Decision (or Control) Structures

• Until now, we only wrote programs with a sequential flow of execution (line n was always executed after line n-1 and before line n+1).
• But we can control the flow using conditions, to perform selection (“execute this part of the code if this, that part if that”) and iteration (“repeat this part of the code until something happens”).
• The novel keyword we will introduce correspond to decision structures, and are if, else, switch, while, do...while and for.
• Decision structures always have to know if something is the case, a notion expressed in programs thanks to the notion of conditions, and captured by the boolean datatype.

Boolean and Conditions

A condition is either true or false. We can store if something is true or false in a (boolean) flag, which is simply a variable of type boolean.

We can declare, assign, initialize and display it as any other variable:

bool flag = true;
Console.WriteLine(true);

But the only two possible values are true and false, and we will study three operations on them: “and” (&&, the conjonction), “or” (||, the disjunction) and “not” (!, the negation). They have the expected meaning that the condition “A and B” is true if and only if A is true, and B is true. Similarly, “A or B” is false if and only if A is false, and B is false (that is, it takes only one to make their disjunction true).

We present this behavior with truth tables, as follows:

We could also have represented those tables in 2-dimensions, and will do so in lab.

Equality and Relational Operators

We test numerical value for equality, as well as string, char and bool!

Console.WriteLine(3 == 4);
Console.WriteLine(myStringVar == "Train");
Console.WriteLine(myCharVar == 'b');

We can also test if a value is greater than another, using the following relational operators.

We can also compare char, but the order is a bit complex (you can find it, for instance, at https://stackoverflow.com/a/14967721/).

The precedence, that we will study in lab, is as follows:

! (* / %) (+ -) (< > <= >=) (== !=) && ||

if Statement

Console.WriteLine("Enter your age");
int age = int.Parse(Console.ReadLine());
if (age >= 18)
{
    Console.WriteLine("You can vote!");
}

if (<condition>)
{
    <statement block>
}

if-else Statements

if (<condition>)
{
    <statement block 1>
}
else
{
    <statement block 2>
}

Nested if-else Statements

<statement block> can actually be an if-else statement itself!

bool usCitizen = true;
int age = 19;

if (usCitizen == true)
{
    if (age > 18)
    {
        Console.WriteLine("You can vote!");
    }
    else
    {
        Console.WriteLine("You are too young!");
    }
}
else
{
    Console.WriteLine("Sorry, only citizens can vote");
}

Note that

• There is a simpler way to write usCitizen == true: simply write usCitizen!
• We could remove the braces
• We could have a similar flavor with only if: if(age > 18 && usCitizen) … else …, but the messages would be less accurate.

if-else-if Statements

if (<condition 1>)
{
    <statement block> // Executed if condition 1 is true
}
else if (<condition 2>)
{
   <statement block> // Executed if condition 1 is false and condition 2 is true
}
...
else if (<condition N>)
{
    <statement block> // Executed if all the previous conditions are false and condition N is true
}
else
{
    <statement block>  // Executed if all the conditions are false
}

Note that the conditions could be really different, not even testing the same thing!

if (age > 12)
    x = 0;
else if (charVar == 'c')
    x = 1;
else if (boolFlag)
    x = 2;
else 
    x = 3;

?: Operator

There is an operator for if else statements for particular cases (assignment, call, increment, decrement, and new object expressions):

condition ? first_expression : second_expression;

int price = adult ? 5 : 3;

We will have a brief look at it if time allows, otherwise you can read about it at https://docs.microsoft.com/en-us/dotnet/csharp/language-reference/operators/conditional-operator.