Loops: for, while, do-while and foreach

Why loops?

Computers are good at doing the same thing many times without getting tired. Printing 1,000 receipts, adding up a month of sales, or asking for a PIN until it is right are all loops. Without loops you would copy the same lines again and again.

The for loop

A for loop has three parts in its brackets, separated by semicolons: the start (int i = 1), the condition checked before every round (i <= 5), and the step done after every round (i++). Each round is called an iteration.

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

for (int i = 10; i > 0; i -= 3)
    Console.Write(i + " ");

The while loop

while checks its condition BEFORE each round. If the condition is false at the start, the body never runs. Use it when you do not know in advance how many rounds you need.

decimal balance = 10000m;
int years = 0;
while (balance < 20000m)
{
    balance *= 1.08m;   // 8% interest per year
    years++;
}
Console.WriteLine($"Doubles in {years} years");

The do-while loop

do-while checks its condition AFTER each round, so the body always runs at least once. It fits menus and input checks, where you must ask at least once.

int pin;
do
{
    Console.Write("Enter 4-digit PIN: ");
} while (!int.TryParse(Console.ReadLine(), out pin) || pin < 1000 || pin > 9999);
Console.WriteLine("PIN accepted");

The foreach loop

foreach visits every item of an array or collection, in order, without an index. It is the clearest loop when you just need each item.

string[] cities = { "Delhi", "Mumbai", "Kolkata", "Chennai" };
foreach (string city in cities)
{
    Console.WriteLine(city.ToUpper());
}

break and continue

int[] amounts = { 120, -5, 300, 0, 9999, 80 };
foreach (int a in amounts)
{
    if (a <= 0) continue;     // skip bad values
    if (a > 5000) break;      // stop at a suspicious amount
    Console.WriteLine($"Processing Rs {a}");
}

Nested loops

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

Common mistakes and fixes