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

If you use one equal sign rather than two, you don’t get an error; how­ ever, the program is wrong. The nearby Technical Stuff sidebar attempts to explain why.

If you have programmed in other computer languages, keep in mind that the C language has no 2ewd or fi word. The final curly brace signals to the compiler that the if statement has ended.

Also, no then word is used with if, as in the if-then thing they have in the BASIC or Pascal programming language.

A question of formatting the if statement

The if statement is your first “complex” C language statement. The C lan­ guage has many more, but if is the first and possibly the most popular, though I doubt that a popularity contest for programming language words has ever been held (and, then again, if would be great as Miss Congeniality but definitely come up a little thin in the swimsuit competition).

Though you probably have seen the if statement used only with curly braces, it can also be displayed as a traditional C language statement. For example, consider the following — one of the modifications from the GENIE1 program:

if(number==5)

{

printf(“That number is 5!\n”);

}

In C, it’s perfectly legitimate to write this as a more traditional type of state­ ment. To wit:

if(number==5) printf(“That number is 5!\n”);

This line looks more like a C language statement. It ends in a semicolon. Everything still works the same; if the value of the number variable is equal to 5, the printf() statement is executed. If number doesn’t equal 5, the rest of the statement is skipped.

Although all this is legal and you aren’t shunned in the C programming com­ munity for using it, I recommend using curly braces with your if statements until you feel comfortable reading the C language.

Chapter 12: C the Mighty if Command 155

Clutter not thy head with this comparison nonsense

The comparison in the if statement doesn’t have to use any symbols at all! Strange but true. What the C compiler does is to figure out what you have put between the parentheses. Then it weighs whether it’s true or false.

For a comparison using <, >, ==, or any of the horde in Table 12-1, the compiler figures out whether the comparison is true or false. However, you can stick just about anything — any valid C statement — between the paren­ theses and the compiler determines whether it works out to true or false. For example:

if(input=1)

This if statement doesn’t figure out whether the value of the input variable is equal to 1.

No, you need two equal signs for that. Instead, what happens between these parentheses is that the numeric variable input is given the value 1. It’s the same as

input=1;

The C compiler obeys this instruction, stuffing 1 into the input variable. Then, it sits back and strokes its beard and thinks, “Does that work out to be true or false?” Not knowing any better, it figures that the statement must be true. It tells the if keyword, and the cluster of statements that belong to the if statement are then executed.

The final solution to the income-tax problem

I have devised what I think is the fairest and most obviously well-intentioned way to decide who must pay the most in income taxes. You should pay more taxes if you’re taller and more taxes if it’s warmer outside. Yessir, it would be hard to dodge this one.

This problem is ideal for the if keyword to solve. You pay taxes based on either your height or the temperature outside, multiplied by your favorite number and then 10. Whichever number is higher is the amount of tax you pay. To figure out which number is higher, the program TAXES.C uses the if keyword with the greater-than symbol. It’s done twice — once for the height value and again for the temperature outside:

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

#include <stdio.h> #include <stdlib.h>

int main()

{

int tax1,tax2;

char height[4],temp[4],favnum[5];

printf(“Enter your height in inches:”); gets(height);

printf(“What temperature is it outside?”); gets(temp);

printf(“Enter your favorite number:”); gets(favnum);

tax1 = atoi(height) * atoi(favnum); tax2 = atoi(temp) * atoi(favnum);

if(tax1>tax2)

{

printf(“You owe $%d in taxes.\n”,tax1*10);

}

if(tax2>=tax1)

{

printf(“You owe $%d in taxes.\n”,tax2*10);

}

return(0);

}

This program is one of the longer ones in this book. Be extra careful when you’re typing it. It has nothing new in it, but it covers almost all the informa­ tion I present in the first several chapters. Double-check each line as you type it into your editor.

Save the file to disk as TAXES.C.

Compile TAXES.C. Fix any errors you see.

Run the program:

Enter your height in inches:

Type your height in inches. Five feet is 60 inches; six feet is 72 inches. The average person is 5'7" tall or so — 67 inches. Press Enter.

What temperature is it outside?

Right now, in the bosom of winter in the Pacific Northwest, it’s 18 degrees. That’s Fahrenheit, by the way. Don’t you dare enter the smaller Celsius number. If you do, the IRS will hunt you down like a delinquent country music star and make you pay, pay, pay.

Chapter 12: C the Mighty if Command 157

Enter your favorite number:

Type your favorite number. Mine is 11. Press Enter.

If I type 72 (my height), 18, and 11, for example, I see the following result, due April 15:

You owe $7920 in taxes.

Sheesh! And I thought the old system was bad. I guess I need a smaller favorite number.

The second if comparison is “greater than or equal to.” This catches the case when your height is equal to the temperature. If both values are equal, the values of both the tax1 and tax2 variables are equal. The first if comparison, “tax1 is greater than tax2,” fails because both are equal. The second comparison, “tax1 is greater than or equal to tax2,” passes when tax1 is greater than tax2 or when both values are equal.

If you enter zero as your favorite number, the program doesn’t say that you owe any tax. Unfortunately, the IRS does not allow you to have zero — or any negative numbers — as your favorite number. Sad, but true.

If It Isn’t True, What Else?

Hold on to that tax problem!

No, not the one the government created. Instead, hold on to the TAXES.C source code introduced in the preceding section. If it’s already in your text editor, great. Otherwise, open it in your editor for editing.

The last part of the TAXES.C program consists of two if statements. The second if statement, which should be near Line 23 in your editor, really isn’t necessary. Rather than use if in that manner, you can take advantage of another word in the C language, else.

Change Line 23 in the TAXES.C program. It looks like this now:

if(tax2>=tax1)

Edit that line: Delete the if keyword and the comparison in parentheses and replace it with this:

else

That’s it — just else by itself. No comparison and no semicolon, and make sure that you type it in lowercase.