Basics

Looping
A loop is a programming language construction that allows the programmer to instruct the computer to perform a certain instruction, or set of instructions over and over again.

There are many different types, but some are omitted from programming languages by virtue of the fact that they are variations on an existing loop type, and as such the same behavior can be achieved without offering a specific loop type to cover it.

However, we cover the three types which are common to most programming languages:

Condition Tested Loops
Counted Loops
Endless Loops

Of these, only the condition tested loop is vital; the other two just give a more convenient way of achieving the same effect.


For Loop

Program that uses for "int i" loops [C#]

using System;

class Program
{
    static void Main()
    { 
//
// Shows five 'for int i' loops.
//
Console.WriteLine("--- For 1 ---");
 for (int i = 0; i < 10; i++)
 {
     Console.WriteLine(i);
 }
 Console.WriteLine("--- For 2 ---");
 for (int i = 10 - 1; i >= 0; i--)
 {
     Console.WriteLine(i);
 }
 Console.WriteLine("--- For 3 ---");
 for (int i = 0; i < 10; i += 2)
 {
     Console.WriteLine(i);
 }
 Console.WriteLine("--- For 4 ---");
 for (int i = 10 - 1; i >= 0; i -= 2)
 {
     Console.WriteLine(i);
 }
 Console.WriteLine("--- For 5 ---");
 for (int i = 0; i < (20 / 2); i += 2)
 {
     Console.WriteLine(i);
 }
    }
}

Output

--- For 1 ---
0
1
2
3
4
5
6
7
8
9
--- For 2 ---
9
8
7
6
5
4
3
2
1
0
--- For 3 ---
0
2
4
6
8
--- For 4 ---
9
7
5
3
1
--- For 5 ---
0
2
4
6
8

While Loop

Program that uses while loop with condition [C#]
using System;

class Program
{
    static void Main()
    {
 
// Continue in while loop until index is equal to ten.
int i = 0;
 while (i < 10)
 {
     Console.Write("While statement ");
     
// Write the index to the screen.
Console.WriteLine(i);
     
// Increment the variable.
i++;
 }
    }
}

 Output

While statement 0
While statement 1
While statement 2
While statement 3
While statement 4
While statement 5
While statement 6
While statement 7
While statement 8
While statement 9

Do While Loop

Program that uses do while loop [C#]

class Program
{
    static void Main()
    {
 int[] ids = new int[] { 6, 7, 8, 10 };
 
//  
// Use do while loop to sum numbers in 4-element array.  
//
int sum = 0;
 int i = 0;
 do
 {
     sum += ids[i];
     i++;
 } while (i < 4);

 System.Console.WriteLine(sum);
    }
}

Output

31

Foreach Loop

Program that uses foreach over array [C#]

using System;

class Program
{
    static void Main()
    {
 
// Use a string array to loop over.
string[] ferns =
 {
     "Psilotopsida",
     "Equisetopsida",
     "Marattiopsida",
     "Polypodiopsida"
 };
 
// Loop with the foreach keyword.
foreach (string value in ferns)
 {
     Console.WriteLine(value);
 }
    }
}

Output

Psilotopsida
Equisetopsida
Marattiopsida
Polypodiopsida

If Statement
The if statement in the C# language is a selection statement. It is implemented with intermediate language instructions
called branches. With if, we can make a logical decision based on a parameter or input from a user. Here, we look at if.

This program simply computes the value of an expression and then tests it in an if-statement. The condition inside the if-statement is evaluated
to a boolean value and if the value is true, the inner block is executed.

Program that uses if-statement [C#]

using System;

class Program
{
    static void Main()
    {
 
// Call method with embedded if-statement three times.
int result1 = Test(0);
 int result2 = Test(50);
 int result3 = Test(-1);

 
// Print results.
Console.WriteLine(result1);
 Console.WriteLine(result2);
 Console.WriteLine(result3);
    }

    static int Test(int value)
    {
 if (value == 0)
 {
     return -1;
 }
 else if (value <= 10)
 {
     return 0;
 }
 else if (value <= 100)
 {
     return 1;
 }
 else 
// See note on "else after return"
{
     return 2;
 }
    }
}

Output

-1
1
0