Question

. What is the output of the following C++ code? vector intList(10); for (int i = 0; i < 10; i++) intList[i] = 2 * i + 5; cout << intList.front() << " " << intList.back() << endl;

Short answer

The output of the code is `5 23`. The first element is 5 and the last element is 23.

Step-by-step solution

Understanding Vector Initialization

The vector `intList` is initialized with size 10, which means it can hold 10 integer elements. At this point, all elements are default-initialized to 0.

Evaluating the For Loop

The for loop runs from `i = 0` to `i = 9`. In each iteration, the element `intList[i]` is assigned the value `2*i + 5`. The loop initializes elements such that: - When `i = 0`, `intList[0] = 5` - When `i = 1`, `intList[1] = 7` - When `i = 2`, `intList[2] = 9` - Continue this pattern up to `i = 9`, where `intList[9] = 23`.

Calculating Front and Back Elements

The `intList.front()` method returns the first element of the vector, which is `intList[0] = 5`. The `intList.back()` method returns the last element of the vector, which is `intList[9] = 23`.

Understanding the Output Statement

The `cout` statement outputs `intList.front()` followed by a space and then `intList.back()`. So, it displays `5 23`.

Key concepts

Vector Initialization

In C++, vectors are dynamic arrays provided by the Standard Template Library (STL). They are versatile and can grow or shrink in size. Initialization is a crucial first step when working with vectors. Consider the line: `vector intList(10);`. This initializes a vector named `intList` with a capacity to hold 10 integer elements. Initially, all elements are set to 0 due to the default initialization of integer types. This is useful because it lets you start with a known state before you fill in the elements with specific values.

For Loop

The `for` loop is a common way to iterate over elements in C++, allowing you to perform operations across a series of elements efficiently. Take a look at this code:

• `for (int i = 0; i < 10; i++) intList[i] = 2 * i + 5;`

The loop executes with the counter `i` starting at 0 and increasing by 1 each iteration until it reaches 9. Each element of `intList` is assigned a value computed as `2 * i + 5`. This operation populates the vector with calculated values, changing its state from all zeros to: - `intList[0] = 5` - `intList[1] = 7` - Continues until `intList[9] = 23`. This loop not only initializes the vector but ensures each element has the correct value as per the given formula.

Vector Methods

Vectors come with several handy methods that make it easy to perform common tasks. Two important methods demonstrated here are `front()` and `back()`. These methods provide quick access to the first and last elements of the vector. For instance:

• `intList.front()` returns the first element, `intList[0]`, which is 5 in this case.
• `intList.back()` returns the last element, `intList[9]`, which is calculated to be 23.

These methods are useful in scenarios where only the boundary elements are needed, allowing efficient access without looping through the vector.

Output in C++

In C++, output can be achieved using the `cout` stream, which is straightforward for displaying values. Consider this line:

• `cout << intList.front() << " " << intList.back() << endl;`

Here, it outputs the values retrieved by `intList.front()` and `intList.back()`, separated by a space. This results in `5 23` being printed to the console, reflecting the initial and final elements of the vector. The `endl` part signals the end of the line, ensuring any subsequent output starts on a new line. Using `cout` in this way is a simple and effective method for displaying vector elements in C++.