C#
L02&L03
Objectives
• C# Variables
• C# Display Variables
• C# Identifiers
• C# Data Types
• C# User Input
C# Variables
Variables are containers for storing data values.
In C#, there are different types of variables (defined with different keywords),
for example:
•int - stores integers (whole numbers), without decimals, such as 123 or -123
•double - stores floating point numbers, with decimals, such as 19.99 or -19.99
•char - stores single characters, such as 'a' or 'B'. Char values are surrounded by
single quotes
•string - stores text, such as "Hello World". String values are surrounded by
double quotes
•bool - stores values with two states: true or false
Declaring (Creating) Variables
• To create a variable, you must specify the type and
assign it a value:
type variableName = value;
• Where type is a C# type (such as int or string), and variableName is
the name of the variable (such as x or name). The equal sign is used
to assign values to the variable.
Declaring (Creating) Variables
• To create a variable that should store text, look at the following
example:
• Example
• Create a variable called name of type string and assign it the value
"John":
• string name = "John";
• Console.WriteLine(name);
Declaring (Creating) Variables
• To create a variable that should store a number, look at
the following example:
• Example
• Create a variable called myNum of type int and assign it the value 15:
• int myNum = 15;
• Console.WriteLine(myNum);
Declaring (Creating) Variables
• You can also declare a variable without assigning the value, and assign
the value later:
• int myNum;
• myNum = 15;
• Console.WriteLine(myNum);
Declaring (Creating) Variables
• Note that if you assign a new value to an existing
variable, it will overwrite the previous value:
• Example
• Change the value of myNum to 20:
• int myNum = 15;
• myNum = 20; // myNum is now 20
• Console.WriteLine(myNum);
Declaring (Creating) Variables
• Other Types
• A demonstration of how to declare variables of other types:
• int myNum = 5;
• double myDoubleNum = 5.99D;
• char myLetter = 'D';
• bool myBool = true;
• string myText = "Hello";
C# Display Variables
• The WriteLine() method is often used to display variable
values to the console window.
• To combine both text and a variable, use the + character:
string name = "John";
Console.WriteLine("Hello " + name);
C# Display Variables
• You can also use the + character to add a variable to another
variable:
string firstName = "John ";
string lastName = "Doe";
string fullName = firstName + lastName;
Console.WriteLine(fullName);
C# Display Variables
• For numeric values, the + character works as a mathematical operator (notice
that we use int (integer) variables here):
int x = 5;
int y = 6;
Console.WriteLine(x + y); // Print the value of x + y
• From the example above, you can expect:
• x stores the value 5
• y stores the value 6
• Then we use the WriteLine() method to display the value of x + y, which is 11
C# Multiple Variables
• To declare more than one variable of the same type, use a comma-
separated list:
int x = 5, y = 6, z = 50;
Console.WriteLine(x + y + z);
C# Multiple Variables
• You can also assign the same value to multiple variables in one line:
int x, y, z; x = y = z = 50;
Console.WriteLine(x + y + z);
C# Identifiers
• All C# variables must be identified with unique names.
• These unique names are called identifiers.
• Identifiers can be short names (like x and y) or more descriptive names
(age, sum, totalVolume).
• Note: It is recommended to use descriptive names in order to create
understandable and maintainable code:
// Good
int minutesPerHour = 60;
// OK, but not so easy to understand what m actually is
int m = 60;
C# Identifiers
The general rules for naming variables are:
• Names can contain letters, digits, and the underscore character (_)
• Names must begin with a letter or underscore
• Names should start with a lowercase letter and it cannot contain
whitespace
• Names are case sensitive ("myVar" and "myvar" are different variables)
• Reserved words (like C# keywords, such as int or double) cannot be
used as names
C# Data Types
• As explained in the variables chapter, a variable in C#
must be a specified data type:
int myNum = 5; // Integer (whole number)
double myDoubleNum = 5.99D; // Floating point number
char myLetter = 'D'; // Character
bool myBool = true; // Boolean
string myText = "Hello"; // String
C# Data Types
• A data type specifies the size and type of variable
values.
• It is important to use the correct data type for the
corresponding variable:
• to avoid errors,
• to save time and memory,
• but it will also make your code more maintainable and
readable.
C# Data Types
The most common data types are:
Data Type Size Description
int 4 bytes Stores whole numbers from -2,147,483,648 to 2,147,483,647
long 8 bytes Stores whole numbers from -9,223,372,036,854,775,808 to
9,223,372,036,854,775,807
float 4 bytes Stores fractional numbers. Sufficient for storing 6 to 7 decimal digits
double 8 bytes Stores fractional numbers. Sufficient for storing 15 decimal digits
bool 1 bit Stores true or false values
char 2 bytes Stores a single character/letter, surrounded by single quotes
string 2 bytes per Stores a sequence of characters, surrounded by double quotes
character
C# Data Types
• Number types are divided into two groups:
• Integer types stores whole numbers, positive or negative (such as 123
or -456), without decimals. Valid types are int and long. Which type
you should use, depends on the numeric value.
• Floating point types represents numbers with a fractional part,
containing one or more decimals. Valid types are float and double.
C# Data Types
• Int
• The int data type can store whole numbers from -2147483648 to
2147483647.
• In general, the int data type is the preferred data type when we
create variables with a numeric value.
int myNum = 100000;
Console.WriteLine(myNum);
C# Data Types
• Long
• The long data type can store whole numbers from -
9223372036854775808 to 9223372036854775807.
• This is used when int is not large enough to store the value. Note that
you should end the value with an "L":
long myNum = 15000000000L;
Console.WriteLine(myNum);
C# Data Types
• Floating Point Types
• You should use a floating point type whenever you need a number
with a decimal, such as 9.99 or 3.14515.
• The float and double data types can store fractional numbers.
float myNum = 5.75;
Console.WriteLine(myNum);
double myNum = 17.15;
Console.WriteLine(myNum);
C# Data Types
• Booleans
• A boolean data type is declared with the bool keyword and can only
take the values true or false:
bool isCSharpFun = true;
bool isFishTasty = false;
Console.WriteLine(isCSharpFun); // Outputs True
Console.WriteLine(isFishTasty); // Outputs False
C# Data Types
• Characters
• The char data type is used to store a single character.
• The character must be surrounded by single quotes, like 'A' or 'c’:
char myGrade = 'B’;
Console.WriteLine(myGrade);
C# Data Types
• Strings
• The string data type is used to store a sequence of characters (text).
String values must be surrounded by double quotes:
string greeting = "Hello World";
Console.WriteLine(greeting);
Type Conversion Methods
• It is possible to convert data types by using built-in methods, such as
Convert.ToBoolean, Convert.ToDouble, Convert.ToString,
Convert.ToInt32 (int), and Convert.ToInt64 (long):
int myInt = 10;
double myDouble = 5.25;
bool myBool = true;
Console.WriteLine(Convert.ToString(myInt)); // convert int to string
Console.WriteLine(Convert.ToDouble(myInt)); // convert int to double
Console.WriteLine(Convert.ToInt32(myDouble)); // convert double to int
Console.WriteLine(Convert.ToString(myBool)); // convert bool to string
C# User Input
• Get User Input
• Console.WriteLine() is used to output (print) values.
• Console.ReadLine() to get user input.
• In the following example, the user can input his username, which is
stored in the variable userName. Then we print the value of
userName:
C# User Input
// Type your username and press enter
Console.WriteLine("Enter username:");
// Create a string variable and get user input from the keyboard and store it in the variable
string userName = Console.ReadLine();
// Print the value of the variable (userName), which will display the input value
Console.WriteLine("Username is: " + userName);
C# User Input
• User Input and Numbers
• The Console.ReadLine() method returns a string. Therefore, you cannot get
information from another data type, such as int. The following program will cause
an error:
Console.WriteLine("Enter your age:");
int age = Console.ReadLine();
Console.WriteLine("Your age is: " + age);
• The error message will be something like this:
Cannot implicitly convert type 'string' to 'int'
C# User Input
• You can convert any type explicitly, by using one of the Convert.To
methods:
Console.WriteLine("Enter your age:");
int age = Convert.ToInt32(Console.ReadLine());
Console.WriteLine("Your age is: " + age);
