Question

What is the output of the following code? int x; int *p; int *q; p = new int[10]; q = p; *p = 4; for (int j = 0; j < 10; j++) { x = *p; p++; *p = x + j; } for (int k = 0; k < 10; k++) { cout << *q << " "; q++; } cout << endl;

Short answer

The output is: 4 4 5 7 10 14 19 25 32 40

Step-by-step solution

Analyze Memory Allocation and Initialization

The line `p = new int[10];` allocates memory for an array of 10 integers. `q = p;` makes `q` point to the same array. The statement `*p = 4;` sets the first element of this array to 4.

Fill the Array Using the Loop

The loop `for (int j = 0; j < 10; j++)` iterates through the array. Initially, `x` is set to 4, then `p` is incremented, so `*p` (the current pointed location) is assigned `x + j`. The array will be filled as 4, 4, 5, 7, 10, 14, 19, 25, 32, 40.

Output the Array

The loop `for (int k = 0; k < 10; k++)` starts with `q` pointing to the start of the array, printing each element followed by a space. As `q` is incremented inside the loop, `*q` outputs each array value in order.

Key concepts

Dynamic Memory Allocation

In C++, dynamic memory allocation is a powerful tool that allows you to reserve memory space during the runtime of a program. This is done using the `new` keyword. In the provided code, `p = new int[10];` is used to allocate memory for an array of 10 integers. This dynamic allocation occurs during the program's execution, which offers flexibility in cases where the size of data isn't known at compile-time.

Dynamic memory allocation contrasts with static memory allocation, where the size and structure of data must be known before program execution. Here are a few points to remember about dynamic memory allocation:

• It's crucial to later deallocate this memory using `delete` or `delete[]` to avoid memory leaks.
• Dynamic memory is stored on the heap, a region of your program's memory that supports variable memory allocation.
• The pointers `p` and `q` both reference the same memory location initially.

Understanding these principles is essential for efficient memory management in C++.

Pointer Arithmetic

Pointer arithmetic allows for traversing or manipulating memory addresses in C++. When a pointer is incremented, it doesn't simply move to the next byte but advances by the size of the data type it points to. In the code example, notice how `p++` and `q++` are used to progress through the array.

Since `p` is a pointer to an integer, incrementing `p` moves it by `sizeof(int)` bytes to the next integer in the allocated memory. This allows for easy iteration over arrays using pointers.

• Ensure correct pointer arithmetic to avoid accessing invalid memory locations which can cause errors.
• Pointer arithmetic is especially useful when dealing with dynamically allocated arrays.
• Pointers can be used to perform arithmetic operations like addition, subtraction, and comparison.

Mastering pointer arithmetic is vital for efficient low-level data manipulation and is a hallmark of proficient C++ programming.

Array Manipulation

In C++, arrays are fundamental data structures used to store collections of elements. In the example code, an integer array of 10 elements is dynamically allocated. It is then populated with values by manipulating pointers. This is a classic demonstration of array manipulation using pointers.

Each element of the array is accessed and modified via pointer dereferencing and arithmetic inside the loop. Here's a useful look at the pertinent concepts:

• The value of `*p` is initially set to 4, and other elements are filled using a formula that incrementally modifies `p` along the array.
• The loop dynamically updates elements, demonstrating both read and write operations.
• Unlike Java or C# arrays, C++ arrays do not have out-of-bound checks, so careful management is necessary.

Using pointers for array manipulation might initially seem complex but offers great control and efficiency for precise data handling.

For Loop Control

The `for` loop is a vital control structure in many programming languages, including C++. It is used to repeat a block of code a known number of times. In the provided code, the `for` loop is instrumental in initializing the array and printing out its elements.

The loop `for (int j = 0; j < 10; j++)` sets conditions that allow for iterating over each element of the dynamically allocated array. The structure of a `for` loop typically includes:

• An initialization statement that usually sets up loop control variables.
• A termination condition (`j < 10` in this case) that tells the loop when to stop.
• An increment expression (`j++`) that updates the loop control variable.

In the code, loops manage the flow, ensuring that each element is methodically processed. They're crucial for systematic data updates and outputs, making them indispensable in C++ programming.