Question

What is the output of the following C++ code? vector intList(5); int i; for (i = 0; i < 5; i++) intList[i] = 2 * i + 1; for (i = 0; i < 5; i++) cout << intList.at(i) << " "; cout << endl;

Short answer

The output is: 1 3 5 7 9

Step-by-step solution

Initialize the vector

The code begins by creating a vector named `intList` which is of type `int`. The vector is initialized with a size of 5, meaning it will store 5 integers.

Populate the vector

The first for loop runs with `i` starting from 0 up to, but not including, 5. In each iteration, `intList[i]` is assigned the value `2 * i + 1`. This results in `intList` being populated as follows: - When `i = 0`, `intList[0] = 1` - When `i = 1`, `intList[1] = 3` - When `i = 2`, `intList[2] = 5` - When `i = 3`, `intList[3] = 7` - When `i = 4`, `intList[4] = 9`.

Print the vector

The second for loop also runs with `i` from 0 to 4. In each iteration, the element `intList.at(i)` is printed followed by a space. Consequently, each element of `intList`, which are 1, 3, 5, 7, and 9, is printed sequentially. Finally, `cout << endl;` prints a newline character.

Key concepts

C++ Programming

C++ is a powerful and versatile programming language widely used for various applications. It is known for its efficiency and control over system resources. One of C++'s key strengths is its ability to work with both low-level memory manipulation and high-level abstractions through features like classes and templates.
C++ supports object-oriented programming, a paradigm that allows for organizing code into reusable parts. This makes it easier to manage large projects. Additionally, C++ has robust standard libraries that provide common functions and data structures, helping programmers avoid rewriting common routines.
Understanding the syntax and semantics of C++ is crucial. It includes using correct identifiers, type declarations, and operators, all of which play a significant role in writing efficient and readable code. Proper use of loops, conditional statements, and functions is also important to harness the full power of C++.

Vectors in C++

Vectors in C++ are a part of the Standard Template Library (STL). They are dynamic arrays that can resize automatically when elements are added or removed. Unlike traditional arrays, vectors offer functionality to handle their size, which can make code more efficient and easier to manage.
A vector is defined by specifying its type inside angle brackets, like `vector`. You can initialize a vector with a specific size, as seen in the code snippet where `intList` is initialized with a size of 5. This means the vector will initially store five elements.

• Vectors provide functions such as `push_back()` to add elements, and `size()` to retrieve the current number of elements.
• The elements can be accessed using either `[]` operator or `.at()` method, where the latter provides bounds checking.
• Vectors are particularly useful for situations where the size of the dataset can vary during the lifetime of the application.

C++ for loops

For loops in C++ are an essential control structure designed to iterate over a range or data set repeatedly. The loop consists of three main parts: initialization, condition, and increment.
In the first loop of the code, `for (i = 0; i < 5; i++)`, the initialization part establishes the variable `i` starting at 0. The condition `i < 5` keeps the loop running as long as `i` is less than 5, and the increment `i++` increases `i` by one after each loop iteration.

• The first loop populates the vector `intList` by assigning values based on the expression `2 * i + 1`.
• The second loop, `for (i = 0; i < 5; i++)`, does not modify the vector but instead, prints the contents of `intList`.

For loops are versatile because they allow iteration over arrays and vectors, providing control over how many times a section of code is executed. They are especially helpful when tasks need to be repeated a specific number of times.

C++ Output

Output in C++ is commonly handled using the `cout` object, which is part of the standard library's iostream. It allows printing messages and variables to the console, facilitating user interaction or program feedback.
The code demonstrates how output is generated using `cout << intList.at(i) << " "`. Each element of the vector `intList` is printed followed by a space. Finally, `cout << endl;` is used to move the cursor to a new line, ensuring that the next output starts on a fresh line.

• `cout` can be combined with formatting options, like fixed decimals or widths, to control display output.
• `endl` not only moves to a new line but also flushes the stream, ensuring all output is immediately displayed.
• Knowing how to effectively use `cout` is crucial for debugging and user communication in C++ programs.

Mastering `cout` and other output functions allows for clear and informative programs, enhancing the experience for both the programmer and the end users.