Lesson 5 - Conditions (branching)

C# .NET Basic constructs Conditions (branching)

In the previous lesson, More on the type system: Data types, we discussed C# .NET data types in details. We need to react somehow to different situations if we want to program something. It may be, for example, a value entered by the user, according to which we would like to change the running of the program. We metaphorically say that the program branches, and for branching we use conditions. We will pay attention to those in today's article. We're going create a program which calculates square roots, and we're going to improve our calculator.

Conditions

In C#, conditions are exactly the same as in all C-like languages, either way, I will explain everything for beginners. Advanced programmers will probably be bored for a moment :)

Conditions are written using the if keyword, which is followed by a logical expression. If the expression is true, the following statement will be executed. If it's not true, the following statement will be skipped, and the program will continue with the next statement. Let's try it out:

if (15 > 5)
        Console.WriteLine("True");
Console.WriteLine("The program continues here...");
Console.ReadKey();

The output:

Console application
True
The program continues here...

If the condition is true, a command which writes a text to the console will be executed. In both cases the program continues. Of course, a variable can also be part of the expression:

Console.WriteLine("Enter a number");
int a = int.Parse(Console.ReadLine());
if (a > 5)
        Console.WriteLine("The number you entered is greater than 5!");
Console.WriteLine("Thanks for the input!");
Console.ReadKey();

Let's look at the relational operators which can be used in expressions:

Meaning Operator
Equal to ==
Greater than >
Less than <
Greater than or equal to >=
Less than or equal to <=
Not equal !=
Negation !

We use the == operator for equality to avoid confusing it with a normal assignment to a variable (the = operator). If we want to negate an expression, we write it in parentheses and we write an exclamation mark before the actual expression within the parentheses. If you want to execute more than one command, you have to insert commands into a block of curly brackets:

Console.WriteLine("Enter some number and I'll calculate a square root:");
int a = int.Parse(Console.ReadLine());
if (a > 0)
{
        Console.WriteLine("The number you entered is greater than 0, so I can calculate it!");
        double root = Math.Sqrt(a);
        Console.WriteLine("The square root of " + a + " is " + root);
}
Console.WriteLine("Thanks for the input");
Console.ReadKey();

Console application
Enter some number and I'll calculate a square root:
144
You've entered a number greater than 0, I can calculate it!
Square root of 144 is 12
Thanks for the input

The program retrieves a number from the user, and if it is greater than 0, it calculates a square root. We have used the Math class, which contains plenty of useful mathematical methods. At the end of this course, we'll learn more about them. Sqrt() returns the value as a double data type. It would be nice if our program warned us if we entered a negative number. With what we know up until now, we'll able to write something like this:

Console.WriteLine("Enter a number and I'll get its square root:");
int a = int.Parse(Console.ReadLine());
if (a > 0)
{
        Console.WriteLine("The number you entered is greater than 0, so I can calculate it!");
        double root = Math.Sqrt(a);
        Console.WriteLine("The square root of " + a + " is " + root);
}
if (a <= 0)
        Console.WriteLine("I can't calculate the square root of a negative number!");
Console.WriteLine("Thanks for the input!");
Console.ReadKey();

We must keep in mind the case where a == 0, but also when it is less than 0. The code can be greatly simplified by using the else keyword which executes the following statement or block of statements if the condition was not true:

Console.WriteLine("Enter a number and I'll get its square root:");
int a = int.Parse(Console.ReadLine());
if (a > 0)
{
        Console.WriteLine("The number you entered is greater than 0, so I can calculate it!");
        double root = Math.Sqrt(a);
        Console.WriteLine("The square root of " + a + " is " + root);
}
else
        Console.WriteLine("I can't calculate the square root of a negative number!");
Console.WriteLine("Thanks for the input!");
Console.ReadKey();

The code is much clearer, and we don't have to make up the negate condition which could be very difficult with complex conditions sometimes. In the case of multiple commands, there would be a {} block again after the else keyword.

Else is also used when we need to set a variable from the condition up so we can't evaluate it later again. The program remembers that the condition didn't apply and it'll move to the else branch. Let's look at an example: Consider a number which value will be either 0 or 1 and we'll be asked to swap those values (if there is 0, we put a 1 there, and the other way around). Naively, we could write a code like this:

int a = 0; // the variable is initialized with a value of 0

if (a == 0) // if the value is 0, we change its value to 1
        a = 1;
if (a == 1) // if the value is 1, we change its value to 0
        a = 0;

Console.WriteLine(a);
Console.ReadKey();

It doesn't work, does it? Let's take a closer look at the program. At the very beginning, a contains value 0, the first condition is undoubtedly fulfilled and it assigns 1 into a. Well, suddenly, the second condition becomes true. What should we do? When we swap the conditions, we'll have the same problem with 1. Now, how do we solve this? You guessed it, using else!

int a = 0; // the variable is initialized with a value of 0

if (a == 0) // if the value is 0, we change its value to 1
        a = 1;
else // if the value is 1, we change its value to 0
        a = 0;

Console.WriteLine(a);
Console.ReadKey();

Conditions can be composed by using two basic logical operators:

Operator C-like syntax
Logical AND &&
Logical OR ||

Let's take a look at the example:

Console.WriteLine("Enter a number between 10-20:");
int a = int.Parse(Console.ReadLine());
if ((a >= 10) && (a <= 20))
        Console.WriteLine("The condition has been met.");
else
        Console.WriteLine("You did it wrong.");
Console.ReadKey();

Of course operators can be combined with parentheses:

Console.WriteLine("Enter a number between 10-20 or 30-40:");
int a = int.Parse(Console.ReadLine());
if (((a >= 10) && (a <= 20)) || ((a >=30) && (a <= 40)))
        Console.WriteLine("The condition has been met.");
else
        Console.WriteLine("You did it wrong.");
Console.ReadKey();

Switch

Switch is a construct taken from the C language, like most of C#'s syntax. It allows us to relatively simplify the usage of if-else command sequences. Let's remember our calculator from the first lesson, which had read two numbers and calculated all 4 operations. Now, we want to single out an operation. Without the switch, we would write the code like this:

Console.WriteLine("Welcome to our calculator");
Console.WriteLine("Enter the first number:");
double a = double.Parse(Console.ReadLine());
Console.WriteLine("Enter the second number:");
double b = double.Parse(Console.ReadLine());
Console.WriteLine("Choose one of the following operations:");
Console.WriteLine("1 - addition");
Console.WriteLine("2 - subtraction");
Console.WriteLine("3 - multiplication");
Console.WriteLine("4 - division");
int option = int.Parse(Console.ReadLine());
double result = 0;
if (option == 1)
        result = a + b;
else
if (option == 2)
        result = a - b;
else
if (option == 3)
        result = a * b;
else
if (option == 4)
        result = a / b;
if ((option > 0) && (option < 5))
        Console.WriteLine("result: {0}", result);
else
        Console.WriteLine("Invalid option");
Console.WriteLine("Thank you for using our calculator. Press any key to end the program.");
Console.ReadKey();

Console application
Welcome to our calculator
Enter the first number:
3.14
Enter the second number:
2.72
Choose one of the following operations:
1 - addition
2 - subtraction
3 - multiplication
4 - division
2
result: 0.42
Thank you for using our calculator. Press any key to end the program.

Notice that we've declared a variable result at the beginning, so we could later assign something to it. If we declared it at every assignment, C# would not compile the code and report an error since the variable would be already declared. A variable can be declared only once. C# is not able to tell whether a value is really assigned to the variable result. It would report an error on the line where we're printing to the console because C# doesn't like the fact that the variable being printed is not guaranteed to contain a value. For this reason, we have to assign zero to variable result at the beginning. Another trick is validating the user's choice. The program should still work the same even without all the elses (but why keep on asking if we already have a result).

Now here's the same program using a switch:

Console.WriteLine("Welcome to our calculator");
Console.WriteLine("Enter the first number:");
double a = double.Parse(Console.ReadLine());
Console.WriteLine("Enter the second number:");
double b = double.Parse(Console.ReadLine());
Console.WriteLine("Choose one of the following operations:");
Console.WriteLine("1 - addition");
Console.WriteLine("2 - subtraction");
Console.WriteLine("3 - multiplication");
Console.WriteLine("4 - division");
int option = int.Parse(Console.ReadLine());
double result = 0;
switch (option)
{
        case 1:
                result = a + b;
        break;
        case 2:
                result = a - b;
        break;
        case 3:
                result = a * b;
        break;
        case 4:
                result = a / b;
        break;
}
if ((option > 0) && (option < 5))
        Console.WriteLine("Result: {0}", result);
else
        Console.WriteLine("Invalid option");
Console.WriteLine("Thank you for using our calculator. Press any key to end the program");
Console.ReadKey();

As you can see, the code is a bit clearer now. If we needed to execute multiple commands in any branch of the switch, surprisingly, we wouldn't write it into the {} block, but just under the first command. The {} block is replaced by a break command that causes a jumping out from the entire switch. Beside of case x:, the switch can also contain the default: branch, which will be executed if neither of the cases applied. It's up to you whether you use a switch or not. Generally, it's useful only for a larger amount of branches and you always could always it with if-else sequences. Don't forget about breaks. Obviously, you can use a switch for string variables as well.

That is all for today. In the next lesson, Loops in C# .NET, we'll take a look at arrays and loops, i.e. finish up with the absolute basics of the C# language. Look forward to it :)


 

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Article has been written for you by David Capka
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