Question

Write a complete \(\mathrm{C}_{+}+\) program that outputs the numbers 1 to 20 , one per line. The program does nothing else.

Short answer

### Short Answer To create a simple C++ program that outputs the numbers 1 to 20, one per line, a loop that iterates 20 times and an output statement are needed. The loop must be implemented using a "for" loop to set an initialization statement, a condition, and an update statement. The iostream library should be included, and the main function should be defined. Inside the main function, the "for" loop should be created with a variable that iterates from 1 to 20. Then, the program should output the current value of the variable followed by a newline character using "cout." The final code should look like this: ```cpp #include using namespace std; int main() { for (int i = 1; i <= 20; ++i) { cout << i << endl; } return 0; } ```

Step-by-step solution

Include necessary headers

To start with, we need to include the standard input-output library, i.e., "iostream," which provides us with the necessary functions for printing output. Moreover, we need to use the "std" namespace to avoid writing "std::" before every standard library function or class. To include the necessary headers: ```cpp #include using namespace std; ```

Define the main function

Now, we need to define the main function, which is the entry point of any C++ program. Here's how to define the main function: ```cpp int main() { } ```

Implement the loop that prints numbers 1 to 20

Inside the main function, we'll implement a loop that iterates 20 times. For this, we can use the "for" loop, which takes three arguments: an initialization statement, a condition, and an update statement. We'll initialize a variable "i" with the value 1, set the condition for continuation as "i <= 20," and update the variable by incrementing it in each iteration. Within the loop, we'll print the current value of "i" followed by a newline character to achieve the desired output. Here's how to implement this: ```cpp for (int i = 1; i <= 20; ++i) { cout << i << endl; } ```

Complete the program

Finally, put all the code snippets together to create the complete C++ program. This is how the final program should look: ```cpp #include using namespace std; int main() { for (int i = 1; i <= 20; ++i) { cout << i << endl; } return 0; } ``` This program will output the numbers 1 to 20, one per line, as required by the exercise.

Key concepts

for loop

A `for loop` is a control flow statement for specifying iteration, which habitually allows code to be executed repeatedly. In C++, it is a convenient way to loop over a range of numbers, such as printing numbers from 1 to 20. The `for loop` consists of three parts within its brackets, separated by semicolons:

• Initialization: This sets a starting point, often a variable defined as `int i = 1`.
• Condition: Here, you specify a condition for the loop to continue, e.g., `i <= 20`. The loop runs as long as this condition is true.
• Increment/Update: This alters the loop variable, here increasing `i` by 1 after each iteration using `++i`.

Executing this loop gives a methodical procedure to repeatedly run code (in our case, `cout << i << endl;`) to print each number on a new line until `i` becomes greater than 20.

iostream library

The `iostream` library in C++ provides essential functionalities for input-output operations. These operations represent reading from the input device (like a keyboard) and writing to an output device (like a screen).

The standard input-output functions include:

• `cin`: Reads input from the user.
• `cout`: Displays output to the user.

Including the library is done with the line `#include `, which must be present at the very beginning of the C++ program. Without this, you can't use `cin` or `cout`, making it impossible to print numbers from 1 to 20, or get input from users effectively.

main function

The `main` function is vital because it marks the program's entry point; that is, it is where your program begins and ends execution. In C++ programming, no code runs outside a function, and the `main` function is special.

Some traits of the `main` function include:

• Return Type: It has an integer return type `int`, because it returns an integer to the operating system upon completing execution.
• Body: Contains curly braces `{}` within which all the executable statements, including loop logic and output commands, are placed.
• Return Statement: Typically, after completing the tasks, a `return 0;` statement is used to indicate the program ended successfully.

If omitted, on completion, the compiler assumes the program should return 0. However, explicitly stating it helps avoid confusion and maintain consistency in programming style.

standard namespace

Namespaces are used in C++ to organize code and avoid naming conflicts. The `standard namespace` is particularly important because it includes all the names from the C++ standard library.

When using standard library components like `cout` and `cin`, prefixing with `std::` becomes repetitive. For instance, you would have to write `std::cout` for console output.
To ease this, the line `using namespace std;` is included after `#include `. It tells the compiler to check the standard namespace for any unresolved functions or classes, allowing direct use of `cout`, `cin`, and other library functions without repeating `std::`.

While this is handy, it is a broad declaration leading to possible conflicts in larger programs. It's acceptable in smaller programs or exercises, like printing numbers from 1 to 20, to make code cleaner and easier to read.