C# 04 Study Notes Decision Statement

4.Decision Statement

4.1. Declare bool Variable

📌Romantic interpretation of Boolean

In the world of C# programming (unlike in the real world), everything is black or white, right or wrong, true or false.

📌true, false

bool areYouReady;
areYouReady = true;
Console.WriteLine(areYouReady);
//output is true

4.2. Use bool Variable

📌equality operator

They are ==, !=.

Operator	Meaning	Example	Output(if age=42)
==	Equal to	age == 100	false
!=	Not Equal to	age != 0	true

📌relational operator

Operator	Meaning	Example	Output
<	Less than	age < 21	false
<=	Less than or equal to	age <= 18	false
>	Greater than	age > 16	true
>=	Greater than or equal to	age >= 33	true

📌conditional logic operator

&& , logic AND

||, logic OR.

📌Bad habit using logic operator🚨

❌ Error:

xxxxxxxxxx
bool flag = num >= 0 && <= 100;  //ERROR!!!

This is hell wrong! When playing with logical operator, the variable can only appear once at a time.

😶 Not good but OK:

xxxxxxxxxx
bool flag = num >= 0 && num <= 100;

😄 Clear:

xxxxxxxxxx
bool flag = (num>=0)&&(num<=100);

📌Short-circuiting⭐️

This is very useful for boosting the performance of the codes🚀:

Write the easier and computation-less condition on the LEFT side of the operator.

xxxxxxxxxx
int num = -10;
bool flag = (num>=0) && (num<=100);

xxxxxxxxxx
int num = -10;
bool flag = (num<=100) || (num>=0);

Apparently, putting the easier codes on the left is more efficient as they will jump out of the condition soon.

📌operator precedence and associativity⭐️

The operators higher up in the table take precedence over operators lower down:

https://docs.microsoft.com/en-us/dotnet/csharp/language-reference/operators/

4.3. if-else Statement

📌sample of if

xxxxxxxxxx
if(flag)
{
    //..
}
else if(flag2)
{
    //..
}
else
{
    //..
}

📌good and bad examples

❌Error:

xxxxxxxxxx
int num = 10;
if(num)
{
    //..
}

Since num is not boolean, it can't be placed into the ().

👍Recommend:

xxxxxxxxxx
bool flag = true;
if(flag)
{
    //..
}

😶OK, but not recommend:

xxxxxxxxxx
bool flag = true;
if(flag==true)
{
    //..
}

4.4. switch Statement

📌When should we use switch?

Use it when you have multiple and parallel conditions.

xxxxxxxxxx
int day = 5;
string dayName = "";
switch (day)
{
    case 0:
        dayName = "Sunday";
        break;
    case 1:
        dayName = "Monday";
        break;
    case 2:
        dayName = "Tuesday";
        break;
    case 3:
        dayName = "Wednesday";
        break;
    //..
    default:
        dayName = "Unknown";
        break;
}

📌Rules Using switch

• case must be unique.
• every case should be ended with break

📌What is fall-through?

xxxxxxxxxx
switch(flag)
{
    case Hearts:
    case Diamonds:
        color="Red";
        break;
    case Clubs:
    case:Spades:
        color="Black";
        break;
    default:
        color=null;
        break;
}

In short, it combines conditions with same behaviors.