Task

You will write a program that checks whether the user entered either yes or no. All other inputs will be considered incorrect (see Figures 17-1 and 17-2).

Figure 17-1 Acceptable answer but sad

Figure 17-2 Yes!

Solution

Here’s the code:

static void Main(string[] args)
{
    // Input
    Console.Write("Do you love me? ");
    string input = Console.ReadLine();

    // Evaluating
    string inputInSmall = input.ToLower();
    if (inputInSmall == "yes" || inputInSmall == "no")
    {
        Console.WriteLine("OK.");
    }
    else
    {
        Console.WriteLine("Say it straight!");
    }

    // Waiting for Enter
    Console.ReadLine();
}

Discussion

Note the following:

• To disregard the difference in case, you convert the input to lowercase letters.

• The condition used is a compound condition. It consists of two partial conditions connected using the conditional OR operator, which is typed as || (two vertical lines).

• The condition is fulfilled (and the if branch is executed) if at least one of the partial conditions is fulfilled. This means the user entered either yes or no. In this case, the alternatives are mutually exclusive. However, you will encounter cases when both conditions can be fulfilled simultaneously.

• The condition is not fulfilled (and the else branch is executed) if neither the first nor the second partial condition is fulfilled. In other words, the user entered something besides yes or no .

Task

You will write a program that checks whether the user entered the correct username (Orwell) and at the same time the correct password (Catalonia). The username is case-insensitive, meaning it can be lowercase and uppercase (see Figures 17-3 and 17-4).

Figure 17-3 Correct username but incorrect password

Figure 17-4 Correct username and password

Solution

Here is the code:

static void Main(string[] args)
{
    // Correct values
    string correctUsername = "Orwell";
    string correctPassword = "Catalonia";

    // Inputs
    Console.Write("User name: ");
    string enteredUserName = Console.ReadLine();

    Console.Write("Password: ");
    string enteredPassword = Console.ReadLine();

    // Evaluating
    if (enteredUserName.ToLower() == correctUsername.ToLower() &&
        enteredPassword == correctPassword)
    {
        Console.WriteLine("Thanks for your books!");
    }
    else
    {
        Console.WriteLine("Could not log you in.");
    }

    // Waiting for Enter
    Console.ReadLine();
}

Discussion

Note the following:

• The condition used is a compound condition again. Its partial conditions are connected using the conditional AND operator, which is typed as && (two ampersands).

• The condition is fulfilled (and the if branch is executed) if both partial conditions are fulfilled simultaneously. This means the user has to enter both the correct username and the correct password.

• To not fulfill the condition (and thus execute the else branch), it is enough to not fulfill either one of the partial conditions .

Task

You will modify the previous task to allow two possible users to log in. Both will have their own passwords.

Solution

Here is the code:

static void Main(string[] args)
{
    // Correct values
    string correctUsername1 = "Orwell";
    string correctPassword1 = "Catalonia";

    string correctUsername2 = "Blair";
    string correctPassword2 = "1984";

    // Inputs
    Console.Write("User name: ");
    string enteredUsername = Console.ReadLine();

    Console.Write("Password: ");
    string enteredPassword = Console.ReadLine();

    // Evalulating
    if (enteredUsername.ToLower() == correctUsername1.ToLower() &&
        enteredPassword == correctPassword1 ||
        enteredUsername.ToLower() == correctUsername2.ToLower() &&
        enteredPassword == correctPassword2)
    {
        Console.WriteLine("Thanks for your books!");
    }
    else
    {
        Console.WriteLine("Could not log you in.");
    }

    // Waiting for Enter
    Console.ReadLine();
}

Discussion

Note the following:

• You can combine both conditional operators: AND with OR.

• Fulfillment of the complete condition requires the user to enter the correct first username and the correct first password or the correct second username and the correct second password.

• The condition intentionally uses a higher priority (precedence) for the AND operator compared to the OR operator. Specifically, both potential users are evaluated first, and the partial results are ORed afterward.

• If you need a different evaluation order, just use parentheses (round brackets) similarly to mathematics.

Task

The task is the same as the previous one, but the solution will be different.

Solution

Here is the code:

static void Main(string[] args)
{
    // Correct values
    string correctUsername1 = "Orwell";
    string correctPassword1 = "Catalonia";

    string correctUsername2 = "Blair";
    string correctPassword2 = "1984";

    // Inputs
    Console.Write("User name: ");
    string enteredUsername = Console.ReadLine();

    Console.Write("Password: ");
    string enteredPassword = Console.ReadLine();

    // Evaluating
    bool user1ok = enteredUsername.ToLower() == correctUsername1.ToLower() &&
                    enteredPassword == correctPassword1;
    bool user2ok = enteredUsername.ToLower() == correctUsername2.ToLower() &&
                    enteredPassword == correctPassword2;
    if (user1ok || user2ok)
    {
        Console.WriteLine("Thanks for your books!");
    }
    else
    {
        Console.WriteLine("Could not log you in.");
    }

    // Waiting for Enter
    Console.ReadLine();
}

Discussion

Note the following:

• You check both users one after another. The main condition can then be written in a clear and concise way.

• Partial conditions are precalculated into variables of type bool. When a condition is fulfilled, the corresponding variable has its value set to true.

Task

You will return to the “Yes or No” project from the beginning of this chapter once again. For the purpose of practicing compound conditions, think about how you would reverse the original condition to swap the if and else branches.

Solution

Here is the code:

static void Main(string[] args)
{
    // Input
    Console.Write("Do you love me? ");
    string input = Console.ReadLine();

    // Evaluating
    string inputInSmall = input.ToLower();
    if (inputInSmall != "yes" && inputInSmall != "no")
    {
        Console.WriteLine("Say it straight!");
    }
    else
    {
        Console.WriteLine("OK.");
    }

    // Waiting for Enter
    Console.ReadLine();
}

Discussion

Note the following:

• Instead of checking the correct input, you now check the incorrect one.

• The input is incorrect when it neither equals yes nor no.

• Reversing the condition caused the OR operator to change into the AND operator. It also caused the equalities to change into inequalities.

Task

The user enters a grade of a student. The program will check then whether the entered number is in the set of possible values 1, 2, 3, 4, or 5 (see Figure 17-5 and Figure 17-6).

Figure 17-5 Within the range

Figure 17-6 Not within the range

Solution

The condition can be formulated enumerating the individual alternatives. For the sake of simplicity, I do not check for possible non-numeric input. You can handle this yourself using try-catch as usual.

Here is the code:

static void Main(string[] args)
{
    // Input
    Console.Write("Enter a grade: ");
    string input = Console.ReadLine();
    int grade = Convert.ToInt32(input);

    // Evaluating
    if (grade == 1 ||
        grade == 2 ||
        grade == 3 ||
        grade == 4 ||
        grade == 5)
    {
        Console.WriteLine("Input OK.");
    }
    else
    {
        Console.WriteLine("Incorrect input.");
    }

    // Waiting for Enter
    Console.ReadLine();
}

Task

The task is to solve the previous exercise using a range check.

Solution

A number belongs to a range given by its lower and upper bounds when it is greater than or equal to the lower bound and at the same time it is less than or equal to the upper bound.

Here is the code:

static void Main(string[] args)
{
    // Input
    Console.Write("Enter a grade: ");
    string input = Console.ReadLine();
    int grade = Convert.ToInt32(input);

    // Evaluating
    if (grade >= 1 && grade <= 5)
    {
        Console.WriteLine("Input OK.");
    }
    else
    {
        Console.WriteLine("Incorrect input.");
    }

    // Waiting for Enter
    Console.ReadLine();
}