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
*/

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
  

Assignments

Assignments about this topic can be found here