Dev Blog

Examples and common mistakes ​

Example programs ​

Group grades ​

The program processes an array of student scores: it computes the average, maximum and its position, and the number of scores above average, and displays the scores in descending order without modifying the original array.

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

int[] scores = [78, 92, 65, 88, 54, 97, 71, 83];

int sum = 0;
int max = scores[0];
foreach (int s in scores)
{
    sum += s;
    max = Math.Max(max, s);
}
double average = (double)sum / scores.Length;

int aboveAverage = 0;
for (int i = 0; i < scores.Length; i++)
{
    if (scores[i] > average)
    {
        aboveAverage++;
    }
}

int[] sorted = (int[])scores.Clone();   // the original is unchanged
Array.Sort(sorted);
Array.Reverse(sorted);                  // in descending order

Console.WriteLine($"Students: {scores.Length}");
Console.WriteLine($"Average score: {average:F2}");
int position = Array.IndexOf(scores, max) + 1;
Console.WriteLine($"Maximum: {max}, student no. {position}");
Console.WriteLine($"Above average: {aboveAverage}");
Console.WriteLine($"Descending: {string.Join(", ", sorted)}");
Console.WriteLine($"Top three: {string.Join(", ", sorted[..3])}");
Console.WriteLine($"Lowest: {sorted[^1]}");

The sum is divided after casting to double; otherwise, the average would be an integer. The Clone copy is needed because Array.Sort modifies the array, while we search for the maximum’s position in the original order. Output:

Students: 8
Average score: 78,50
Maximum: 97, student no. 6
Above average: 4
Descending: 97, 92, 88, 83, 78, 71, 65, 54
Top three: 97, 92, 88
Lowest: 54

Sales matrix ​

The program stores four months of sales for three stores in a rectangular double[,] array and displays a table with totals for each store, each month, and a grand total.

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

string[] shops = ["Center", "London", "Brussels"];
string[] months = ["Jan", "Feb", "Mar", "Apr"];

// Rows are stores, columns are months (thousands of UAH).
double[,] sales =
{
    { 120.5, 98.0, 134.2, 141.7 },
    { 87.3, 91.6, 102.4, 99.8 },
    { 64.0, 70.5, 68.9, 81.2 },
};

int rows = sales.GetLength(0);
int cols = sales.GetLength(1);

Console.Write($"{"Store",-14}");
for (int j = 0; j < cols; j++)
{
    Console.Write($"{months[j],8}");
}
Console.WriteLine($"{"Total",9}");

double[] monthTotals = new double[cols];
for (int i = 0; i < rows; i++)
{
    double shopTotal = 0;
    Console.Write($"{shops[i],-14}");
    for (int j = 0; j < cols; j++)
    {
        Console.Write($"{sales[i, j],8:F1}");
        shopTotal += sales[i, j];
        monthTotals[j] += sales[i, j];
    }
    Console.WriteLine($"{shopTotal,9:F1}");
}

Console.Write($"{"Total",-14}");
double grandTotal = 0;
foreach (double total in monthTotals)
{
    Console.Write($"{total,8:F1}");
    grandTotal += total;
}
Console.WriteLine($"{grandTotal,9:F1}");

The row (store) total accumulates in shopTotal, which resets to zero for each row, while column (month) totals accumulate in a separate monthTotals array. The rectangular array is initialized with nested braces: collection expressions […] are not supported for multidimensional arrays. Output:

Store              Jan     Feb     Mar     Apr    Total
Center           120,5    98,0   134,2   141,7    494,4
London            87,3    91,6   102,4    99,8    381,1
Brussels          64,0    70,5    68,9    81,2    284,6
Total            271,8   260,1   305,5   322,7   1160,1

Sentence analysis ​

The program splits a sentence into words, finds the longest word, counts vowels, and capitalizes each word.

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

Console.WriteLine("Enter a sentence:");
string sentence = Console.ReadLine() ?? "";

char[] separators = [' ', ',', '.', '!', '?', ';', ':', '–'];
string[] words = sentence.Split(separators,
    StringSplitOptions.RemoveEmptyEntries);

if (words.Length == 0)
{
    Console.WriteLine("No words found.");
    return;
}

string longest = words[0];
int vowels = 0;
string[] titled = new string[words.Length];

for (int i = 0; i < words.Length; i++)
{
    string word = words[i];
    if (word.Length > longest.Length)
    {
        longest = word;
    }
    foreach (char c in word)
    {
        if ("aeiou".Contains(char.ToLower(c)))
        {
            vowels++;
        }
    }
    titled[i] = char.ToUpper(word[0]) + word[1..].ToLower();
}

Console.WriteLine($"Words: {words.Length}");
Console.WriteLine($"Longest word: {longest} ({longest.Length})");
Console.WriteLine($"Vowels: {vowels}");
Console.WriteLine($"Capitalized: {string.Join(" ", titled)}");

The hyphen and apostrophe are not separators, so “object-oriented” remains one word. The expression char.ToUpper(word[0]) + word[1..] combines a character and a string: the result has type string. Output:

Enter a sentence:
C# – modern, object-oriented and VERY popular language!
Words: 7
Longest word: object-oriented (15)
Vowels: 17
Capitalized: C# Modern Object-oriented And Very Popular Language

Report using StringBuilder ​

The program builds a bordered product table in StringBuilder and compares the time needed to build a 20,000-character string using concatenation and Append.

cs
using System.Diagnostics;
using System.Text;

Console.OutputEncoding = Encoding.UTF8;

string[] items = ["Keyboard", "Mouse", "Monitor 27\"", "Headphones"];
int[] counts = [3, 5, 2, 4];
decimal[] prices = [1250m, 480m, 9999m, 1299.5m];

var report = new StringBuilder();
report.AppendLine("┌────────────────────┬──────┬───────────┐");
report.AppendLine($"│{"Item",-20}│{"Count",6}│{"Amount",11}│");
report.AppendLine("├────────────────────┼──────┼───────────┤");

decimal total = 0;
for (int i = 0; i < items.Length; i++)
{
    decimal sum = counts[i] * prices[i];
    total += sum;
    report.AppendLine($"│{items[i],-20}│{counts[i],6}│{sum,11:N2}│");
}

report.AppendLine("├────────────────────┴──────┼───────────┤");
report.AppendLine($"│{"Total",-27}│{total,11:N2}│");
report.Append("└───────────────────────────┴───────────┘");
Console.WriteLine(report);

// Speed comparison: 20,000 string appends.
const int N = 20_000;
var watch = Stopwatch.StartNew();
string text = "";
for (int i = 0; i < N; i++)
{
    text += "x";
}
watch.Stop();
long concatMs = watch.ElapsedMilliseconds;

watch.Restart();
var builder = new StringBuilder();
for (int i = 0; i < N; i++)
{
    builder.Append('x');
}
string built = builder.ToString();
watch.Stop();

Console.WriteLine($"Concatenation:  {concatMs} ms");
Console.WriteLine($"StringBuilder: {watch.ElapsedMilliseconds} ms");
Console.WriteLine($"Equal strings: {text == built}");

Three parallel arrays, items, counts, and prices, describe products: elements at the same index belong to one product (Topic 7 groups such data into a class). The Stopwatch class measures time. Output (timing depends on the computer):

┌────────────────────┬──────┬───────────┐
│Item                │ Count│     Amount│
├────────────────────┼──────┼───────────┤
│Keyboard            │     3│   3 750,00│
│Mouse               │     5│   2 400,00│
│Monitor 27"         │     2│  19 998,00│
│Headphones          │     4│   5 198,00│
├────────────────────┴──────┼───────────┤
│Total                      │  31 346,00│
└───────────────────────────┴───────────┘
Concatenation:  29 ms
StringBuilder: 0 ms
Equal strings: True

Common mistakes ​

Table 4.3 lists the most common mistakes when working with arrays and strings.

Table 4.3. Common mistakes when working with arrays and strings

ProblemCause and fix
IndexOutOfRangeExceptioncondition i <= a.Length or index −1; loop while i < a.Length, the last element is a[^1]
modifying the “copy” changes the originalb = a copies the reference; create a copy with Clone, a[..], or Array.Copy
NullReferenceException for j[i][k]a jagged array row has not been created; first use j[i] = new int[n]
the string is unchanged after s.Replace(…)string methods return a new string; s = s.Replace(…)
System.Int32[] is displayedConsole.WriteLine(a) does not display elements; use string.Join(", ", a)
empty elements after Splitconsecutive separators; use RemoveEmptyEntries
"London" == "london" gives Falsecomparison is case-sensitive; use StringComparison.OrdinalIgnoreCase
building a large string is slow+= in a loop copies the entire string each time; use StringBuilder
BinarySearch gives the wrong resultthe array is not sorted; call Array.Sort first