Loops: for, while, do-while, foreach

Why loops?

If you need to do something 100 times, you don't write the line 100 times; you write a loop. The power of computers comes from repeating without getting bored.

The for loop

If you know how many times to repeat, for is ideal. It has three parts: the start, the condition, and the step after each round.

for (int i = 1; i <= 5; i++)
{
    Console.WriteLine($"{i} x 7 = {i * 7}");
}

1 x 7 = 7
2 x 7 = 14
3 x 7 = 21
4 x 7 = 28
5 x 7 = 35

The flow is like this: i starts as 1; if the condition (i <= 5) is true, the body runs; then i++ increases it by one; when the condition becomes false, the loop ends.

The while loop

When you don't know how many rounds there will be and want to continue as long as a condition holds, you use while. Example: a capacitor discharging. Let the voltage halve at every step, and let's count until it drops below 0.1 V.

double voltage = 10.0;
int steps = 0;

while (voltage >= 0.1)
{
    voltage /= 2;
    steps++;
}
Console.WriteLine($"After {steps} steps the voltage fell to {voltage:F4} V.");

After 7 steps the voltage fell to 0.0781 V.

Check the calculation: 10 → 5 → 2.5 → 1.25 → 0.625 → 0.3125 → 0.15625 → 0.078125. After the seventh halving it drops below 0.1.

do-while: run at least once

do-while checks the condition at the end, so the body runs at least once. It is tailor-made for asking the user for valid input:

int number;
bool isValid;

do
{
    Console.Write("Enter a number between 1 and 10: ");
    isValid = int.TryParse(Console.ReadLine(), out number) && number >= 1 && number <= 10;
}
while (!isValid);

Console.WriteLine($"The number you entered: {number}");

foreach: walking through a collection

It is used to process every element of a list or array in turn. You don't have to manage a counter. (We will look at arrays in detail in the next lessons.)

double[] resistors = { 100, 220, 330 };
double series = 0;
double inverse = 0;

foreach (double r in resistors)
{
    series += r;
    inverse += 1 / r;
}

Console.WriteLine($"Series: {series} ohm");
Console.WriteLine($"Parallel: {1 / inverse:F2} ohm");

Series: 650 ohm
Parallel: 56.90 ohm

In a series connection, the resistances add up. In parallel, the reciprocal of the equivalent resistance is the sum of the reciprocals of the resistances: 1/R = 1/100 + 1/220 + 1/330, and the result is about 56.90 ohm.

break and continue

break ends the loop completely, while continue skips the current round and moves on to the next.

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

1 3 5 7

Nested loops

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

1	2	3
2	4	6
3	6	9

Try it yourself

Find the sum of the numbers from 1 to 100 with for (expected: 5050). Then add up only the ones divisible by 3. After that, write a program that prints the numbers from 1 to 20 and prints "Beep" in place of the numbers divisible by 3.