Question

What is the output of the following C++ code? int *tempList; int num = 3; tempList = new int[7]; tempList[6] = 4; for (int j = 5; j >= 0; j--) tempList[j] = tempList[j + 1] + j * num; for (int j = 0; j < 7; j++) cout << tempList [j] << " "; cout << endl;

Short answer

The output is: 49 49 46 40 31 19 4

Step-by-step solution

Initialize Pointers and Variables

We start by declaring a pointer `tempList` that will hold the address of dynamic memory of type integer and an integer variable `num` which is initialized to 3.

Allocate Dynamic Memory

Next, dynamic memory allocation is done using `new int[7]`, which creates an array of seven integers and assigns its address to `tempList`.

Set Initial Value in Array

The last index of the array, `tempList[6]`, is assigned the value 4.

Populate the Array with Loop

A loop runs backwards from 5 to 0, updating each element in the array using the formula `tempList[j] = tempList[j + 1] + j * num`. This modifies each element based on the next element to the right plus the product of the current index (`j`) and `num`.

Calculate Each Element's Value

- For `j = 5`: `tempList[5] = tempList[6] + 5 * 3 = 4 + 15 = 19` - For `j = 4`: `tempList[4] = tempList[5] + 4 * 3 = 19 + 12 = 31` - For `j = 3`: `tempList[3] = tempList[4] + 3 * 3 = 31 + 9 = 40` - For `j = 2`: `tempList[2] = tempList[3] + 2 * 3 = 40 + 6 = 46` - For `j = 1`: `tempList[1] = tempList[2] + 1 * 3 = 46 + 3 = 49` - For `j = 0`: `tempList[0] = tempList[1] + 0 * 3 = 49 + 0 = 49`

Output the Array

A loop iterates from 0 to 6 and prints each element in `tempList` followed by a space. The newline is printed after exiting the loop.

Key concepts

Dynamic Memory Allocation

In C++ programming, dynamic memory allocation is used to allocate memory at runtime. This means we are requesting a specific amount of memory from the system's heap while the program is executing. Using `new` in C++, we can allocate memory dynamically.
For instance, `tempList = new int[7];` allocates an array of integers that can hold 7 elements. This memory is not tied to the program's stack, which disappears when a function exits. Instead, it persists until explicitly deallocated using `delete` or `delete[]`.

• Key to dynamic memory allocation is efficient management, preventing leaks.
• Always ensure allocated memory is deallocated to avoid memory wastage.

Pointer Manipulation

Pointers are a fundamental concept in C++ that allow handling memory and array elements efficiently. Pointers store the address of a variable or a dynamically allocated block of memory. In this example, `int *tempList;` declares `tempList` as a pointer to an integer.
Initially, `tempList` points to dynamic memory allocated by `new`. This allows direct manipulation of the memory block, which is especially useful for arrays.

• Using a pointer, we can modify elements directly using arithmetic operations.
• Pointers help navigate through complex data structures like arrays within loops.

Loop Iteration

Loop iteration is fundamental in programming, allowing repetitive operations efficiently. In this exercise, loop structures are used to populate and later print the array elements.
The first loop goes backward from 5 to 0. It updates each element by referencing the element to its right (`tempList[j + 1]`) and performing calculations. The second loop runs forwards from 0 to 6, printing each element.

• The backward loop modifies values dynamically, creating dependency on previously calculated values.
• The forward loop ensures all results are displayed in a sequence, showcasing changes due to earlier computations.

Array Initialization

Array initialization is about setting up arrays before they are used. In C++, arrays can be dynamically allocated and initialized using loops or direct assignments.
After memory allocation like `tempList = new int[7];`, initializing array elements ensures each index has an expected value. In this exercise, specific initialization happens with `tempList[6] = 4;`.

• Initial value setting affects subsequent computations.
• Proper initialization helps prevent accessing uninitialized or junk values, ensuring predictable outcomes.