csharp-basics/2-variables-and-types.md

marp	paginate	math	theme	title
true	true	mathjax	buutti	2. Variables and Types

Variables and Types

Overview

• Variables
• Comments
• Data Types
• Arithmetic Operators
• Increment & Decrement
• Assignment Operators
• Strings
• Character Constants
• String Interpolation

Variables

• A variable can be thought of as a name for a certain address in computer's memory
  • Using this name we can access the value on the computer's memory
  • The value can be read or written
• On more practical terms: We can assign values to named variables.

Declaring variables

• Every variable declaration in C# requires the type and the name of the variable, for example:

  int x;
  

• After declaration, you can assign a value for declared variables:

  x = 25;
  

• Variable declaration with value assignment can be done in one line:

  int x = 25;
  

Printing to console with Console.WriteLine

• We can use the method Console.WriteLine to write, a.k.a. print to the C# console

  Console.WriteLine("Hello World!")
  

• We can also declare variables and print their values like this:

  int example = 123;
  Console.WriteLine(example);
  // prints 123
  

This program prints the value 15:

using System;
namespace MyAwesomeProgram
{
  class Program
  {
    static void Main(string[] args)
    {
      int a = 25;
      int b = 10;
      Console.WriteLine(a - b);
    }
  }
}

Extra: Modifiers

• A common modifier to add in front of a variable is const, short for constant
• If we know that a value of a variable is never going to change during the execution of the script, we can set it to const:

  const int one = 1;
  
  one = 2;  // raises an error
  

• Some programmers prefer using const by default.
• Other modifiers include access modifiers introduced in Lecture 7.

Comments

• There are two basic ways to comment in C#:

  // This is a comment
  
  /* This is a
  multi
  line
  comment */
  

• Any text inside a comment will not be executed
• Use comments to a) explain your code b) comment out actual code for testing and debugging

Data types

What is a data type?

• Data type tells to a computer what type of data is stored in a variable.
• Data types are commonly divided into two categories:
  • Primitive data types
  • Reference data types
• Here we go through the primitive data types
  • We dig deeper on the differences of these data types later in Lecture 7

Primitive data types

Type	Represents	Range	Default
bool	Boolean value	true or false	false
int	32-bit signed integer	-2147483648 to 2147483647	0
float	32-bit single-precision float	±1.5 \cdot 10^{-45} to ±3.4 \cdot 10^{38}	0.0F
double	64-bit double-precision float	±5.0 \cdot 10^{-324} to ±1.7 \cdot 10^{308}	0.0D
decimal	128-bit precise decimal values	±1.0 \cdot 10^{-28} to ±7.9228 \cdot 10^{28}	0.0M
char	16-bit Unicode character	U+0000 to U+ffff	\0
byte	8-bit unsigned integer	0 to 255	0

More types listed in the C# reference!

Data type examples

double airPressure = 1.2;         // Use for most decimal numbers
decimal accountBalance = 1.2m;    // Use for accuracy (e.g. financial applications)
float bulletSpeed = 1.2f;         // Use only when you know its precision will be enough
bool loggedIn = false;
char previousInput = 'b';

• char is only used for single characters, multi-character strings will be introduced in a bit.

Extra: Casting data types

Data types can be cast to another either...

...implicitly:

double valueAddedTax = 25.5;
decimal valueAddedTaxDecimal = valueAddedTax;

...explicitly:

double valueAddedTax = 25.5;
decimal valueAddedTaxDecimal = (decimal)valueAddedTax;

• Casting is useful when, for example, when we want to sum a double and a decimal together:

  double a = 1.0;
  decimal b = 2.1m;
  Console.WriteLine(a + (double)b);
  Console.WriteLine((decimal)a + b);
  

• C# Guide: Casting and type conversions

Assignments (variables)

Assignments about this topic can be found here

Assignments (data types)

Assignments about this topic can be found here

Arithmetic operations?

• Arithmetic operations are common mathematical operations:
  • Addition
  • Subtraction
  • Multiplication
  • Division
  • Modulus (remainder, in Finnish jakojäännös)
• The operations are represented by arithmetic operators

Arithmetic Operators

Operator	Name	Example	Description
+	Addition	a + b	Adds together two values
-	Subtraction	a - b	Subtracts one value from another
*	Multiplication	a * b	Multiplies two values
/	Division	a / b	Divides one value by another
%	Modulus	a % b	Returns the division remainder
++	Increment	a++	Increases the value by 1
--	Decrement	a–-	Decreases the value by 1

Exercise 1: Trying Out Variables

1. Create a new console application and declare two variables of type double.
2. Assign different values for those variables.
3. Print the sum, difference, fraction and product of those values to the console.

The assignment operator

We have used the assignment operator = for assigning values for variables:

int x;
x = 25;

• Note the difference between = and == introduced in Lecture 3!
  • = is used for assigning values for variables, == is used for comparing values

Assignment operators

Operator	Example	Same As
=	x = 5	x = 5
+=	x += 5	x = x + 5
-=	x -= 5	x = x - 5
*=	x *= 5	x = x * 5
/=	x /= 5	x = x / 5
%=	x %= 5	x = x % 5

• As shown here, there are some assignment operators that work as shorthands for longer assignments
• Particularly useful when the variable name is longer, so you don't have to write it twice when its value is changed

Assignment operators: An example

int uppercaseLetters = 2;
uppercaseLetters += 4;          // is now 6

int specialCharacters = 2;
specialCharacters *= 2;         // is now 4

Console.WriteLine(uppercaseLetters);
Console.WriteLine(specialCharacters);

Increment and decrement operations

• You can increment or decrement a variable value by 1 with dedicated short-hands
  • Most programming languages implement these!

• Addition example:

  int a = 3;
  
  a = a + 1;  // a is now 4
  a += 1;     // a is now 5
  a++;        // a is now 6
  

• Subtraction example:

  int a = 3;
  
  a = a - 1;  // a is now 2
  a -= 1;     // a is now 1
  a--;        // a is now 0
  

• ++ and -- are called the increment and decrement operators

Extra: Increment/decrement operation precedence

• Note that incrementing can be written as prefix (++i) or a postfix (i++)
• In this example, a++ and ++a do exactly the same:

  int a = 3;
  a++; // a is now 4
  ++a; // a is now 5
  

• Their exact difference is complicated, and in some cases, using either prefix or postfix form can produce different results:

int a = 3;
int b = ++a;
Console.WriteLine(b); // 4

Assignment of b happens after ++, so its value is 4

int a = 3;
int b = a++;
Console.WriteLine(b); // 3

Assignment of b happens before ++, so its value is 3

Strings

• string is a special type that contains an array of characters.

  string name = "Sini Aalto";
  

• You can concatenate (i.e., combine) multiple strings with the + operator:

  string firstName = "Sini";
  string lastName = "Aalto";
  
  string fullName = firstName + " " + lastName;
  
  Console.WriteLine(fullName);    // Outputs "Sini Aalto"
  

Character Constants

• Character constants are preceded by a backslash \ and can be used for formatting strings
• \n represents a newline

  string firstName = "Johannes";
  string lastName = "Kantola";
  string fullName = firstName + "\n" + lastName;
  Console.WriteLine(fullName);
  
  /* This outputs
  Johannes
  
  Kantola
  */
  

• All character constants are listed here

String Interpolation

• Concatenating multiple variables into one string with the + operator quickly becomes tedious
• It's much easier to use string interpolation by prefixing your target string with $ and inserting the variables inside curly brackets { }:

  string animal = "Dog";
  string sound = "Woof";
  
  Console.WriteLine($"{animal} says {sound}!");
  // Outputs "Dog says Woof!"
  

String Formatting

• You can add format strings to change the way variables are interpolated into a string
• Add the format string after your variable, separated by a colon (:)
• You can find an overview of format strings and a handy list of both standard and custom strings here

  double pi = 3.141592653;
  Console.WriteLine($"Pi to three digits: {pi:G3}");
  // Outputs "Pi to three digits: 3.14"
  

User input with Console.ReadLine()

• For the next exercise, you'll need the Console.ReadLine() method
• The method pauses the program, waits for an input stream from the console that pops up, and returns the value of the input:

  string userInput = Console.ReadLine();
  Console.WriteLine(userInput);
  

Exercise 2: Weekday survey

1. Create a console application that asks the user which weekday it is and assigns the answer to a string variable.
2. Print Have a nice <weekday> to the console where <weekday> is replaced with the string the user wrote.

Assignments (arithmetic operations)

Assignments about this topic can be found here