Tutorial 3:Control Statements - Selection

The if Statement:
using System;

class IfSelect
{
public static void Main()
{
string myInput;
int myInt;

Console.Write("Please enter a number: ");
myInput = Console.ReadLine();
myInt = Int32.Parse(myInput);

// Single Decision and Action with braces
if (myInt > 0)
{
Console.WriteLine("Your number {0} is greater than zero.", myInt);
}

// Single Decision and Action without brackets
if (myInt < 0)
Console.WriteLine("Your number {0} is less than zero.", myInt);

// Either/Or Decision
if (myInt != 0)
{
Console.WriteLine("Your number {0} is not equal to zero.", myInt);
}
else
{
Console.WriteLine("Your number {0} is equal to zero.", myInt);
}

// Multiple Case Decision
if (myInt < 0 || myInt == 0)
{
Console.WriteLine("Your number {0} is less than or equal to zero.", myInt);
}
else if (myInt > 0 && myInt <= 10)
{
Console.WriteLine("Your number {0} is in the range from 1 to 10.", myInt);
}
else if (myInt > 10 && myInt <= 20)
{
Console.WriteLine("Your number {0} is in the range from 11 to 20.", myInt);
}
else if (myInt > 20 && myInt <= 30)
{
Console.WriteLine("Your number {0} is in the range from 21 to 30.", myInt);
}
else
{
Console.WriteLine("Your number {0} is greater than 30.", myInt);
}
}
}

The switch Statement:
using System;

class SwitchSelect
{
public static void Main()
{
string myInput;
int myInt;

begin:

Console.Write("Please enter a number between 1 and 3: ");
myInput = Console.ReadLine();
myInt = Int32.Parse(myInput);

// switch with integer type
switch (myInt)
{
case 1:
Console.WriteLine("Your number is {0}.", myInt);
break;
case 2:
Console.WriteLine("Your number is {0}.", myInt);
break;
case 3:
Console.WriteLine("Your number is {0}.", myInt);
break;
default:
Console.WriteLine("Your number {0} is not between 1 and 3.", myInt);
break;
}

decide:

Console.Write("Type \"continue\" to go on or \"quit\" to stop: ");
myInput = Console.ReadLine();

// switch with string type
switch (myInput)
{
case "continue":
goto begin;
case "quit":
Console.WriteLine("Bye.");
break;
default:
Console.WriteLine("Your input {0} is incorrect.", myInput);
goto decide;
}
}
}


The while Loop:
using System;

class WhileLoop
{
public static void Main()
{
int myInt = 0;

while (myInt < 10)
{
Console.Write("{0} ", myInt);
myInt++;
}
Console.WriteLine();
}
}

The do Loop:
using System;

class DoLoop
{
public static void Main()
{
string myChoice;

do
{
// Print A Menu
Console.WriteLine("My Address Book\n");

Console.WriteLine("A - Add New Address");
Console.WriteLine("D - Delete Address");
Console.WriteLine("M - Modify Address");
Console.WriteLine("V - View Addresses");
Console.WriteLine("Q - Quit\n");

Console.WriteLine("Choice (A,D,M,V,or Q): ");

// Retrieve the user's choice
myChoice = Console.ReadLine();

// Make a decision based on the user's choice
switch(myChoice)
{
case "A":
case "a":
Console.WriteLine("You wish to add an address.");
break;
case "D":
case "d":
Console.WriteLine("You wish to delete an address.");
break;
case "M":
case "m":
Console.WriteLine("You wish to modify an address.");
break;
case "V":
case "v":
Console.WriteLine("You wish to view the address list.");
break;
case "Q":
case "q":
Console.WriteLine("Bye.");
break;
default:
Console.WriteLine("{0} is not a valid choice", myChoice);
break;
}

// Pause to allow the user to see the results
Console.Write("press Enter key to continue...");
Console.ReadLine();
Console.WriteLine();
} while (myChoice != "Q" && myChoice != "q"); // Keep going until the user wants to quit
}
}

The for Loop:
using System;

class ForLoop
{
public static void Main()
{
for (int i=0; i < 20; i++)
{
if (i == 10)
break;

if (i % 2 == 0)
continue;

Console.Write("{0} ", i);
}
Console.WriteLine();
}
}

The foreach Loop:
A foreach loop is used to iterate through the items in a list. It operates on arrays or collections such as ArrayList, which can be found in the System.Collections namespace. The syntax of a foreach loop is foreach ( in ) { }. The type is the type of item contained in the list. For example, if the type of the list was int[] then the type would be int.
The iteration variable is an identifier that you choose, which could be anything but should be meaningful. For example, if the list contained an array of people's ages, then a meaningful name for item name would be age.
The in keyword is required.
using System;

class ForEachLoop
{
public static void Main()
{
string[] names = {"Cheryl", "Joe", "Matt", "Robert"};

foreach (string person in names)
{
Console.WriteLine("{0} ", person);
}
}
}

Summary:
Loops allow you to execute a block of statements repeatedly. C# offers several statements to construct loops with, including the while, do, for, and foreach loops. while loops execute a block of statements as long as an expression is true, do loops execute a block of statements at least once and then keep going as long as a condition is true, for loops execute a block of statements a specified amount of times, and foreach loops execute a block of statements for each item in a collection. Normally a block of statements will execute from beginning to end. However, the normal flow of a loop can be changed with the break and continue statements.