Question

a. Write a statement that declares sales to be a pointer to a pointer of type double. b. Write a C++ code that dynamically creates a two-dimensional array of five rows and seven columns and sales contains the base address of that array. c. Write a C++ code that inputs data from the standard input device into the array sales. d. Write a C++ code that deallocates the memory space occupied by the two- dimensional array to which sales points.

Short answer

Declare: `double **sales;`. Allocate with loops to create a 5x7 array, input data with `cin`, and deallocate using `delete` for each pointer and the pointer array.

Step-by-step solution

Declare the Pointer

To declare 'sales' as a pointer to a pointer of type double, you will use the following code in C++: ```cpp double **sales; ``` This code creates a variable 'sales' which can be used to store the address of another pointer, which in turn will store the address of a double type value. This setup is useful for creating dynamic 2D arrays.

Allocate Memory for the Array

Next, we'll write C++ code to dynamically create a 2D array and store its base address in 'sales'. We want to create a 2D array with 5 rows and 7 columns: ```cpp sales = new double*[5]; // Create an array of 5 double pointers for (int i = 0; i < 5; ++i) { sales[i] = new double[7]; // For each row, create a new double array with 7 elements } ``` This code first allocates memory for an array of 5 pointers to double. Each pointer then gets assigned to a new array of 7 doubles, effectively creating a 5x7 matrix.

Input Data into the Array

To input data into the array 'sales', loop through each element and use 'cin' to receive user input: ```cpp for (int i = 0; i < 5; ++i) { for (int j = 0; j < 7; ++j) { std::cout << "Enter value for sales[" << i << "][" << j << "]: "; std::cin >> sales[i][j]; } } ``` This nested loop iterates over each row and column, prompting the user to enter a value for each element in the 2D array.

Deallocate the Memory

Finally, to avoid memory leaks, we need to deallocate the dynamically allocated memory: ```cpp for (int i = 0; i < 5; ++i) { delete[] sales[i]; // Delete each row } delete[] sales; // Delete the array of pointers ``` This code iterates through each row of the matrix, deleting the memory allocated for each array of doubles. After deleting all the rows, it deletes the memory allocated for the array of pointers.

Key concepts

Pointer Arithmetic

Pointer arithmetic is a crucial concept in programming, especially when dealing with dynamic memory allocation. In C++, a pointer is a variable that stores the memory address of another variable. When you declare a pointer to a pointer, like the statement `double **sales;`, you're setting up a scenario where the pointer can point to another pointer, which then points to an actual data type. This double layer is essential for creating 2D arrays dynamically.

Pointer arithmetic allows you to move through memory locations that a pointer references. For example, if you increment a pointer, it goes to the next memory location that type variable would occupy. For a double type, it will move as many bytes as a double uses. This arithmetic underpins the indexing of arrays, where you can iterate over the array by incrementing pointers. This mechanism helps in accessing elements based on their index in the virtual grid of a 2D array.

2D Arrays

Two-dimensional arrays, or 2D arrays, are arrays of arrays. They model a grid-like structure with rows and columns. In scenarios where dimensions are not known at compile time, we create these arrays dynamically. For example, the code snippet: ```cpp double **sales; sales = new double*[5]; for (int i = 0; i < 5; ++i) { sales[i] = new double[7]; } ``` This code dynamically builds a structure to store data. Here, `sales` points to an array of 5 pointers, with each pointer leading to an array of 7 `double` elements. This essentially forms a 5x7 matrix in memory.

• Creating 2D arrays involves determining the number of rows and columns.
• Each row is allocated separately, allowing flexible memory usage compared to static arrays.

This flexibility is one of the reasons dynamic memory allocation is often employed for 2D arrays.

Memory Deallocation

Memory management is a critical aspect when working with dynamically allocated structures. If you do not properly manage memory, your program could suffer from memory leaks—where memory that is no longer needed is not returned to the system. In the context of the dynamic 2D array, each row of data, as well as the array of row pointers, needs deallocation.

The code: ```cpp for (int i = 0; i < 5; ++i) { delete[] sales[i]; } delete[] sales; ``` Deallocates memory row by row, first cleaning up each row of data, then clearing the array of pointers. Doing so frees the memory that was previously occupied, ensuring that the system can reallocate it elsewhere. Memory deallocation is as vital as its allocation to maintain efficiency and program stability.

User Input Handling

Gathering user input efficiently is an integral part of interactive programs. In the solution exercise, input management ensures that each cell in our 2D array stores user-provided data. The loop structure used for input: ```cpp for (int i = 0; i < 5; ++i) { for (int j = 0; j < 7; ++j) { std::cout << "Enter value for sales[" << i << "][" << j << "]: "; std::cin >> sales[i][j]; } } ``` showcases a simple yet effective way to fill a 2D array with user data.

Effective input handling involves several best practices:

• Providing clear prompts to the user so they know what to enter.
• Validating input for correctness to prevent erroneous or unexpected data.
• Handling input errors gracefully to maintain program stability.

These practices ensure data integrity and improve the user experience, making robust input handling a cornerstone of program design.