Loops in C#

Why loops

Printing the numbers 1 to 5 by hand is five lines. Printing 1 to 5000 is not something you want to type. A loop runs the same block over and over until a condition says stop.

The for loop

Use for when you know how many times to repeat, or when you need a counter:

for (int i = 1; i <= 5; i++)
{
    Console.WriteLine(i);
}

The header has three parts separated by semicolons:

1. Initialiser: int i = 1 runs once at the start.
2. Condition: i <= 5 is checked before every pass. When false, the loop ends.
3. Iterator: i++ runs after every pass.

You can count down, or step by more than one:

for (int i = 10; i >= 0; i -= 2)
{
    Console.Write(i + " ");   // 10 8 6 4 2 0
}

The counter i only exists inside the loop.

Looping over an array with for

string[] names = { "Asha", "Ravi", "Meera" };

for (int i = 0; i < names.Length; i++)
{
    Console.WriteLine($"{i}: {names[i]}");
}

Arrays start at index 0, so the last index is Length - 1. That is why the condition is < and not <=. Getting this wrong throws an IndexOutOfRangeException.

The foreach loop

When you just want every item and do not care about the index, foreach is simpler and safer:

string[] names = { "Asha", "Ravi", "Meera" };

foreach (string name in names)
{
    Console.WriteLine(name);
}

It works on arrays, lists, dictionaries, strings and anything else that can be enumerated. You cannot modify the collection while iterating it, and you cannot assign to the loop variable. Both are features.

foreach is the loop you will write most often in C#.

The while loop

Use while when you do not know in advance how many passes you need. The condition is checked before each pass:

int balance = 100;
int week = 0;

while (balance > 0)
{
    balance -= 30;
    week++;
}

Console.WriteLine($"Ran out in week {week}");   // week 4

If the condition is false at the start, the body never runs at all.

Reading input until valid

int number;

while (!int.TryParse(Console.ReadLine(), out number))
{
    Console.WriteLine("Please enter a whole number:");
}

Console.WriteLine($"Thanks, you entered {number}");

The do while loop

do while checks the condition after each pass, so the body always runs at least once. It fits menus and prompts:

string answer;

do
{
    Console.Write("Continue? (y/n) ");
    answer = Console.ReadLine() ?? "n";
} while (answer == "y");

break and continue

break leaves the loop immediately. continue skips the rest of the current pass and moves to the next one:

for (int i = 1; i <= 10; i++)
{
    if (i % 2 == 0) continue;   // skip even numbers
    if (i > 7) break;           // stop after 7
    Console.Write(i + " ");     // 1 3 5 7
}

In a nested loop, break only exits the inner loop. If you need to leave both, put the loops in a method and return.

Nested loops

A loop inside a loop is how you handle grids and tables:

for (int row = 1; row <= 3; row++)
{
    for (int col = 1; col <= 3; col++)
    {
        Console.Write(row * col + "\t");
    }
    Console.WriteLine();
}

Keep nesting shallow. Two levels is normal, three is a sign that some of the work belongs in its own method.

Infinite loops

A loop whose condition is always true never stops unless something inside breaks out. Sometimes that is intentional, for example a game loop or a server that waits for requests:

while (true)
{
    string? command = Console.ReadLine();
    if (command == "quit") break;
    Console.WriteLine($"You typed {command}");
}

Accidental infinite loops usually come from forgetting to change the variable the condition depends on.

Choosing the right loop

• Known count or need the index: for.
• Every item in a collection: foreach.
• Repeat until something happens: while.
• Must run at least once: do while.

Quick recap

• for has an initialiser, a condition and an iterator, and it is ideal for counting.
• foreach visits every item and is the most common loop in C#.
• while checks first, do while checks after, so do while always runs once.
• break exits the loop and continue skips to the next pass.
• Array indexes run from 0 to Length - 1.