Question

The following program is supposed to read two numbers from a file named input.dat and write the sum of the numbers to a file named output.dat. However, it fails to do so. Rewrite the program so that it accomplishes what it is intended to do. (Also, include statements to close the files.) #include #include using namespace std; int main() { int num1, num2; ifstream infile; outfile.open("output.dat"); infile >> num1 >> num2; outfile << "Sum = " << num1 + num2 << endl; return 0; }

Short answer

Fix the file opening errors and ensure all files are properly closed.

Step-by-step solution

Identify Missing Initialization

The program does not open the input file before reading. Add the correct statement to open 'input.dat' using the ifstream object, infile.

Open Input File

Insert the line 'infile.open("input.dat");' before attempting to read from infile.

Correct Output File Opening

The output file is opened without defining `outfile` first. Add an ofstream declaration: 'ofstream outfile;' before the outfile.open statement.

File Closing Statements

Ensure that both files are closed properly by using `infile.close();` and `outfile.close();` before the return statement.

Complete and Corrected Code

```cpp #include #include using namespace std; int main() { int num1, num2; ifstream infile; ofstream outfile; infile.open("input.dat"); outfile.open("output.dat"); if (infile.is_open() && outfile.is_open()) { infile >> num1 >> num2; outfile << "Sum = " << num1 + num2 << endl; } else { cout << "Unable to open file(s)" << endl; } infile.close(); outfile.close(); return 0; } ```

Key concepts

ifstream and ofstream usage

In C++, files are handled with the help of the `ifstream` and `ofstream` classes. These classes are part of the `` header, which needs to be included at the start of your program. - **`ifstream` (input file stream):** This is used for reading data from files. You create an instance of `ifstream` and use its `open()` method to access the file you want to read. - **`ofstream` (output file stream):** This is used for writing data to files. Similarly, this requires an instance of `ofstream` which also uses the `open()` method to point to the desired file to write into. **Steps to engage these streams:** - Instantiate the stream object. For example, `ifstream infile;` and `ofstream outfile;`. - Open the file using `open()`, such as `infile.open("input.dat");`. - Use the object to read from or write to the file using the `>>` operator for input or the `<<` operator for output. Finally, don't forget to close the files using `close()`, which is crucial for freeing resources and avoiding data corruption.

Error handling with file streams

Error handling is essential when working with file streams. The success of operations like opening a file isn't guaranteed, so you need to check if the file was opened successfully. You can use the `is_open()` method on file stream objects to verify whether a file was successfully opened. - Checking **`is_open()`** helps ensure your program doesn't attempt to read or write when the file isn't ready. Here's how to handle errors effectively during file operations: - **Use Conditional Statements:** After opening a file, immediately check its status using an `if` condition. For example: ```cpp if (infile.is_open()) { // Proceed with file operations } else { cout << "Unable to open input.dat" << endl; } ``` This condition ensures that you only proceed if `infile` was opened successfully. - **Print Error Messages:** Clearly indicate to the user if a file failed to open. This helps debug your program and alerts you to potential issues. Error handling not only avoids runtime errors but also provides a seamless user experience by gracefully managing file-related issues.

Basic C++ syntax and structure

Understanding basic C++ syntax and structure is crucial for constructing effective programs. At its simplest, a C++ program consists of: - **Preprocessor Directives:** These include libraries and define constants, noted by the `#include <...>` statement at the top. For file operations, including `` and `` is necessary. - **`main()` Function:** This is the starting point of any C++ program. Its return type is `int`, and it usually ends with `return 0;` to indicate successful execution. In the program given, basic syntax elements like variable declaration (`int num1, num2;`) and using standard input and output (`cin` and `cout`) are paramount. - **Variable Declaration:** Before using any variable, it needs to be declared with a type, such as `int`, `float`, or `char`. - **Namespace Usage:** The `using namespace std;` directive allows the omission of `std::` prefix for standard library objects and functions. - **Control Structures:** Conditional statements (`if` or `else`) play a key role, especially when handling file operations for error checking. Adherence to C++ syntax and understanding program structure not only makes code comprehensible but also ensures its effectiveness in performing the intended tasks.