Chapter 14

Iffy C Logic

In This Chapter

Helping your program make sense

Using logical comparisons in an if statement

Understanding logical OR, ||

Understanding logical AND, &&

There’s more to the if command than you can C, or than you may have seen in earlier chapters in this book. That extra stuff involves good old-

fashioned Mr. Spock-like logic.

Unlike with math, most people don’t go running and screaming from logic. That’s because people don’t have the foggiest idea what logic really is. If they did, no one would ever play the lottery or visit Las Vegas. That’s because it’s illogical to assume that odds of 1 in 80 million mean anything. But I digress.

You can apply two types of logic in a computer program. First is the logic of properly laying out the decision that if makes: whether to use < or >= and other silly things like that. But beyond that is the logic of either-or and andboth and various mixtures of each one. It’s enough to drive you insane! This chapter helps prevent that.

Exposing Flaws in Logic

Sometimes, programmers get so involved in whether their programs run that they forget about using the programs.

Once upon a time, a study was done on the “usability” of a particular program. In the program, users were asked a vital question and had to respond by press­ ing the Tab key. Normally, it’s the Enter key that users expect to press, but with that one program, pressing the Enter key caused something odd to happen, which required three or four steps to undo and returned users to the original question.

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

The odd thing is that the users tended to blame themselves for pressing Enter rather than Tab. They blamed themselves rather than assume that the pro­ gram behaved in an illogical way. Had the programmer been using logic, the program would have used the Enter key rather than Tab. But, as the budding programmer you’re becoming, even you know that such a thing is really easy to change.

Take a moment to review the flow of the BLOWUP1.C program, which I show you how to create in Chapter 13. The program asks a vital question:

Would you like your computer to explode?

I believe that it’s safe to assume in most cases that the user would not want their computer to explode. Yet the program was written so that only the N key press — one key of many possible keys — prevents the computer from blowing up. This is a great example of misapplied logic.

When giving the user a choice and one option is better than another, make it very easy for them to accidentally select the best option. In the case of the BLOWUP1.C program, the code should probably look like this:

printf(“Would you like your computer to explode?”); c=getchar();

if(c==’Y’)

{

printf(“OK: Configuring computer to explode now.\n”); printf(“Bye!\n”);

}

else

{

printf(“Okay. Whew!\n”);

}

The only option in this example that truly blows up the computer is pressing the Y key. Any other key causes the computer not to blow up — the better option.

Of course, the best option is to limit input precisely. To do that, you need to know a few more if command tricks, which are introduced starting in the next section.

You may have noticed this illogic in Chapter 13, when you first ran the BLOWUP1 program. Pressing even the little N key causes the blow-up message to be displayed.

Most Yes or No questions wait for either Y or N to be pressed and ignore any other key (except for maybe Esc).

Chapter 14: Iffy C Logic 177

If, And, Or, But

The BLOWUP1.C program also presents an interesting problem. No, not how to blow up the computer. That’s easy! Instead, I mean what about the little Y key?

In the preceding section, when the program runs, the user can only press the Y key to blow up the computer. But, how can you be certain that a capital Y is pressed rather than a baby Y? Or vice versa: What if only the little Y is being checked for?

A solution (but not the best one)

This section shows modified source code for BLOWUP1.C:

#include <stdio.h>

int main()

{

char c;

printf(“Would you like your computer to explode?”); c=getchar();

if(c==’Y’)

{

printf(“OK: Configuring computer to explode now.\n”); printf(“Bye!\n”);

}

else if(c==’y’)

{

printf(“OK: Configuring computer to explode now.\n”); printf(“Bye!\n”);

}

else

{

printf(“Okay. Whew!\n”);

}

return(0);

}

Edit the original source code for BLOWUP1.C and make the changes shown in this example. Primarily, you’re checking for Y to begin with, and an else if block is added to confirm checking for baby Y. Oh, and the printf() state­ ments are all switched around from the first program example to improve the program’s overall logic.

Save the modified source code to disk as BLOWUP2.C.

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

Compile and run.

Anything you type, other than a Y or a (highlight), is “safe.” But, if you type a Y or a y, the computer blows up (supposedly).

Why check for both upperand lowercase? Logic! You never know whether the user has the Caps Lock key turned on.

By making the program specifically check for the Y/y key press, you make every other key on the keyboard a No selection. That’s wise. Logical, even.

For another improvement, tell users about their options. For example, the following modification to Line 7 adds a common key-choice gizmo to the question:

printf(“Would you like your computer to explode? (Y/N)”);

This line lets users know that they can press Y or N. (This is fine despite the fact the program doesn’t specifically check for N; you don’t need to!)

A better solution, using logic

I want to believe that there’s no right or wrong way to program a computer, but there are more efficient ways. For example, the BLOWUP2.C source code has several statements that are repeated:

{

printf(“OK: Configuring computer to explode now.\n”);

printf(“Bye!\n”);

}

Whenever you see repetition like that in a program, it generally means that you have a better way to do things. In this case, the problem lies with the two if commands: one to look for ‘Y’ and the other to look for ‘y’. A better option is to use the OR logical operator so that the if command reads like this:

If variable a is equal to ‘Y’ or if variable a is equal to

‘y’

which becomes

if(a==’Y’ OR a==’y’)

Then, all you need to know is that, in C, the logical operator for OR is two pipe characters: ||. The equation becomes

if(a==’Y’ || a==’y’)

Chapter 14: Iffy C Logic 179

This line reads as just described: “If variable a is equal to ‘Y’ or if variable a is equal to ‘y’.” If either one is true, the set of statements belonging to the if command is executed.

Here’s your updated source code:

#include <stdio.h>

int main()

{

char c;

printf(“Would you like your computer to explode?”); c=getchar();

if(c==’Y’ || c==’y’)

{

printf(“OK: Configuring computer to explode now.\n”); printf(“Bye!\n”);

}

else

{

printf(“Okay. Whew!\n”);

}

return(0);

}

The major modification is with the if condition, which now compares two operations with a logical OR. That cuts down on the program’s redundancy.

Save the changes to disk as BLOWUP3.C. Compile. Fix any errors. Then run it:

It’s logically cured twice!

The pipe character is also known as the vertical bar. It’s on the same key as the backslash character, usually right above the Enter or Return key, on the main part of the keyboard.

Sometimes the pipe character has a break in the middle, and sometimes it’s a solid line.

Two pipe characters mean OR in the C language, a logical choice between two different conditions.

A condition must be on either side of the ||, logical OR, comparison.

The || means that either condition specified must be true for the if command to be true. Each condition must be its own comparison, using the operators in Table 12-1, over in Chapter 12.

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

The if command’s logical friends

You can use the logical operators && and || to help the if command make multiple decisions in one statement:

The && is the logical AND operator.

The || is the logical OR operator.

Table 14-1 explains both.

Table 14-1	Logical Operators Used in if Comparisons
Operator	Meaning	“True” Examples
||	Or	true || true
		true || false
		false || true
&&	And	true || true

The logical operator is used on two standard if command comparisons. For example:

if(temperature>65 && temperature<75)

{

printf(“My, but it’s nice weather outside\n”);

}

In this example, the if command makes two comparisons: temperature>65 and temperature<75. If both are true, the &&, logical AND condition is also true and the entire statement passes; the printf() function then displays the string. Table 14-2 shows the possibilities of how you can figure it out.

Table 14-2	Figuring Out a Logical AND Operation
Temperature	temperature>65	(and)	temperature<75	Logical AND result
45	45>65		45<75
	FALSE	&&	TRUE	FALSE
72	72>65		72<75

Chapter 14: Iffy C Logic 181

Temperature	temperature>65	(and)	temperature<75	Logical AND result
	TRUE	&&	TRUE	TRUE
90	90>65		90<75
	TRUE	&&	FALSE	FALSE

According to Tables 14-1 and 14-2, both conditions that if evaluates must be true for the logical AND to work. (Not shown, and not possible in the given pro­ gram, is when both conditions are FALSE. In that case, the AND test also fails.)

Here’s a logical OR puzzle, which is used in an if statement to test two conditions:

if(score>100 || cheat_code==’Y’)

{

printf(“You have defeated the level boss!\n”);

}

The logical OR tests two different conditions — making two utterly different comparisons. That’s fine; as long as they’re valid if command comparisons, they’ll work. Table 14-3 plods through the potential results.

Table 14-3	Figuring Out a Logical OR Operation
Score, cheat_code	score>100	(or)	cheat_code==’Y’	Logical OR result
50,Z	50>100		‘Z’==’Y’
	FALSE	||	FALSE	FALSE
200,Z	200>100		‘Z’==’Y’
	TRUE	||	FALSE	TRUE
50,Y	50>100		‘Y’==’Y’
	FALSE	||	TRUE	TRUE
200,Y	200>100		‘Y’==’Y’
	TRUE	||	TRUE	TRUE

Table 14-3 shows how the logical OR judges comparisons. If only one of them is TRUE, the entire statement is proven true. Unlike the logical AND, where both (or all) conditions must be true, it takes only one comparison to make the entire if statement true.

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

Most of the time, you probably compare only two conditions with && or ||. You can, however, compare more than one. It works, though it can look sloppy. All the following examples are legal, though they may take some mental work to figure out:

if(key==’A’ || key==’B’ || key==’C’)

In the preceding line, the if statement is true if the value of the character vari­ able key is either A, B, or C. Any one that’s true works.

In the following example, all three conditions must work out TRUE for the entire statement to be true: The temperature value must be greater than 70 and the value of sun must equal the value of shining and the value of weekend must equal the constant YES. If all three are true, the statement is true.

if(temperature>70 && sun==shining && weekend==YES)

The next one takes a little brain grease:

if(status==1 || user==ROOT && timer<MAX)

Logical statements read from left to right. Each condition is evaluated as you read it: status can be 1, or the user can be ROOT — either one must be true. But then that is evaluated against timer being less than MAX, which must be true for the entire statement to be true. Whew!

Yeah: If you’re brain is fried now, please do take a break.

You can easily remember that the && means “logical AND” because the ampersand character is used to mean “and” in English.

It’s not easy to remember that || means “logical OR.”

Multiple && and || conditions are evaluated from left to right (just like you read). Any single condition that fails the text makes the entire statement false.

There’s a difference between && and & (a single ampersand). The & serves a different purpose in C, so be sure to use && when making logical deci­ sions in an if statement. The same goes for || and |; use || in an if statement.

The | and & are bit-wise operators, performing logical operations on bits. These may or may not be covered in this book’s companion, C All-in-One Desk Reference For Dummies (Wiley).

The ampersand itself is a contraction of the Latin et, which means and. The & character is simply a stylized combination of an e and a t.