type))$

cpp

int *p = &arr[0];
int *p2 = p + 2; // p2 now points to arr[2]

3. Subtracting Pointers

If you have two pointers pointing to elements of the same array, you can subtract them. The result is not the number of bytes, but the number of elements between them.

cpp

int *p_start = &arr[0];
int *p_end = &arr[2];
int distance = p_end - p_start; // Result: 2

4. Relationship Between Pointers and Arrays

In C++, the arr[i] notation is just "syntactic sugar" for a pointer operation. The compiler always translates it to:

*(arr + i)

This means you can iterate through an array using just the pointer, which is often required in low-level tasks.

cpp

const int N = 3;
int numbers[N] = {100, 200, 300};
int *ptr = numbers;

for(int i = 0; i < N; i++) {
    std::cout << *(ptr + i) << " "; // Same as ptr[i] or numbers[i]
}

Exam Pitfalls

Multiplication and Division: You cannot multiply or divide pointers. Operations like ptr * 2 will cause a compilation error.
Adding Pointers: You cannot add a pointer to another pointer (ptr1 + ptr2). It makes no logical sense.
Out of Bounds: Arithmetic allows you to set a pointer outside the array. C++ won't stop you, but attempting to dereference (*) such an address will crash the program.
void Pointers*: You cannot perform arithmetic on void* pointers because the compiler doesn't know the size to shift the address by.

Example: Reversing an Array with Pointers

cpp

void reverse(int *left, int *right) {
    while (left < right) {
        int temp = *left;
        *left = *right;
        *right = temp;
        left++;
        right--;
    }
}

Pointer Arithmetic: Navigating Through Memory

1. Incrementation and Decrementation (ptr++, ptr--)

When you increment a pointer by 1 (ptr++), you are not moving by one byte in memory. You move by as many bytes as the data type occupies (i.e., by sizeof(type)).

If an int takes 4 bytes, ptr++ increases the address by 4.
If a double takes 8 bytes, ptr++ increases the address by 8.

cpp

int arr[] = {10, 20, 30};
int *p = arr; // points to arr[0]

p++; // p now points to arr[1]
std::cout << *p; // Prints 20

2. Adding and Subtracting Integers

You can move a pointer by any number of elements by adding or subtracting an integer.

$New\\_Address = Old\\_Address + (n \\cdot sizeof(type))$

cpp

int *p = &arr[0];
int *p2 = p + 2; // p2 now points to arr[2]

3. Subtracting Pointers

If you have two pointers pointing to elements of the same array, you can subtract them. The result is not the number of bytes, but the number of elements between them.

cpp

int *p_start = &arr[0];
int *p_end = &arr[2];
int distance = p_end - p_start; // Result: 2

4. Relationship Between Pointers and Arrays

In C++, the arr[i] notation is just "syntactic sugar" for a pointer operation. The compiler always translates it to:

*(arr + i)

This means you can iterate through an array using just the pointer, which is often required in low-level tasks.

cpp

const int N = 3;
int numbers[N] = {100, 200, 300};
int *ptr = numbers;

for(int i = 0; i < N; i++) {
    std::cout << *(ptr + i) << " "; // Same as ptr[i] or numbers[i]
}

Exam Pitfalls

Multiplication and Division: You cannot multiply or divide pointers. Operations like ptr * 2 will cause a compilation error.
Adding Pointers: You cannot add a pointer to another pointer (ptr1 + ptr2). It makes no logical sense.
Out of Bounds: Arithmetic allows you to set a pointer outside the array. C++ won't stop you, but attempting to dereference (*) such an address will crash the program.
void Pointers*: You cannot perform arithmetic on void* pointers because the compiler doesn't know the size to shift the address by.

Example: Reversing an Array with Pointers

cpp

void reverse(int *left, int *right) {
    while (left < right) {
        int temp = *left;
        *left = *right;
        *right = temp;
        left++;
        right--;
    }
}

Stage 11: Pointer Arithmetic in C++

Pointer Arithmetic: Navigating Through Memory

1. Incrementation and Decrementation (ptr++, ptr--)

2. Adding and Subtracting Integers

3. Subtracting Pointers

4. Relationship Between Pointers and Arrays

Exam Pitfalls

Example: Reversing an Array with Pointers

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Stage 10: Pointers, Address-of, and Dereference Operators

Stage 3: Automatic One-Dimensional Arrays in C++

Stage 6: C-Style String Handling (char[])

Stage 11: Pointer Arithmetic in C++

Pointer Arithmetic: Navigating Through Memory

1. Incrementation and Decrementation (ptr++, ptr--)

2. Adding and Subtracting Integers

3. Subtracting Pointers

4. Relationship Between Pointers and Arrays

Exam Pitfalls

Example: Reversing an Array with Pointers

You might also like

Stage 10: Pointers, Address-of, and Dereference Operators

Stage 3: Automatic One-Dimensional Arrays in C++

Stage 6: C-Style String Handling (char[])