The simplest program lists commands one after another in a sequence, much like following the steps of a recipe. Follow a recipe step by step and you always create the same dish. If a program lists commands step by step, the computer always produces the same result.
In some cases, you may want a program to do the exact same thing over and over again, such as a simple program to display traffic signals. However for most programs, you want the computer to react to outside data. To make a computer respond in different ways, a program needs to offer two or more choices for the computer to follow.
When you quit a program, the program may ask, "Do you really want to exit?" At this point, the program is giving the computer a choice of two possible actions to take based on your answer.
If you answer Yes, the computer quits the program. If you answer No, the computer keeps running the program.
When a program gives the computer a choice of two or more commands to follow, that's called a branching or decision statement.
All branching statements work the same way:
A comparison operator (or a conditional expression) compares an expression (such as A > 45) to determine a
TrueorFalsevalue.The branching statement offers at least two groups of commands for the computer to follow based on whether its comparison is
TrueorFalse.
The simplest branching statement is an IF-THEN statement, which looks like this:
IF (Something is True or False) THEN Command
The IF-THEN checks if something is True or False:
If something is
True, theIF-THENcommand tells the computer to run exactly one command.If something is
False, the computer doesn't run this command.
An example of a simple IF-THEN statement might occur while playing a video game, such as
IF (Player hit the Pause button) THEN Pause game
If the player hit the pause button (True), you want the computer to pause the game. If the player doesn't hit the pause button (False), you don't want to pause the game, as shown in Figure 4-1.
The simple IF-THEN statement runs only one command if a certain condition is True. What if you want to run two or more commands? In that case, you must define a list of commands to run. A group of commands is sometimes called a block of commands or just a block.
So if you want to run more than one command in an IF-THEN statement, you must define a block of commands. In the curly bracket language family, such as C, you use curly brackets to define the beginning and end of a block of commands, such as
if (True or False)
{
command #1
command #2
.
.
command #3
)The curly brackets tell the IF-THEN statement to run the entire block of commands enclosed within the curly brackets.
In other languages, the IF-THEN statement itself defines the start of a block and then you use an END IF command to define the end of a block, such as this BASIC language example:
IF (True or False) THEN Command #1 Command #2 . . Command #3 END IF
Finally, some languages, such as Pascal, force you to explicitly declare the beginning and end of a block of commands with the begin and end keywords, such as
If (True or False) then
Begin
Command #1
Command #2
.
.
Command #3
End;No matter what language you use, the idea is the same; you must define the beginning and end of all the commands you want the IF-THEN statement to run.
The simple IF-THEN statement either runs a command (or block of commands) or it doesn't. But what if you want the computer to take one action if something is True and a completely different action if something is False? In that case, you must use a variation — an IF-THEN-ELSE statement.
The IF-THEN-ELSE statement gives the computer a choice of two mutually exclusive choices, as shown in Figure 4-2.
Like the simple IF-THEN statement, the IF-THEN-ELSE statement can run a single command or a block of commands, such as
if (True or False) then
{
command #1
command #2
.
.
command #3
}
else
{
command #1command #2 . . command #3 )
The IF-THEN-ELSE statement tells the computer, "Check if something is True. If so, follow this set of commands. Otherwise, follow this second set of commands."
One problem with the IF-THEN-ELSE statement is that it only checks a single condition. If that single condition is False, it always runs its second set of commands, such as
IF (Salary > 100000) THEN TaxRate = 0.45 ELSE TaxRate = 0.30 END IF
In this BASIC language example, if the value of the Salary variable is greater than 100000, the TaxRate variable is always set to 0.45.
However, if the Salary variable isn't greater than 100000 (it's less than or equal to 100000), the ELSE portion of the IF-THEN-ELSE statement always sets the TaxRate variable to 0.30.
The IF-THEN-ELSE always gives the computer a choice of exactly two, mutually exclusive choices. What if you want to give the computer three or more possible choices? Then you must use the IF-THEN-ELSEIF statement.
The IF-THEN-ELSEIF statement offers two advantages over the IF-THEN-ELSE statement:
You can check a condition for each set of commands.
You can define three or more separate sets of commands for the computer to follow.
Warning
Not all programming languages, such as C/C++, offer the IF-THEN-ELSEIF statement.
The IF-THEN-ELSEIF statement only runs a command (or block of commands) if some condition is True, as shown in Figure 4-3.
If every conditional expression is False, the IF-THEN-ELSE statement doesn't run any commands. Only if one of its conditional expressions is True does the IF-THEN-ELSE statement run exactly one set of commands, such as
IF (Salary > 100000) THEN TaxRate = 0.45 ELSEIF (Salary > 50000) THEN TaxRate = 0.30 END IF
In this example, the computer has three possible choices:
If Salary > 100000, set TaxRate = 0.45.
If Salary > 50000 (but less than 100000), set TaxRate = 0.30.
If Salary <= 50000, do nothing.
The first choice checks if the value of the Salary variable is greater than 100000, such as 250000. If so, it tells the computer to set the TaxRate variable to 0.45 and immediately exit out of the entire IF-THEN-ELSEIF statement.
The second choice only checks if the value of the Salary variable is greater than 50000. What happens if the Salary value is 150000? In that case, the first choice runs (Salary > 100000), so this second choice would've never been checked at all.
So although the second choice might seem to run if the Salary variable is greater than 50000, it really won't run unless Salary > 50000 and the first choice did not run, which means that the Salary variable must be less than or equal to 100000.
If the value of the Salary variable is equal or less than 50000, the third choice isn't to run any additional commands at all. Unlike the IF-THEN-ELSE statement, which always runs at least one set of commands, it's possible for an IF-THEN-ELSEIF statement to run zero commands.
The advantage of the IF-THEN-ELSEIF statement is that you can check for multiple conditions and give the computer three or more possible commands (or block of commands) to follow.
To give the computer additional choices, you can just keep tacking on additional ELSEIF statements, such as
IF (True or False) THEN Command ELSEIF (True or False) THEN Command ELSEIF (True or False) THEN Command END IF
This example now gives the computer three possible sets of commands to follow. If none of these conditions is True, the IF-THEN-ELSEIF statement may do nothing.
To keep checking for additional conditions, you have to add additional ELSEIF statements, such as
IF (True or False) THEN Command ELSEIF (True or False) THEN Command ELSEIF (True or False) THEN Command
ELSEIF (True or False) THEN Command ELSEIF (True or False) THEN Command END IF
This example gives the computer five possible commands (or blocks of commands) for the computer to follow, although it's possible that the computer still follows zero commands.
Warning
The IF-THEN-ELSEIF statement makes the computer run exactly zero or one command (or block of commands), no matter how many additional ELSEIF statements you add on.
If you want to make sure the IF-THEN-ELSEIF statement always runs one command (or block of commands), you can tack on the ELSE statement at the very end, such as
IF (True or False) THEN Command ELSEIF (True or False) THEN Command ELSEIF (True or False) THEN Command ELSEIF (True or False) THEN Command ELSE Command END IF
The ELSE statement at the end insures that the entire IF-THEN-ELSEIF statement always runs at least one command. Notice that the ELSE statement doesn't check a condition because it runs only if all preceding conditions are False, such as in the following example:
IF (Age > 65) THEN Status = Retired ELSEIF (Age > 20) THEN Status = Working ELSE Status = Bum END IF
In this example, the IF-THEN-ELSEIF statement gives the computer three possible choices:
Set Status = Retired only if Age > 65
Set Status = Working only if Age > 20 (and less than or equal to 65)
Set Status = Bum only if Age is less than or equal to 20 (which means the other two conditions are False)
To make a decision in an IF-THEN statement, the computer must use a conditional expression that's either True or False. Simple conditional expressions might be
Age = 55 Salary <= 55000 Name <> "John Smith"
You can also use Boolean operators (AND, OR, NOT, and XOR) to calculate multiple conditions. Suppose you want to check if a variable falls within a range of values, such as being greater than 20 but less than 65:
Warning
In most programming languages, you can type a Boolean operator in lowercase (and), uppercase (AND), or a mix of both upper and lowercase (And). Whichever style you like best, use it consistently throughout your program.
(Age > 20) AND (Age <= 65)
Warning
Chapter 3 of this mini-book contains more information about how Boolean operators work.
Table 4-1 shows how different values for the Age variable determine the value of the preceding Boolean expression.
Table II.4-1. Multiple Boolean Expressions Ultimately Evaluate to a Single True or False Value
Value of the Age Variable | Value of (Age > 20) Expression | Value of (Age <= 65) Expression | Value of Complete Boolean Expression |
|---|---|---|---|
15 | False | True | False |
35 | True | True | True |
78 | True | False | False |
Because multiple Boolean expressions ultimately evaluate to a single True or False value, you can use multiple Boolean expressions in any IF-THEN statements, such as
IF (Age > 20) AND (Age <= 65) THEN Status = Working ELSE Status = Bum END IF
There's no limit to the number of Boolean expressions you can combine with Boolean operators. The following is a perfect valid Boolean expression that ultimately evaluates to a single True or False value:
(Age > 20) AND (Age <= 65) OR (Age = 72) OR (Name = "John")
Tip
The more Boolean expressions you string together with Boolean operators, the more confusing everything gets so it's generally best to use no more than two Boolean expressions and a single Boolean operator (AND, OR, NOT, or XOR) at a time.
The IF-THEN-ELSEIF statement can check multiple conditions and offer two or more choices for the computer to follow. However, the more choices available, the harder the IF-THEN-ELSEIF statement can be to understand, as shown in the following example:
IF (Age = 65) THEN Status = Retired ELSEIF (Age = 21) THEN Status = Working ELSEIF (Age = 15) THEN Status = Student ELSE Status = Baby END IF
For two or three choices, the IF-THEN-ELSE statement may be easy to understand, but after you need to offer four or more choices, the IF-THEN-ELSEIF statement can start getting clumsy. As an alternative, most programming languages offer a SELECT CASE statement:
SELECT CASE Variable CASE X Command #1 CASE Y Command #2 END SELECT
The SELECT CASE statement examines a variable and if it's equal to a specific value, the computer follows a command (or block of commands). The preceding SELECT CASE statement is equivalent to the following IF-THEN-ELSEIF statement:
If Variable = X THEN Command #1 ELSEIF Variable = Y THEN Command #2 END IF
The basic idea behind the SELECT CASE statement is to make it easier to list multiple choices. Both an IF-THEN-ELSEIF statement and a SELECT CASE or switch statement perform the same function; it's just that the SELECT CASE statement is easier to read and understand.
Consider the following IF-THEN-ELSEIF statement:
IF (Age = 65) THEN Status = Retired ELSEIF (Age = 21) THEN Status = Working ELSEIF (Age = 15) THEN Status = Student END IF
Rewriting this as a SELECT CASE statement might look like this:
SELECT CASE Age CASE 65 Status = Retired CASE 21 Status - Working CASE 15 Status = Student END SELECT
As you can see, the SELECT CASE statement is much less cluttered and easier to read and understand than the IF-THEN-ELSEIF statement.
Instead of using a SELECT CASE statement, curly bracket languages, like C, use a switch statement. The equivalent SELECT CASE statement written as a switch statement in C looks like this:
switch (Variable)
{
case X: Command #1;
break;
case Y: Command #2;
}A SELECT CASE statement in BASIC might look like this:
SELECT CASE Age CASE 65 Status = Retired CASE 21 Status - Working CASE 15 Status = Student END SELECT
The equivalent switch statement in C might look like this:
switch (age)
{
case 65: status = retired;
break;
case 21: status = working;
break;
case 15: status - student;
}Warning
The most crucial difference between the SELECT CASE statement in other languages and the switch statement in the curly bracket languages is the use of the break command. If you omit the break command, the switch statement doesn't know when to stop running commands.
In the preceding example, the break command stops the computer from running the other commands stored in the rest of the switch statement. So if the value of the age variable is 65, the preceding C program does the following:
Set the
statusvariable toretired.Stop running the
switchstatement.
Suppose you didn't include the break command, as follows:
switch (age)
{
case 65: status = retired;
case 21: status = working;
case 15: status - student;
}If the value of the age variable is 65, this is how this C program works:
Set the
statusvariable toretired.Set the
statusvariable toworking.Set the
statusvariable tostudent.
Without the break command, the curly bracket languages, like C, simply run every command all the way through the switch statement until it reaches the bottom, which probably isn't what you want.
Warning
When using the switch statement in C (and other curly bracket languages), always use the break command unless you specifically don't need it, as I explain in the following section.
One major limitation of the SELECT CASE statement is that it only checks if a variable matches a single value, such as
SELECT CASE Age CASE 65 Status = Retired CASE 21 Status - Working CASE 15 Status = Student END SELECT
This SELECT CASE statement doesn't do anything unless the value of the Age variable is exactly 65, 21, or 15. If the value of the Age variable is 66, 23, or 17, the preceding SELECT CASE statement does nothing.
Matching exact values may be useful, but sometimes you may want to run the same command (or block of commands) if a variable matches one or more values. For example, rather than match the number 65 exactly, you might want the SELECT CASE statement to match 65, 66, or 67. In that case, you can write the SELECT CASE statement like this:
SELECT CASE Age CASE 65, 66, 67 Status = Retired CASE 21 Status - Working CASE 15 Status = Student END SELECT
With a switch statement in a curly bracket language, like C, you can do the following:
switch (age)
{
case 67:
case 66:
case 65: status = retired;
break;
case 21: status = working;
break;
case 15: status - student;
}By not using the break command if the value of the age variable is 67 or 66, the computer just continues down, line by line, until it runs the command if the age variable was 65. Then it hits the break command directly under the status = retired command and stops.
Tip
The switch command can be easier to read because all the matching values (67, 66, 65, 21, and 15) appear in a vertical column. The equivalent SELECT CASE statement can be slightly harder to read because all the values don't line up in a single vertical column.
The problem with the SELECT CASE statement is that it needs to match a value exactly. Although you could type in all possible values to match, that can get clumsy, as in the following that sets Status = retired if the Age variable is between 65 and 75:
SELECT CASE Age CASE 65, 67, 68, 69, 70, 71, 72, 73, 74, 75 Status = Retired CASE 21 Status - Working CASE 15 Status = Student END SELECT
To avoid this problem, many languages let you check for a range of values. So if you want to check if a variable is equal or greater than 65 and less than or equal to 75, you could define the range of 65 TO 75 like this:
SELECT CASE Age CASE 65 TO 75 Status = Retired CASE 21 Status - Working
CASE 15 Status = Student END SELECT
Warning
The curly bracket languages, like C and C++, don't let you check for a range of values in a switch statement.
Listing a range of values can be useful, but what if there's no upper (or lower) limit? For example, anyone over the age of 65 might be considered retired, so you need to use a comparison operator to check a variable with a value, such as Age >= 65.
To use a comparison operator in a SELECT CASE statement, languages such as BASIC use the following syntax:
SELECT CASE Age CASE IS >= 65 Status = Retired CASE 21 TO 64 Status - Working CASE 15 Status = Student END SELECT
In this example, the first part of the SELECT CASE statement tells the computer to check if the value in the Age variable is (note the IS keyword) >= 65.
The second part of the SELECT CASE statement checks if the Age variable falls within the range of 21 to 64.
The third part of the SELECT CASE statement checks if the Age variable is exactly equal to 15.
As you can see, each part of a SELECT CASE statement can check a value by matching it exactly, checking a range of values, or using a comparison operator.
Warning
The curly bracket languages, like C and C++, don't let you use comparison operators in a switch statement.
It's possible for a SELECT CASE statement to run zero commands if the CASE statement can't match a variable to any specific value, such as
SELECT CASE Age CASE 65 Status = Retired CASE 21 Status - Working CASE 15 Status = Student END SELECT
The preceding SELECT CASE statement doesn't do anything if the Age variable is 13, 25, or 81. To make sure the SELECT CASE statement always runs at least one command, you must add the ELSE statement, such as
SELECT CASE Age CASE 65 Status = Retired CASE 21 Status - Working CASE 15 Status = Student ELSE Status = Bum END SELECT
In this example, if the value of the Age variable is 24 or 5, it doesn't match any of the specific values, so the command under the ELSE statement runs instead (Status = Bum).
Instead of using the ELSE statement, the curly bracket languages use a default statement, such as
switch (age)
{
case 65: status = retired;
break;
case 21: status = working;
break;
case 15: status - student;
break;
default: status = bum;
}Both the ELSE and default statements force the SELECT CASE (or switch) statement to always do something.
As a general rule, use the IF-THEN statements for making the computer choose one or more commands (or blocks of commands). If you need the computer to choose from three or more commands (or blocks of commands), the SELECT CASE (switch) statement may be easier to read and write instead.
Branching simply gives the computer multiple options to use when running. By accepting outside information and comparing its value, a branching statement can help the computer choose an appropriate response out of many possible responses.