Loops

Why Loops?

Imagine you need to print "Hello" 100 times. You could write 100 Console.WriteLine statements... or you could write a loop and let the computer do the boring stuff. Loops are your code's repeat button.

C# has four types of loops, each with its sweet spot:

for — when you know exactly how many times to repeat
while — when you repeat until something changes
do-while — like while, but guaranteed to run at least once
foreach — when you want to visit every item in an array or collection

for and while Loops

// for loop — "do this N times"
// (start; keep going while; after each loop)
Console.WriteLine("Countdown:");
for (int i = 5; i >= 1; i--)
{
    Console.Write($"{i}... ");
}
Console.WriteLine("Liftoff!");

// while loop — "keep going until this is false"
int fuel = 100;
Console.WriteLine("\nBurning fuel:");
while (fuel > 0)
{
    fuel -= 30;
    Console.WriteLine($"  Fuel remaining: {Math.Max(fuel, 0)}%");
}

// do-while — runs AT LEAST once
int attempts = 0;
do
{
    attempts++;
    Console.WriteLine($"Attempt #{attempts}");
} while (attempts < 3);

Output

Countdown:
5... 4... 3... 2... 1... Liftoff!

Burning fuel:
  Fuel remaining: 70%
  Fuel remaining: 40%
  Fuel remaining: 10%
  Fuel remaining: 0%
Attempt #1
Attempt #2
Attempt #3

foreach — The Collection Whisperer

foreach is the friendliest loop. It says "give me each item in this collection, one at a time." You don't need to track an index or worry about going out of bounds. It just works.

Use foreach whenever you're iterating over arrays, lists, strings (yes, strings are collections of characters!), or any other collection.

foreach, break, and continue

// foreach with an array
string[] heroes = { "Spider-Man", "Batman", "Wonder Woman", "Iron Man" };
foreach (string hero in heroes)
{
    Console.WriteLine($"  Hero: {hero}");
}

// foreach with a string (each char!)
string word = "C#";
foreach (char c in word)
{
    Console.Write($"[{c}]");
}
Console.WriteLine();

// break — emergency exit
Console.WriteLine("\nSearching for Batman:");
foreach (string hero in heroes)
{
    if (hero == "Batman")
    {
        Console.WriteLine("  Found him!");
        break;  // stop the loop right now
    }
    Console.WriteLine($"  Not {hero}...");
}

// continue — skip this one, keep going
Console.WriteLine("\nOdd numbers only:");
for (int i = 1; i <= 10; i++)
{
    if (i % 2 == 0)
        continue;  // skip even numbers
    Console.Write($"{i} ");
}
Console.WriteLine();

Output

  Hero: Spider-Man
  Hero: Batman
  Hero: Wonder Woman
  Hero: Iron Man
[C][#]

Searching for Batman:
  Not Spider-Man...
  Found him!

Odd numbers only:
1 3 5 7 9

Nested Loops — Loops Inside Loops

// Print a multiplication table
Console.WriteLine("Multiplication Table (1-4):");
for (int row = 1; row <= 4; row++)
{
    for (int col = 1; col <= 4; col++)
    {
        Console.Write($"{row * col,4}"); // ,4 pads to 4 chars
    }
    Console.WriteLine();
}

// Build a triangle
Console.WriteLine("\nTriangle:");
for (int i = 1; i <= 5; i++)
{
    Console.WriteLine(new string('*', i));
}

Output

Multiplication Table (1-4):
   1   2   3   4
   2   4   6   8
   3   6   9  12
   4   8  12  16

Triangle:
*
**
***
****
*****

Why Loops?

for and while Loops

foreach — The Collection Whisperer

foreach, break, and continue

Nested Loops — Loops Inside Loops

Quick check