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:

Assignments

Assignments about this topic can be found here

| Comparison | Syntax | |:-------------------------|:---------| | Less than | `a < b` | | Less than or equal to | `a <= b` | | Greater than | `a > b` | | Greater than or equal to | `a >= b` | | Equal to | `a == b` | | Not equal to | `a != b` |