TopolearnEvery algorithm is built from choosing between paths and repeating steps.
if / else if / else takes a bool condition; C# does not treat numbers or null as true/false, so if (count) does not compile. Combine conditions with &&, || and !, which short-circuit. The conditional operator cond ? a : b picks a value.
A switch statement compares one value against cases. Each section must end with break, return or throw; there is no fall-through between non-empty sections. Cases can be constants or patterns (case > 100:).
Loops:
for (int i = 0; i < n; i++)when you need the index.foreach (var item in items)to walk any collection; you cannot reassignitemand must not add or remove items from aListwhile iterating it.while (cond)checks first;do { } while (cond);runs at least once.
break leaves the innermost loop, continue skips to the next iteration. To leave nested loops, move them into a method and return.
For many value-to-result mappings, the switch expression (x switch { 1 => "one", _ => "many" }) is usually clearer than a switch statement; it is covered in the pattern matching lesson.
Ranges of numbers are produced with for or Enumerable.Range(start, count), which works nicely with foreach.
Example
int[] scores = [72, 95, 40, 88];
foreach (var s in scores)
{
string grade;
if (s >= 90) grade = "A";
else if (s >= 70) grade = "B";
else grade = "F";
Console.Write($"{s}:{grade} "); // 72:B 95:A 40:F 88:B
}
Console.WriteLine();
for (int i = 0; i < scores.Length; i++)
{
if (scores[i] < 50) continue;
if (scores[i] > 90) break;
Console.Write(i + " "); // 0
}
switch (DateTime.Now.DayOfWeek)
{
case DayOfWeek.Saturday:
case DayOfWeek.Sunday: Console.WriteLine("\nweekend"); break;
default: Console.WriteLine("\nweekday"); break;
}Remember
- Conditions must be bool: no truthy numbers or null
- foreach walks any collection; for when you need the index
- switch sections cannot fall through; end each with break or return
Watch out for
- Modifying a List inside a foreach over it, which throws InvalidOperationException.