Question

What is the output of the following code? int *p; int *q; int i; p = new int[5]; p[0] = 5; for (i = 1; i < 5; i++) p[i] = p[i - 1] + 2 * i; cout << "Array p: "; for (i = 0; i < 5; i++) cout << p[i] << " "; cout << endl; q = new int[5]; for (i = 0; i < 5; i++) q[i] = p[4 - i]; cout << "Array q: "; for (i = 0; i < 5; i++) cout << q[i] << " "; cout << endl;

Short answer

Array p: 5 7 11 17 25; Array q: 25 17 11 7 5.

Step-by-step solution

Initialize and Allocate Arrays

Two integer pointers, `p` and `q`, are declared. An integer `i` is also declared. Memory is dynamically allocated for an array of 5 integers using `p`.

Populate Array p

Array `p` is initialized with `p[0] = 5`. A loop is used to fill in the rest of the array from `p[1]` to `p[4]` using the formula: `p[i] = p[i - 1] + 2 * i`. This results in `p` being [5, 7, 11, 17, 25].

Display Array p

The program outputs "Array p:" followed by each element of `p`, resulting in the sequence: `5 7 11 17 25`.

Populate Array q

Memory is allocated for another array of 5 integers using `q`. A loop is used to populate `q` such that each element of `q` is the reverse of `p`. Therefore, `q` becomes [25, 17, 11, 7, 5].

Display Array q

The program outputs "Array q:" followed by each element of `q`, resulting in the sequence: `25 17 11 7 5`. This confirms that `q` contains the elements of `p` in reverse order.

Key concepts

Pointers in C++

In C++, a pointer is a variable that stores the memory address of another variable. Pointers are essential in C++ programming as they allow for dynamic memory management, array manipulation, and efficient handling of large data structures. Dynamic memory allocation using pointers is key for creating flexible programs.
In the exercise, we use the `int *p` and `int *q` to point to arrays in memory. The `new` keyword is used to dynamically allocate memory for an array of integers. For example, `p = new int[5];` allocates space for storing 5 integers, which `p` points to.
By manipulating pointers, you can directly access and modify the values stored in the allocated memory. This is evident as we use `p[i] = p[i - 1] + 2 * i` to modify the contents of the array. Remember:

• Pointers must be carefully managed to prevent memory leaks (unused memory that is not deallocated).
• Always delete dynamically allocated memory using `delete[]` to free up space when done.
• Pointers can enhance performance but require careful handling to avoid segmentation faults.

Array Manipulation

Arrays in C++ are collections of elements of the same type stored in contiguous memory locations. They offer an efficient way to store and manage multiple items using a single variable name and index numbers.
In our example, we use arrays to store sequences of integers for `p` and `q`. The process of manipulating arrays includes initializing, accessing, and updating their elements. The operation `p[i] = p[i - 1] + 2 * i` adds a calculated value to each element, modifying `p` based on its previous values.
For `q`, we create an array that stores the same elements as `p`, but in reverse order. This demonstrates not just simple initialization but also advanced manipulation techniques.

• By using loops, elements of an array can easily be accessed and modified.
• Array indices start from 0, so the first element is at index 0 and the last element at `length - 1`.
• C++ does not check array bounds, so you need to be cautious to prevent accessing invalid positions.

Loop Structures in C++

Loop structures are used to execute a block of code repeatedly, based on a condition. C++ provides several types of loops: `for`, `while`, and `do-while`. Each serves different use cases but shares the goal of repeating code efficiently.
Our exercise predominantly uses `for` loops, which are a concise way to iterate over arrays. We initiate the loop by setting the starting point (`i = 1` or `i = 0`) and specifying a condition for continuation (`i < 5`). The third part of a `for` loop is the increment, updating `i` after each iteration.
Here, the loops are used to:

• Populate `p` by building upon each previous element using `p[i] = p[i - 1] + 2 * i`.
• Reverse the order of elements from `p` to create `q`.

Loops offer:

• Efficiency in handling repetitive tasks, especially with arrays.
• Flexibility in traversing any sequence or range of values.

Choosing the correct loop structure is important for clarity and performance of your program. In our case, `for` loops offer clear, concise, and efficient iterations, making them ideal for the task.