Bonus Mini Project: Engineer's Calculator

The goal

In the last lesson of the course, we put the pieces together. We will write a small but real tool: an Engineer's Calculator that calculates according to the operation chosen from a menu and does not crash on bad input. The topics we will use: loops, the switch expression, methods, TryParse, List<T>, LINQ and error handling.

The program will have four options: (1) current calculation with Ohm's law, (2) series and parallel equivalent of two resistors, (3) grade average, (0) exit.

Step 1: Safe number reading

Instead of writing the same TryParse loop everywhere, let's write a single helper method. It keeps asking until the user enters a valid number:

static double ReadNumber(string prompt)
{
    while (true)
    {
        Console.Write(prompt);
        if (double.TryParse(Console.ReadLine(), out double value))
            return value;

        Console.WriteLine("  Invalid number, try again.");
    }
}

Step 2: Calculation methods

Keeping the calculations separate from the user interface is a good habit: the methods only calculate, and the caller handles printing.

static double OhmCurrent(double voltage, double resistance)
{
    if (resistance <= 0)
        throw new ArgumentException("Resistance must be greater than zero.");
    return voltage / resistance;
}

static double Series(double r1, double r2) => r1 + r2;

static double Parallel(double r1, double r2)
{
    if (r1 + r2 == 0)
        throw new ArgumentException("Both resistances cannot be zero at the same time.");
    return r1 * r2 / (r1 + r2);
}

Parallel equivalent: for two resistors, R = R1 × R2 / (R1 + R2). For example, for 100 and 220 ohm: 22000 / 320 = 68.75 ohm.

Step 3: The whole program

Here is the combined version. You can paste it into a single Program.cs file and run it with dotnet run:

bool keepGoing = true;

while (keepGoing)
{
    Console.WriteLine();
    Console.WriteLine("=== ENGINEER'S CALCULATOR ===");
    Console.WriteLine("1) Ohm's law (current)");
    Console.WriteLine("2) Series and parallel resistance");
    Console.WriteLine("3) Grade average");
    Console.WriteLine("0) Exit");
    Console.Write("Your choice: ");

    string? choice = Console.ReadLine();

    try
    {
        switch (choice)
        {
            case "1":
                double v = ReadNumber("Voltage (V): ");
                double r = ReadNumber("Resistance (ohm): ");
                double i = OhmCurrent(v, r);
                Console.WriteLine($"Current: {i:F4} A, power: {v * i:F3} W");
                break;

            case "2":
                double r1 = ReadNumber("1st resistance (ohm): ");
                double r2 = ReadNumber("2nd resistance (ohm): ");
                Console.WriteLine($"Series: {Series(r1, r2):F2} ohm");
                Console.WriteLine($"Parallel: {Parallel(r1, r2):F2} ohm");
                break;

            case "3":
                GradeAverage();
                break;

            case "0":
                keepGoing = false;
                Console.WriteLine("See you!");
                break;

            default:
                Console.WriteLine("Please choose a number from the menu.");
                break;
        }
    }
    catch (ArgumentException ex)
    {
        Console.WriteLine($"Could not calculate: {ex.Message}");
    }
}

static void GradeAverage()
{
    var grades = new List<double>();
    Console.WriteLine("Enter the grades one by one (press Enter on an empty line to finish):");

    while (true)
    {
        Console.Write($"  Grade {grades.Count + 1}: ");
        string? input = Console.ReadLine();
        if (string.IsNullOrWhiteSpace(input)) break;

        if (double.TryParse(input, out double grade) && grade >= 0 && grade <= 100)
            grades.Add(grade);
        else
            Console.WriteLine("  You must enter a number between 0 and 100.");
    }

    if (!grades.Any())
    {
        Console.WriteLine("No grades were entered.");
        return;
    }

    Console.WriteLine($"Average: {grades.Average():F2}");
    Console.WriteLine($"Highest: {grades.Max()}  Lowest: {grades.Min()}");
    Console.WriteLine($"Number of passing grades (>= 60): {grades.Count(n => n >= 60)}");
}

static double ReadNumber(string prompt)
{
    while (true)
    {
        Console.Write(prompt);
        if (double.TryParse(Console.ReadLine(), out double value))
            return value;
        Console.WriteLine("  Invalid number, try again.");
    }
}

static double OhmCurrent(double voltage, double resistance)
{
    if (resistance <= 0)
        throw new ArgumentException("Resistance must be greater than zero.");
    return voltage / resistance;
}

static double Series(double r1, double r2) => r1 + r2;

static double Parallel(double r1, double r2)
{
    if (r1 + r2 == 0)
        throw new ArgumentException("Both resistances cannot be zero at the same time.");
    return r1 * r2 / (r1 + r2);
}

A sample run (on a machine with English settings, the decimal separator is a period):

=== ENGINEER'S CALCULATOR ===
1) Ohm's law (current)
2) Series and parallel resistance
3) Grade average
0) Exit
Your choice: 2
1st resistance (ohm): 100
2nd resistance (ohm): 220
Series: 320.00 ohm
Parallel: 68.75 ohm

Try it yourself

1. Add a new operation: 4) Voltage divider (Vout = Vin × R2 / (R1 + R2)).
2. Make repeating easier: Save the last calculated result into a List<string> history and list it with a "5) History" option.
3. Move it into a class: Put the calculation methods inside a static class Calculations and separate them from the user interface.
4. Write to a file: On exit, save the history to a text file with File.WriteAllLines.

You have finished this course; you now have the foundation to write real tools in C#. The next stops: file operations, async/await, windowed applications with WinForms, and the web with ASP.NET.