Dev Blog

Examples and common mistakes ​

Example programs ​

ECTS grade ​

The program converts a point total to an ECTS grade and a national-scale grade according to the grading scale of Kryvyi Rih National University. Both conversions use switch expressions: the first uses relational patterns, and the second uses constant patterns joined by or.

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

Console.Write("Points (0–100): ");
if (!int.TryParse(Console.ReadLine(), out int points)
    || points is < 0 or > 100)
{
    Console.WriteLine("Error: an integer from 0 to 100 is required.");
    return;
}

string ects = points switch
{
    >= 90 => "A",
    >= 80 => "B",
    >= 71 => "C",
    >= 61 => "D",
    >= 50 => "E",
    >= 30 => "FX",
    _ => "F",
};

string national = ects switch
{
    "A" => "excellent",
    "B" or "C" => "good",
    "D" or "E" => "satisfactory",
    _ => "unsatisfactory",
};

Console.WriteLine($"ECTS grade: {ects}");
Console.WriteLine($"National-scale grade: {national}");

Results for 85 and 47 points and for an invalid value:

Points (0–100): 85
ECTS grade: B
National-scale grade: good
Points (0–100): 47
ECTS grade: FX
National-scale grade: unsatisfactory
Points (0–100): 120
Error: an integer from 0 to 100 is required.

Calculator menu ​

The calculator stores the current value and executes user commands in a do/while loop until the user enters q. A switch statement handles commands, and a branch with a when condition handles division by zero.

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

double result = 0;
string command;

do
{
    Console.WriteLine($"Current value: {result}");
    Console.Write("Command (+, -, *, /, c – reset, q – quit): ");
    command = Console.ReadLine() ?? "q";

    if (command == "c")
    {
        result = 0;
        continue;                 // go to the loop condition check
    }
    if (command is not ("+" or "-" or "*" or "/"))
    {
        if (command != "q")
        {
            Console.WriteLine($"Unknown command “{command}”.");
        }
        continue;
    }

    Console.Write("Number: ");
    if (!double.TryParse(Console.ReadLine(), out double number))
    {
        Console.WriteLine("This is not a number.");
        continue;
    }

    switch (command)
    {
        case "+":
            result += number;
            break;
        case "-":
            result -= number;
            break;
        case "*":
            result *= number;
            break;
        case "/" when number == 0:
            Console.WriteLine("Cannot divide by zero.");
            break;
        case "/":
            result /= number;
            break;
    }
} while (command != "q");

Console.WriteLine($"Result: {result}");

The continue statement in a do/while loop jumps to the condition check, so the q command ends the loop immediately. The ?? operator supplies "q" when input ends (ReadLine returns null). The not ("+" or "-" or "*" or "/") pattern checks that the command is none of these operators. Example session:

Current value: 0
Command (+, -, *, /, c – reset, q – quit): +
Number: 12,5
Current value: 12,5
Command (+, -, *, /, c – reset, q – quit): *
Number: 4
Current value: 50
Command (+, -, *, /, c – reset, q – quit): /
Number: 0
Cannot divide by zero.
Current value: 50
Command (+, -, *, /, c – reset, q – quit): %
Unknown command “%”.
Current value: 50
Command (+, -, *, /, c – reset, q – quit): -
Number: 10
Current value: 40
Command (+, -, *, /, c – reset, q – quit): q
Result: 40

Statistics for entered numbers ​

The program reads numbers until a blank line, skips invalid values, and displays the count, minimum, maximum, and average.

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

int count = 0;
double sum = 0;
double min = double.MaxValue;
double max = double.MinValue;

Console.WriteLine("Enter numbers; a blank line ends input:");
string? line = Console.ReadLine();

while (!string.IsNullOrEmpty(line))
{
    if (double.TryParse(line, out double value))
    {
        count++;
        sum += value;
        min = Math.Min(min, value);
        max = Math.Max(max, value);
    }
    else
    {
        Console.WriteLine($"  “{line}” skipped: not a number");
    }
    line = Console.ReadLine();
}

if (count == 0)
{
    Console.WriteLine("No numbers were entered.");
}
else
{
    Console.WriteLine($"Count: {count}");
    Console.WriteLine($"Minimum: {min}, maximum: {max}");
    Console.WriteLine($"Average: {sum / count:F2}");
}

A line is read before the loop and again at the end of its body, so the while condition checks a new line each time. The string.IsNullOrEmpty method returns true for both an empty string and null (end of input). The minimum starts at the largest possible value, so the first number will lower it. The count == 0 check prevents division by zero. Output:

Enter numbers; a blank line ends input:
12,5
7
seven
  “seven” skipped: not a number
-3
20

Count: 4
Minimum: -3, maximum: 20
Average: 9,12

Multiplication table ​

The program asks for the table size, repeats the prompt until a valid value is entered, and displays a multiplication table using nested for loops. The field width {…,4} right-aligns the numbers.

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

Console.Write("Table size (1–12): ");
int size;
while (!int.TryParse(Console.ReadLine(), out size)
       || size < 1 || size > 12)
{
    Console.Write("Enter an integer from 1 to 12: ");
}

Console.Write("   │");                     // column headings
for (int col = 1; col <= size; col++)
{
    Console.Write($"{col,4}");
}
Console.WriteLine();
Console.WriteLine("───┼" + new string('─', 4 * size));

for (int row = 1; row <= size; row++)     // outer loop
{
    Console.Write($"{row,2} │");
    for (int col = 1; col <= size; col++) // inner loop
    {
        Console.Write($"{row * col,4}");
    }
    Console.WriteLine();
}

The while loop header both reads and validates the number: TryParse stores the result in the size variable declared before the loop. The characters │, ┼, and ─ draw the border. Output for size 9 (the first input, 15, was rejected):

Table size (1–12): 15
Enter an integer from 1 to 12: 9
   │   1   2   3   4   5   6   7   8   9
───┼────────────────────────────────────
 1 │   1   2   3   4   5   6   7   8   9
 2 │   2   4   6   8  10  12  14  16  18
 3 │   3   6   9  12  15  18  21  24  27
 4 │   4   8  12  16  20  24  28  32  36
 5 │   5  10  15  20  25  30  35  40  45
 6 │   6  12  18  24  30  36  42  48  54
 7 │   7  14  21  28  35  42  49  56  63
 8 │   8  16  24  32  40  48  56  64  72
 9 │   9  18  27  36  45  54  63  72  81

Common mistakes ​

Table 3.3 lists the most common mistakes when working with branching and loops.

Table 3.3. Common mistakes when working with branching and loops

ProblemCauseFix
the block after if always executes; warning CS0642a semicolon after the condition if (x > 0);remove ; after the closing parenthesis of the condition
the loop never endsthe body does not change the variable in the condition, or the condition is always trueupdate the counter; step through the condition in the debugger
an extra or missing iteration< instead of <= or vice versa, an incorrect initial valuecheck the first and last iterations manually
CS0163: Control cannot fall through…a switch branch has no breakend the branch with break, return, or continue
CS8509: the switch expression is not exhaustivesome values have no matching branchadd a _ branch
CS8510: the pattern is unreachablea more general pattern appears first (> 0 before > 10)order branches from specific to general
CS0165 after if without elsethe variable is not assigned a value in every branchinitialize the variable or add else
CS0103: the loop variable is inaccessiblethe variable is declared in the for header or in a blockdeclare the variable before the loop
break in switch does not end the loopbreak applies to the nearest switchuse a flag for the loop or return