150 Part III: Giving Your Programs the Ability to Run Amok

The #include <stdlib.h> part is necessary because the program uses the atoi() function.

The if command is followed by parentheses, which contain the compar­ ison that the if keyword tests.

The comparison that if makes tests the value of the variable number with 5. The < symbol between them means “less than.” The test reads “If the value of the variable number is less than 5.” If this is true, the cluster of statements following the if keyword is executed. If the test proves false, the cluster of statements is skipped.

Remember the < — less than — from school? Good!

Notice that the if test isn’t followed by a semicolon! Instead, it’s fol­ lowed by a statement enclosed in curly braces. The statements (there can be more than one) “belong” to the if command and are executed only if the condition is true.

If you see only the line The genie knows all, sees all!, you proba­ bly typed a number greater than 4 (which includes 5 and higher). The reason is that the if statement tests only for values less than 5. If the value is less than 5, That number is less than 5! is displayed. The next section elaborates on how it all works.

No, the computer genie doesn’t know all and see all if you type a number 5 or greater.

Did you notice the extra set of curly braces in the middle of this pro­ gram? That’s part of how the if statement works. Also notice how they’re indented.

The if keyword, up close and impersonal

It’s unlike any other C language word you have seen. The if keyword has a unique format, with plenty of options and room for goofing things up. Yet, it’s a handy and powerful thing that you can put in your programs — something you use a lot.

The if keyword is used to make decisions in your programs. It makes a com­ parison. If the result is true, the rest of the if statement is executed. If the comparison isn’t true, the program skips over the rest of the if statement, as shown in Figure 12-1.

The if statement is a statement “block” that can look like this:

if(comparison)

{

statement; [statement;...]

}

Chapter 12: C the Mighty if Command 151

Program execution

	?	printf("whatever");
		if(chins>3)
False	True	{
		printf("Where is your neck?");
		}
Figure 12-1:		printf("something else");
How if
affects a
program.

if is followed by a set of parentheses in which a comparison is made. The comparison is mathematical in nature, using the symbols shown in Table 12-1. What’s being compared is usually the value of a variable against a constant value, or two variables against each other. (See Table 12-1 for examples.)

If the result of the comparison is true, the statement (or group of statements) between the curly braces is executed. If the result is false, the stuff in the curly braces is conveniently skipped over — ignored like a geeky young lad at his first high school dance and with a zit the size of Houston on his chin.

Yes, the curly braces that follow if can contain more than one statement. And, each of the statements ends with a semicolon. All are enclosed in the curly braces. It’s technically referred to as a code block. It shows you which statements “belong” to if. The whole darn thing is part of the if statement.

Table 12-1	Operators Used in if Comparisons
Comparison	Meaning or Pronunciation	“True” Examples
<	Less than	1	< 5
		8	< 9
==	Equal to	5	== 5
		0	== 0

(continued)

152 Part III: Giving Your Programs the Ability to Run Amok

Table 12-1 (continued)

Comparison	Meaning or Pronunciation	“True” Examples
>	Greater than	8	> 5
		10 > 0
<=	Less than or equal to	4	<= 5
		8	<= 8
>=	Greater than or equal to	9	>= 5
		2	>= 2
!=	Not equal to	1	!= 0
		4	!= 3.99

The GENIE1 program, from the preceding section, uses this if statement:

if(number<5)

{

printf(“That number is less than 5!\n”);

}

The first line is the if keyword and its comparison in parentheses. What’s being compared is the value of the numeric variable number and the constant value 5. The comparison is “less than.” Is number less than 5? If so, the state­ ment in curly braces is executed. If not, the whole deal is skipped over.

Consider these modifications:

if(number==5)

{

printf(“That number is 5!\n”);

}

Now, the comparison is number==5? (Is the number that is entered equal to five?) If it is, the printf() statement displays That number is 5!

These changes compare the value of number with 5 again:

if(number>=5)

{

printf(“That number is more than 4!\n”);

}

Chapter 12: C the Mighty if Command 153

This time, the test is greater than or equal to: Is the number that is entered 5 or more than 5? If the number is greater than or equal to 5, it must be more than 4, and the printf() statement goes on to display that important info on the screen.

The following modification to the GENIE1.C program doesn’t change the if comparison, as in the previous examples. Instead, it shows you that more than one statement can belong to if:

if(number<5)

{

printf(“That number is less than 5!\n”); printf(“By goodness, aren’t I smart?\n”);

}

Everything between the curly braces is executed when the comparison is true. Advanced C programs may have lots of stuff in there; as long as it’s between the curly braces, it’s executed only if the comparison is true. (That’s why it’s indented — so that you know that it all belongs to the if statement.)

The comparison that if makes is usually between a variable and a value. It can be a numeric or single-character variable.

if cannot compare strings. For information on comparing strings, refer to my book C All-in-One Desk Reference For Dummies (Wiley).

Less than and greater than and their ilk should be familiar to you from basic math. If not, you should know that you read the symbols from left to right: The > symbol is greater than because the big side comes first; the < is less than because the lesser side comes first.

The symbols for less than or equal to and greater than or equal to always appear that way: <= and >=. Switching them the other way generates an error.

The symbol for “not” in C is the exclamation point. So, != means “not equal.” What is !TRUE (not-true) is FALSE. “If you think that it’s butter, but it’s !.” No, I do ! want to eat those soggy zucchini chips.

When you’re making a comparison to see whether two things are equal, you use two equal signs. I think of it this way: When you build an if statement to see whether two things are equal, you think in your head “is equal” rather than “equals.” For example:

if(x==5)

Read this statement as “If the value of the x variable is equal to 5, then. . . .” If you think “equals,” you have a tendency to use only one equal sign — which is very wrong.