csharp-basics/3-conditionals.md

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true	true	mathjax	buutti	3. Conditionals

Conditionals

Overview

• Conditionals
• if and else
• Logical Operators

Comparison operators

• Comparison operators are used to compare two variables
  • They return either true or false
• Two variables of any type can be compared with equality operators
  • Equal to: a == b
    • (Do not mix with the assignment operator, see Lecture 2!)
  • Not equal to: a != b
• Two numbers can be further compared with less than/greater than operators:
  • Less than: a < b
  • Less than or equal: a <= b
  • Greater than: a > b
  • Greater than or equal: a >= b

bool data type

• As shown in Lecture 2, bool is a data type for storing truth values true or false
• Because conditionals return true or false, the result can be stored in a variable

  int a = 4;
  int b = 3;
  bool areEqual = a == b;   // outputs False
  bool biggerOrNot = a > b; // outputs True
  

• Useful for making multiple comparisons at once

if, else if and else statements

• if checks truthfulness of a given statement
• If it results in false, we can check if another condition is met with else if
• Multiple else if can be chained indefinitely
• If no statement returns true, the else block is executed
• The statements are checked in order, and the first true condition is executed
  • No other block is executed
• (You can also have just if and else without the else if, or just a single if.)

int a = 2
if (a > 4)
{
  // do something
}
else if (a < 2)
{
  // do something else
}
else if (a < 3)
{
  // do something else
}
else
{
  // a is 4??
}

Conditionals: An example

double temperatureInOulu = 2.3;
double temperatureInIvalo = -10.9;
if (temperatureInOulu > temperatureInIvalo)
{
  Console.WriteLine("Oulu is warmer than Ivalo");
}
else if (temperatureInOulu < temperatureInIvalo)
{
  Console.WriteLine("Ivalo is warmer than Oulu");
}
else
{
  Console.WriteLine
    ("Oulu and Ivalo have the same temperature");
}

This outputs Oulu is warmer than Ivalo.

Not operator !

• The ! ("not") operator flips the boolean value.

Console.WriteLine(!true);     // Outputs false
bool itsColdOutside = true;
if (!itsColdOutside)          // same as checking if (itsColdOutside == false)
{
  Console.WriteLine("It's warm outside.");
}

The switch statement

• The switch statement compares the given expression (in this example, the path variable) with the value of each case
• Only the matching code is executed
• If no match is found, the default code block is executed
• This example outputs Here's the catalogue!
• break ends each case (the code will not compile if omitted!)

string path = "/browse";

switch (path)
{
  case "/browse":
    Console.WriteLine("Here's the catalogue!");
    break;
  case "/contact":
    Console.WriteLine("Here's our contact info.");
    break;
  default:
    Console.WriteLine("Given path doesn't exist!");
    break;
}

Scope of Variables

• Variables declared inside of blocks {} are local to that scope; they are only accessible inside of that block

  if (true)
  {
    int b = 1;        // b is only accessible in this block
    b = 2;            // This works
  }
  
  b = 3;              // This throws an error
  

• Similarly, classes are only defined inside their namespace, and have to be imported to be accessible elsewhere
• Note: Some languages have global variables that are accessible everywhere — as an object-oriented language, C# doesn't have such feature.

Logical Operators

• &&, || and ! are the logical AND, OR and NOT operators
• These are useful when writing complex if statements

int a = 1;
int b = 3;
int c = 5;
if (a < 10)
{
  if (b < 10)
  {
    if (c < 10)
    {
      Console.WriteLine
        ("All are smaller than 10!")
    }
  }
}

int a = 1;
int b = 3;
int c = 5;
if (a < 10 && b < 10 && c < 10)
{
  Console.WriteLine
    ("All are smaller than 10!")
}

• Same functionality achieved in fewer lines!

Common logical operators

Operator	Name	Usage	Description
&&	AND	a && b	Returns true if both variables are true. b is not evaluated if a == false.
||	OR	a || b	Returns true if one or both variables are true. b is not evaluated if a == true.
!	NOT	!a	Negates the boolean value. (true becomes false and vice versa).

Less common logical operators

Operator	Name	Usage	Description
^	XOR	a ^ b	Exclusive OR ("joko tai"): returns true if only either of a or b are true, but not both!
&	Logical AND	a & b	Returns true if both variables are true. Both variables are always evaluated.
|	Logical OR	a | b	Returns true if one or both variables are true. Both variables are always evaluated.

Logical operators: An Example

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

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

if (uppercaseLetters >= 6 && specialCharacters >= 2)
{
  Console.WriteLine("Strong password!");
}
else
{
  Console.WriteLine("Weak password...");
}

This outputs Strong password!

Extra: Single statement if

• If a code block following a statement only has one line of code, it is possible to write the block without curly brackets:

  int baa = 49;
  if (baa > 20)
    Console.WriteLine("Baa");
  else
    Console.WriteLine("Not baa!");
  if (baa > 20) Console.WriteLine("Baa");
  else Console.WriteLine("Not baa!");
  

• You may encounter code like this — however, we highly recommend to refrain from using this syntax as it is highly prone to errors.
  • Can you guess how this syntax can lead to bugs?

Exercise 1

1. Create a console application that asks the user which weekday it is and assigns the answer to a string variable.
2. Using a switch-case expression, calculate the days remaining until next Monday.
3. If the result is more than 3, print Have a nice week!. Otherwise, print Have a nice weekend!.

Exercise 2

1. Create a console application that lets the user input a note as a string.
2. If the length of the note is less than 30, the program prints the current time and the note separated by a tab. Otherwise, the date and the note are printed to a separate line.

Tip: Use DateTime.Now.ToString() for current time. Use .Length after your message variable to get the length of the message.

Reading

Basics covering the syntax in C# are covered here:

• Learn .NET: C# Documentation
• Tutorialspoint: C# tutorial
• W3Schools: C# tutorial

Assignments

Assignments about this topic can be found here