Question

Look at the following array definition. int numbers[5] = { 1, 2, 3 }; What value is stored in numbers[2]? What value is stored in numbers[4]?

Short answer

Answer: The value stored at index 2 is 3, and the value stored at index 4 is 0.

Step-by-step solution

Understanding Array Initialization

In C, when an array is declared with a specific size and initialized with less elements than its size, the remaining elements will be automatically initialized with the value 0. In this case, the array 'numbers' is of size 5, but only three elements are provided (1, 2, and 3). Therefore, the remaining elements (numbers[3] and numbers[4]) will be initialized with the value 0.

Find the Value at Index 2

To find the value stored in numbers[2], we can simply look at the array initialization. The array is defined as int numbers[5] = { 1, 2, 3 }; Since array indices start from 0, the element at index 2 is 3. So, the value stored in numbers[2] is 3.

Find the Value at Index 4

To find the value stored in numbers[4], recall that the remaining elements of the array (after the initial 1, 2, and 3) will be initialized with the value 0, as explained in Step 1. So, the value stored in numbers[4] is 0.

Key concepts

Array Initialization

In C++, creating an array involves specifying its size and optionally initializing it with values. Consider this declaration: ```cpp int numbers[5] = {1, 2, 3}; ``` Here, we are creating an array called `numbers` with 5 elements. The values `{1, 2, 3}` are used to initialize the first few elements of the array. When the number of specified values is less than the array size, the remaining elements must be initialized in some way. This leads to what is known as default initialization, which we'll cover next.
In summary, initializing arrays properly is essential, as it determines what values exist at specific indices, particularly important when utilizing partially filled arrays. This practice not only aids in defect prevention but also maintains predictable program behavior.

Default Values in Arrays

When you initialize an array with fewer values than its specified size, C++ automatically sets the missing indices to default values. For integer arrays, this default value is zero. This behavior occurs because uninitialized variables or elements can lead to undefined states, which might cause unexpected program results or crashes.
For the array `int numbers[5] = {1, 2, 3};`, the elements are initialized as follows:

• `numbers[0]` is 1
• `numbers[1]` is 2
• `numbers[2]` is 3
• `numbers[3]` defaults to 0
• `numbers[4]` defaults to 0

This default initialization ensures that all spaces in the array contain predictable values, which is crucial for debugging and maintaining the program's logic flow.

Indexing in Arrays

Understanding how to access elements in an array using indices is fundamental in C++. Indexing starts at 0, meaning the first element of the array is accessed with the index 0, the second with 1, and so on.
Consider the following example: ```cpp int numbers[5] = {1, 2, 3}; ``` To find the value at a specific index in this array, we use the bracket notation `numbers[index]`. - `numbers[2]` refers to the third element, which is initialized to 3 as per the array declaration. - `numbers[4]`, which refers to the fifth position, is 0 due to default initialization. This indexing allows you to efficiently loop through arrays for processing each element or for modifying array contents. It's a core concept in managing collections of items in programming.