csharp-basics/4-loops.md

marp	paginate	math	theme	title
true	true	mathjax	buutti	4. Loops

Loops

Overview

• While loop
• For loop
• Nested loops
• Breaking out of loops

Why loops?

• Loops are useful when you need to repeat a block of code multiple times and to avoid code like this:

  Console.WriteLine(count);
  ++count;
  Console.WriteLine(count);
  ++count;
  Console.WriteLine(count);
  // Yuck! Nobody wants to see this.
  

• Loops can also be used for iterating through arrays and lists (more on those later)

while loop

• A while loop keeps executing its code block as long as the condition in its statement is true:

  int count = 0;
  while(count < 4)
  {
    Console.WriteLine(count);
    ++count;
  }
  // Outputs '0', '1', '2' and '3'
  

do...while loop

• do...while works the same way as while, but there's a key difference:
• The code is executed at least once and the condition is checked at the end:

  int count = 0;
  do
  {
    Console.WriteLine(count);
    ++count;
  } while (count < 4);
  // Outputs '0', '1', '2' and '3'
  

for loop

• for loops are used when the number of iterations is predefined
• for loop is initialized in three steps:

  for (<initial value>; <condition>; <increment>)
  {
    /* Code to be executed */
  }
  

• The initial value is set before the first iteration
• The condition is checked before each iteration
• The increment is executed after each iteration

For loop: An example

See the following two examples:

for (int i = 0; i < 4; ++i)
{
  Console.WriteLine
    ("Current: " + i);
}

/* Outputs
Current: 0
Current: 1
Current: 2
Current: 3
*/

for (int i = 8; i > 0; i -= 2)
{
  Console.WriteLine
    ("Current: " + i);
}

/* Outputs
Current: 8
Current: 6
Current: 4
Current: 2
*/

---

We start from zero int i = 0

print the current value

add one ++i

until the value is no longer under four i < 4.

for (int i = 0; i < 4; ++i)
{
  Console.WriteLine
    ("Current: " + i);
}

We start from eight int i = 8

print the current value

remove two i -= 2

until the value is no longer over zero i >

for (int i = 8; i > 0; i -= 2)
{
  Console.WriteLine
    ("Current: " + i);
}

Nested Loops

Nothing prevents you from using a loop inside another loop:

int columns = 3;
int rows = 4;
for (int i = 0; i < columns; ++i)
{
  for (int j = 0; j < rows; ++j)
  {
    // Console.Write doesn't add a linebreak after printout
    Console.Write(
      "(" + j.ToString() + "," + i.ToString() + ")"
    );
  }
  Console.WriteLine("");
}

Breaking out of loops with break

• To stop the execution of a loop before its end condition is met, use the break keyword:

  int i = 0;
  while(true)
  {
    ++i;
    if(i > 3)
    break;
  }
  // i is now 4
  

Skipping ahead with continue

• To skip the current iteration, use the continue keyword

  int i = 0;
  while(i < 10)
  {
    ++i;
    if (i % 2 == 0)
    continue;
    Console.WriteLine(i);
  }
  // Prints every odd number from 1 to 10
  

Exercise 1: The main loop

At the heart of every program that doesn't quit right away, there is something called the main loop

1. Create a console application that keeps asking the user for input until the user inputs quit.
2. Expand the program so that if the user inputs help, a help screen appears that explains what both quit and help commands do (See next image).

---

Exercise 2: Prime numbers

A Prime number is a natural number greater than 1 that is not a product of two natural numbers (so it cannot be divided into a whole number).

Create a console application which prints all the prime numbers between 0 and 40.

Assignments

Assignments about this topic can be found here