c# pat 3

Lesson 3: Control Statements - Selection

In the last couple of lessons, every program you saw contained a limited amount of sequential steps and then stopped. There were no decisions you could make with the input and the only constraint was to follow straight through to the end. The information in this lesson will help you branch into separate logical sequences based on decisions you make. More specifically, the goals of this lesson are as follows:

• Learn the if statements.
• Learn the switch statement.
• Learn how break is used in switch statements.
• Understand proper use of the goto statement.

The if Statement

An if statement allows you to take different paths of logic, depending on a given condition. When the condition evaluates to a boolean true, a block of code for that true condition will execute. You have the option of a single if statement, multiple else if statements, and an optional else statement. Listing 3-1 shows how each of these types of if statements work.

Listing 3-1. forms of the if statement: IfSelection.cs

using System;

class IfSelect
{
    public static void Main()
    {
        string myInput;
        int myInt;

        Console.Write("Please enter a number: ");
        myInput = Console.ReadLine();
        myInt = Int32.Parse(myInput);

        // Single Decision and Action with braces
        if (myInt > 0)
        {
            Console.WriteLine("Your number {0} is greater than zero.", myInt);
        }

        // Single Decision and Action without brackets
        if (myInt < 0)
            Console.WriteLine("Your number {0} is less than zero.", myInt);

        // Either/Or Decision
        if (myInt != 0)
        {
            Console.WriteLine("Your number {0} is not equal to zero.", myInt);
        }
        else
       {
            Console.WriteLine("Your number {0} is equal to zero.", myInt);
        }

        // Multiple Case Decision
        if (myInt < 0 || myInt == 0)
        {
            Console.WriteLine("Your number {0} is less than or equal to zero.", myInt);
        }
        else if (myInt > 0 && myInt <= 10)
        {
            Console.WriteLine("Your number {0} is in the range from 1 to 10.", myInt);
        }
        else if (myInt > 10 && myInt <= 20)
        {
            Console.WriteLine("Your number {0} is in the range from 11 to 20.", myInt);
        }
        else if (myInt > 20 && myInt <= 30)
        {
            Console.WriteLine("Your number {0} is in the range from 21 to 30.", myInt);
        }
        else
       {
            Console.WriteLine("Your number {0} is greater than 30.", myInt);
        }
    }
}

The statements in Listing 3-1 use the same input variable, myInt as a part of their evaluations. This is another way of obtaining interactive input from the user. Here's the pertinent code:

        Console.Write("Please enter a number: ");
        myInput = Console.ReadLine();
        myInt = Int32.Parse(myInput);

We first print the line "Please enter a number: " to the console. The Console.ReadLine() statement causes the program to wait for input from the user, who types a number and then presses Enter. This number is returned in the form of a string into the myInput variable, which is a string type. Since we must evaluate the user's input in the form of an int, myInput must be converted. This is done with the command Int32.Parse(myInput). (Int32 and similar types will be covered in another lesson on advanced types) The result is placed into the myInt variable, which is an int type.

Now that we have a variable in the type we wanted, we will evaluate it with if statements. The first statement is of the form if (boolean expression) { statements }, as shown below:

        // Single Decision and Action with braces
        if (myInt > 0)
        {
            Console.WriteLine("Your number {0} is greater than zero.", myInt);
        }

You must begin with the keyword if. Next is the boolean expression between parenthesis. This boolean expression must evaluate to a true or false value. In this case, we are checking the user's input to see if it is greater than (>) 0. If this expression evaluates to true, we execute the statements within the curly braces. (We refer to the structure with curly braces as a "block") There could be one or more statements within this block. If the boolean expression evaluates to false, we ignore the statements inside the block and continue program execution with the next statement after the block.

Note: In other languages, such as C and C++, conditions can be evaluated where a result of 0 is false and any other number is true. In C#, the condition must evaluate to a boolean value of either true or false. If you need to simulate a numeric condition with C#, you can do so by writing it as (myInt != 0), which means that the expression evaluate to true if myInt is not 0.

The second if statement is much like the first, except it does not have a block, as shown here:

        // Single Decision and Action without braces
        if (myInt < 0)
            Console.WriteLine("Your number {0} is less than zero.", myInt);

If its boolean expression evaluates to true, the first statement after the boolean expression will be executed. When the boolean expression evaluates to false, the first statement after the boolean expression will be skipped and the next program statement will be executed. This form of if statement is adequate when you only have a single statement to execute. If you want to execute two or more statements when the boolean expression evaluates to true, you must enclose them in a block.

Most of the time, you'll want to make an either/or kind of decision. This is called an if/else statement. The third if statement in Listing 3-1 presents this idea, as shown below:

        // Either/Or Decision
        if (myInt != 0)
        {
            Console.WriteLine("Your number {0} is not equal to zero.", myInt);
        }
        else
       {
            Console.WriteLine("Your number {0} is equal to zero.", myInt);
        }

When the boolean expression evaluates to true, the statement(s) in the block immediately following the if statement are executed. However, when the boolean expression evaluates to false, the statements in the block following the else keyword are executed.

When you have multiple expressions to evaluate, you can use the if/else if/else form of the if statement. We show this form in the fourth if statement of Listing 3-1, and repeated below:

        // Multiple Case Decision
        if (myInt < 0 || myInt == 0)
        {
            Console.WriteLine("Your number {0} is less than or equal to zero.", myInt);
        }
        else if (myInt > 0 && myInt <= 10)
        {
            Console.WriteLine("Your number {0} is in the range from 1 to 10.", myInt);
        }
        else if (myInt > 10 && myInt <= 20)
        {
            Console.WriteLine("Your number {0} is in the range from 11 to 20.", myInt);
        }
        else if (myInt > 20 && myInt <= 30)
        {
            Console.WriteLine("Your number {0} is in the range from 21 to 30.", myInt);
        }
        else
       {
            Console.WriteLine("Your number {0} is greater than 30.", myInt);
        }

This example begins with the if keyword, again executing the following block if the boolean expression evaluates to true. However, this time you can evaluate multiple subsequent conditions with the else if keyword combination. the else if statement also takes a boolean expression, just like the if statement. The rules are the same, when the boolean expression for the else if statement evaluates to true, the block immediately following the boolean expression is executed. When none of the other if or else if boolean expressions evaluate to true, the block following the else keyword will be executed. Only one section of an if/else if/else statement will be executed.

One difference in the last statement from the others is the boolean expressions. The boolean expression, (myInt < 0 || myInt == 0), contains the conditional OR (||) operator. In both the regular OR (|) operator and the conditional OR (||) operator, the boolean expression will evaluate to true if either of the two sub-expressions on either side of the operator evaluate to true. The primary difference between the two OR forms are that the regular OR operator will evaluate both sub-expressions every time. However, the conditional OR will evaluate the second sub-expression only if the first sub-expression evaluates to false.

The boolean expression, (myInt > 0 && myInt <= 10), contains the conditional AND operator.  Both the regular AND (&) operator and the conditional AND (&&) operator will return true when both of the sub-expressions on either side of the operator evaluate to true.  The difference between the two is that the regular AND operator will evaluate both expressions every time. However, the conditional AND operator will evaluate the second sub-expression only when the first sub-expression evaluates to true.

The conditional operators (&& and ||) are commonly called short-circuit operators because they do not always evaluate the entire expression. Thus, they are also used to produce more efficient code by ignoring unnecessary logic.

The switch Statement

Another form of selection statement is the switch statement, which executes a set of logic depending on the value of a given parameter. The types of the values a switch statement operates on can be booleans, enums, integral types, and strings. Listing 3-2 shows how to use the switch statement with both int and string types.

Listing 3-2. Switch Statements: SwitchSelection.cs

using System;

class SwitchSelect
{
    public static void Main()
    {
        string myInput;
        int myInt;

        begin:

        Console.Write("Please enter a number between 1 and 3: ");
        myInput = Console.ReadLine();
        myInt = Int32.Parse(myInput);

        // switch with integer type
        switch (myInt)
        {
            case 1:
                Console.WriteLine("Your number is {0}.", myInt);
                break;
            case 2:
                Console.WriteLine("Your number is {0}.", myInt);
                break;
            case 3:
                Console.WriteLine("Your number is {0}.", myInt);
                break;
            default:
                Console.WriteLine("Your number {0} is not between 1 and 3.", myInt);
                break;
        }

        decide:

        Console.Write("Type \"continue\" to go on or \"quit\" to stop: ");
        myInput = Console.ReadLine();

        // switch with string type
        switch (myInput)
        {
            case "continue":
                goto begin;
            case "quit":
                Console.WriteLine("Bye.");
                break;
            default:
                Console.WriteLine("Your input {0} is incorrect.", myInput);
                goto decide;
        }
    }
}

Note: Listing 3-2 will throw an exception if you enter any value other than an int. i.e. the letter 'a' would be an error.

Listing 3-2 shows a couple of switch statements. The switch statement begins with the switch keyword followed by the switch expression. In the first switch statement in listing 3-2, the switch expression evaluates to an int type, as follows:

        // switch with integer type
        switch (myInt)
        {
            case 1:
                Console.WriteLine("Your number is {0}.", myInt);
                break;
            case 2:
                Console.WriteLine("Your number is {0}.", myInt);
                break;
            case 3:
                Console.WriteLine("Your number is {0}.", myInt);
                break;
            default:
                Console.WriteLine("Your number {0} is not between 1 and 3.", myInt);
                break;
        }

The switch block follows the switch expression, where one or more choices are evaluated for a possible match with the switch expression. Each choice is labeled with the case keyword, followed by an example that is of the same type as the switch expression and followed by a colon (:). In the example we have case 1:, case 2:, and case 3:. When the result evaluated in the switch expression matches one of these choices, the statements immediately following the matching choice are executed, up to and including a branching statement, which could be either a break, continue, goto , return, or throw statement. table 3-1 summarizes the branching statements.

Table 3-1. C# Branching Statements

Branching statement	Description
break	Leaves the switch block
continue	Leaves the switch block, skips remaining logic in enclosing loop, and goes back to loop condition to determine if loop should be executed again from the beginning.
goto	Leaves the switch block and jumps directly to a label of the form "<labelname>:"
return	Leaves the current method.
throw	Throws an exception.

You may also include a default choice following all other choices. If none of the other choices match, then the default choice is taken and its statements are executed. Although use of the default label is optional, I highly recommend that you always include it. This will help catch unforeseen circumstances and make your programs more reliable.

Each case label must end with a branching statement, as described in table 3-1, which is normally the break statement. The break statement will cause the program to exit the switch statement and begin execution with the next statement after the switch block. There are two exceptions to this: adjacent case statements with no code in between or using a goto statement. Here's an example that shows how to combine case statements:

        switch (myInt)
        {
            case 1:
            case 2:
            case 3:
                Console.WriteLine("Your number is {0}.", myInt);
                break;
            default:
                Console.WriteLine("Your number {0} is not between 1 and 3.", myInt);
                break;
        }

By placing case statements together, with no code in-between, you create a single case for multiple values. A case without any code will automatically fall through to the next case. The example above shows how the three cases for myInt equal to 1, 2, or 3, where case 1 and case 2 will fall through and execute code for case 3.

A case statement can only be an exact match and you can't use logical conditions. If you need to use logical conditions, you can use an if/else if/else statement.

Another way to control the flow of logic in a switch statement is by using the goto statement. You can either jump to another case statement, or jump out of the switch statement. The second switch statement in Listing 3-2 shows the use of the goto statement, as shown below:

        // switch with string type
        switch (myInput)
        {
            case "continue":
                goto begin;
            case "quit":
                Console.WriteLine("Bye.");
                break;
            default:
                Console.WriteLine("Your input {0} is incorrect.", myInput);
                goto decide;
        }

Note: in the current example, "continue", is a case of the switch statement -- not the keyword.

The goto statement causes program execution to jump to the label following the goto keyword. During execution, if the user types in "continue", the switch statement matches this input (a string type) with the case "continue": label and executes the "goto begin:" instruction. The program will then leave the switch statement and start executing the first program statement following the begin: label. This is effectively a loop, allowing you to execute the same code multiple times. The loop will end when the user types the string "quit". This will be evaluated with the case "quit": choice, which will print "Bye." to the console, break out of the switch statement and end the program.

Warning: You should not create loops like this. It is *bad* programming style. The only reason it is here is because I wanted to show you the syntax of the goto statement.

When neither the "continue" nor "quit" strings are entered, the "default:" case will be entered. It will print an error message to the console and then execute the goto decide: command. This will cause program execution to jump to the first statement following the decide: label, which will ask the user if they want to continue or quit. This is effectively another loop.

Clearly, the goto statement is powerful and can, under controlled circumstances, be useful. However, I must caution you strongly on its use. The goto statement has great potential for misuse. You could possibly create a very difficult program to debug and maintain. Imagine the spaghetti code that could be created by random goto statements throughout a program. In the next lesson, I'll show you a better way to create loops in your program.

VARIANTS

Objective – understand how to program single, dual alternative, and nested conditional structures; learn to rely on well-formed data for programs and be able to make one or two-way decisions with an if-statement.

Problem statement – solve 3 problems with the help of the conditional structures.