Loops

A loop is a logical structure used to repeatedly execute a block of code. This repetition can either iterate an exact number of times (definite) or continuously until a condition is met (indefinite).

For Loop

In C# the for loop uses definite repetition to run a block of code a specified number of times and consists of three statements separated by semicolons.

• Statement 1 executes only once, before the code block runs for the first time, and is commonly used to set the initial value of a variable.

• Statement 2 is a conditional statement that must return true for the code block to execute.

• Statement 3 runs after each repetition of the loop and is typically used to alter the value of the variable set in Statement 1.

for (int i = 5; i > 0; i--)
{
  // Repeated code here
}

Note: Be careful to avoid creating an infinite loop, which occurs if the stop condition never returns false.

While Loop

The while loop in C# executes an unspecified number of times until the given condition returns false. The condition is tested before each iteration of the loop. If the condition is false when it is tested the first time, the code block is never run.

int i = 0;
while (i > -5)
{
  // Repeated code here
  i--;
}

Note: The variable tested in the condition should be updated within the code block to avoid an infinite loop.

Do While Loop

This form of loop uses the do keyword, followed by the code block, followed by the while keyword and condition. Unlike the while loop it checks the condition after the code block is executed. This means the loop will always iterate at least once, but the condition must be true for it to continue.

int i = 1;
do
{
  // Repeated code here
  i++;
} while (i <= 3);

Note: The loop in the last example will still run once even though the condition returns false.

For Each Loop

The foreach loop in C# executes a block of code for each element in a given array or collection. The block is run with each element of the array in turn until the array has been exhausted.

It is declared with the foreach keyword, followed by, in parentheses, a variable type, a variable name, the keyword in and the collection to iterate over.

// Array to loop over
int[] numbersArray = {0, 1, 2, 3};

foreach (int number in numbersArray)
{
  System.Console.WriteLine(number);
}

This example outputs the following:

0
1
2
3

Nested Loop

Nested loop is a concept where one loop is put inside another. For every iteration of the outer loop, all iterations of the inner loop are executed.

// Outer loop
for (int i = 0; i < 3; i++)
{
  // Inner loop
  for (int j = 0; j < 2; j++)
  {
    System.Console.WriteLine("i = " + i + ", j = " + j);
  }
}

This example outputs the following:

i = 0, j = 0
i = 0, j = 1
i = 1, j = 0
i = 1, j = 1
i = 2, j = 0
i = 2, j = 1

Inner and outer loops do not require being the same type. For instance, A while loop can be placed inside a for loop, or a do-while loop inside a foreach loop.

int[] numbersArray = { 0, 1, 2, 3 };
// Outer loop
foreach (int number in numbersArray)
{
  // Inner loop
  for (int k = 0; k < 2; k++)
  {
    System.Console.WriteLine("number = " + number + ", k = " + k);
  }
}

This example outputs the following:

number = 0, k = 0
number = 0, k = 1
number = 1, k = 0
number = 1, k = 1
number = 2, k = 0
number = 2, k = 1
number = 3, k = 0
number = 3, k = 1

There is no upper limit on nesting levels. The examples above used two loops, but one can nest even more loops. However, it’s a good practice to avoid deep nesting, as it complicates programs.