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Practice ​

The program finds the highest temperature of the week but prints 21 °C even though the array contains 25. Find and fix the error using the debugger.

cs
Console.OutputEncoding = System.Text.Encoding.UTF8;

int[] temps = [12, 15, 9, 21, 18, 14, 25];

int max = FindMax(temps, out int day);
Console.WriteLine($"Maximum {max} °C on day {day + 1}");

// BUG: the loop does not reach the last element.
static int FindMax(int[] values, out int index)
{
    index = 0;
    for (int i = 1; i < values.Length - 1; i++)
    {
        if (values[i] > values[index])
        {
            index = i;
        }
    }
    return values[index];
}

Output from the buggy program:

Maximum 21 °C on day 4

Debugging procedure:

  1. Set a breakpoint on if (values[i] > values[index]) and run the program (F5).
  2. Press F5 and watch i and index in the Locals window. To avoid stopping on every iteration, set the breakpoint condition to i >= values.Length - 2 (Conditions…).
  3. After stopping at i = 5, press F10 several times: the loop ends, and the condition is never evaluated for i = 6.
  4. Add values.Length - 1 to the Watch window: it equals 6, and the loop condition i < 6 excludes the last index.

The error is in the loop bound: it should be i < values.Length. After the fix, the program prints Maximum 25 °C on day 7. These off-by-one errors most often occur at loop boundaries, so check the first and last iterations.

Example 2. Fraction calculator ​

Write a program that adds fractions entered in a/b format until an empty line is entered, reduces the sum, and handles invalid input, a zero denominator, and overflow without terminating.

cs
Console.OutputEncoding = System.Text.Encoding.UTF8;
Console.InputEncoding = System.Text.Encoding.UTF8;

long numerator = 0, denominator = 1;
Console.WriteLine("Enter fractions a/b (empty line to finish):");

while (true)
{
    string? line = Console.ReadLine();
    if (string.IsNullOrWhiteSpace(line))
    {
        break;
    }
    try
    {
        (long a, long b) = ParseFraction(line);
        numerator = checked(numerator * b + a * denominator);
        denominator = checked(denominator * b);
        long gcd = Gcd(Math.Abs(numerator), denominator);
        numerator /= gcd;
        denominator /= gcd;
        Console.WriteLine($"  sum = {numerator}/{denominator}");
    }
    catch (FormatException ex)
    {
        Console.WriteLine($"  {ex.Message} Try again.");
    }
    catch (DivideByZeroException)
    {
        Console.WriteLine("  The denominator cannot be zero.");
    }
    catch (OverflowException)
    {
        Console.WriteLine("  The sum is too large; fraction skipped.");
    }
}

Console.WriteLine($"Result: {numerator}/{denominator}"
    + $" ≈ {(double)numerator / denominator:F4}");

// Parses “a/b”; moves the sign to the numerator.
static (long, long) ParseFraction(string text)
{
    string[] parts = text.Split('/');
    if (parts.Length != 2
        || !long.TryParse(parts[0], out long a)
        || !long.TryParse(parts[1], out long b))
    {
        throw new FormatException($"“{text}” is not an a/b fraction.");
    }

    if (b == 0)
    {
        throw new DivideByZeroException();
    }
    return b < 0 ? (-a, -b) : (a, b);
}

static long Gcd(long a, long b) =>
    b == 0 ? Math.Max(a, 1) : Gcd(b, a % b);

The ParseFraction method throws FormatException with a message containing the input line, and DivideByZeroException for a zero denominator. The loop catches exceptions separately for each fraction, so an error in one line does not stop the calculation. Calculations run in a checked context: overflowing long causes OverflowException instead of an incorrect sum, and the fraction is then skipped. The sum is reduced by the GCD of the numerator and denominator. Output:

Enter fractions a/b (empty line to finish):
1/2
  sum = 1/2
1/3
  sum = 5/6
2/0
  The denominator cannot be zero.
one quarter
  “one quarter” is not an a/b fraction. Try again.
-1/6
  sum = 2/3
9223372036854775807/2
  The sum is too large; fraction skipped.

Result: 2/3 ≈ 0,6667

Example 3. A robust console menu ​

Write a program with the commands add N (add a number to the list), avg (average), get I (element by index), div A B (integer division), and q (quit) that never crashes: errors in any command go to the error stream, and at the end the program reports the number of failed commands and sets the exit code.

cs
Console.OutputEncoding = System.Text.Encoding.UTF8;
Console.InputEncoding = System.Text.Encoding.UTF8;

int[] numbers = [];
int failures = 0;

Console.WriteLine("Commands: add N, avg, get I, div A B, q");
while (true)
{
    Console.Write("> ");
    string[] cmd = (Console.ReadLine() ?? "q").Split(' ',
        StringSplitOptions.RemoveEmptyEntries);
    if (cmd.Length == 0)
    {
        continue;
    }
    if (cmd[0] == "q")
    {
        break;
    }

    try
    {
        Execute(cmd, ref numbers);
    }
    catch (Exception ex) when (ex is FormatException
        or IndexOutOfRangeException or DivideByZeroException
        or InvalidOperationException)
    {
        failures++;
        Console.Error.WriteLine($"  Error: {ex.Message}");
    }
}

Console.WriteLine($"Failed commands: {failures}");
Environment.ExitCode = failures > 0 ? 1 : 0;

static void Execute(string[] cmd, ref int[] numbers)
{
    switch (cmd[0])
    {
        case "add":
            int value = int.Parse(cmd[1]);  // parse first
            Array.Resize(ref numbers, numbers.Length + 1);
            numbers[^1] = value;
            Console.WriteLine($"  Elements: {numbers.Length}");
            break;
        case "avg":
            if (numbers.Length == 0)
            {
                throw new InvalidOperationException(
                    "The list is empty.");
            }
            double sum = 0;
            foreach (int n in numbers)
            {
                sum += n;
            }
            double avg = sum / numbers.Length;
            Console.WriteLine($"  Average: {avg:F2}");
            break;
        case "get":
            int index = int.Parse(cmd[1]);
            Console.WriteLine($"  [{index}] = {numbers[index]}");
            break;
        case "div":
            int a = int.Parse(cmd[1]), b = int.Parse(cmd[2]);
            Console.WriteLine($"  {a / b}");
            break;
        default:
            throw new FormatException(
                $"Unknown command “{cmd[0]}”.");
    }
}

The Execute method runs commands, and the main loop catches only exceptions that can result from an invalid command (a when filter listing the types). Other exceptions (program bugs) are not caught, so they can be noticed and fixed. In add, the number is parsed before the array is enlarged: if parsing fails, the array stays unchanged. An unknown command is also reported with FormatException. Output:

Commands: add N, avg, get I, div A B, q
> avg
  Error: The list is empty.
> add 10
  Elements: 1
> add x
  Error: The input string 'x' was not in a correct format.
> add
  Error: Index was outside the bounds of the array.
> get 5
  Error: Index was outside the bounds of the array.
> add 25
  Elements: 2
> avg
  Average: 17,50
> div 7 0
  Error: Attempted to divide by zero.
> help
  Error: Unknown command “help”.
> q
Failed commands: 6

The program exits with code 1 because some commands failed.