The Explanation

Operating systems and compilers often use identifiers with one or two initial underscore characters, so it is better to avoid that usage yourself.

C names are case sensitive, meaning an uppercase letter is considered distinct from the corresponding lowercase letter. Therefore, socks is different from Socks or SOCKS .

Four Good Reasons to Declare Variables

• Putting all the variables in one place makes it easier for a reader to grasp what the program is about. This is particularly true if you give your variables meaningful names (such as taxrate instead of r ). If the name doesn't suffice, use comments to explain what the variables represent. Documenting a program in this manner is one of the basic techniques of good programming.

• Thinking about what to put into the variable declaration section encourages you to do some planning before plunging into writing a program. What information does the program need to get started? What exactly do I want the program to produce as output? What is the best way to represent the data?

• Declaring variables helps prevent one of programming's more subtle and hard-to-find bugs , that of the misspelled variable name. For example, suppose that in some language, which lacks declarations, you made the statement

  RADIUS1 = 20.4;
  

  and that elsewhere in the program you mistyped

  CIRCUM = 6.28 * RADIUSl;
  

  You unwittingly replaced the numeral 1 with the letter l . That other language would create a new variable called RADIUSl and use whatever value it had (perhaps zero, perhaps garbage). CIRCUM would be given the wrong value, and you might have a heck of a time trying to find out why. This can't happen in C (unless you were silly enough to declare two such similar variable names) because the compiler will complain when the undeclared RADIUSl shows up.

• Your C program will not compile if you don't declare your variables. If the preceding reasons fail to move you, you should give this one serious thought.

By the way, if you find that your compiler allows you to declare variables anywhere in a block and not just at the beginning, the compiler may be a C++ mode, for C++ does allow that behavior. (See Appendix G, "Answers to Review Questions." )

Assignment

num = 1;

The assignment statement is one of the basic operations in C. This particular example means "assign the value 1 to the variable num ." The earlier int num; line allotted computer space for the variable num , and the assignment line gives it its value. You can assign num a different value later on, if you want; that is why num is termed a variable. Note that the assignment statement assigns a value from the right side to the left side. Also, the statement is completed with a semicolon, as shown in Figure 2.2.

Figure 2.2. The assignment statement is one of the basic C operations.

The printf() Function

printf ("I am a simple ");
printf ("computer.\n");
printf ("My favorite number is %d because it is first.\n",num);

These lines all use a standard C function called printf() . The parentheses signify that printf is a function name. The material enclosed in the parentheses is information passed from the main() function to the printf() function. For example, the first line passes the phrase I am a simple to the printf() function. Such information is called the argument or parameter of a function (see Figure 2.3). What does the function printf() do with this argument? It looks at whatever lies between the double quotation marks and prints that text on the screen.

Figure 2.3. The function printf() with an argument.

This first printf() line is an example of how you call or invoke a function in C. You need type only the name of the function, placing the desired argument(s) within the parentheses. When the program reaches this line, control is turned over to the named function ( printf() in this case). When the function is finished with whatever it does, control is returned to the original (the calling ) function ” main() , in this example.

What about this next printf() line? It has the characters \n included in the quotes, and they didn't get printed! What's going on? The \n symbol means to start a new line. The \n combination (typed as two characters) represents a single character called the newline character . Its meaning is "start a new line at the far left margin." In other words, printing the newline character performs the same function as pressing the Enter key of a typical keyboard. Why not just use the Enter key when typing the printf() argument? Because that would be interpreted as an immediate command to your editor, not as an instruction to be stored in your source code. In other words, when you press the Enter key, the editor quits the current line on which you are working and starts a new one. The newline character, however, affects how the output of the program is displayed.

The newline character is an example of an escape sequence . An escape sequence is used to represent difficult- or impossible -to-type characters. Other examples are \t for tab and \b for backspace . In each case, the escape sequence begins with the backslash character, \ . We'll return to this subject in Chapter 3, "Data and C."

Well, that explains why the three printf() statements produced only two lines: The first print instruction didn't have a newline character in it, but the second and third did.

The final printf() line brings up another oddity: What happened to the %d when the line was printed? As you will recall, the output for this line was

My favorite number is 1 because it is first.

Aha! The digit 1 was substituted for the symbol group %d when the line was printed, and 1 was the value of the variable num . The %d is a placeholder to show where the value of num is to be printed. This line is similar to the following BASIC statement:

PRINT "My favorite number is "; num; " because it is first."

The C version does a little more than this, actually. The % alerts the program that a variable is to be printed at that location, and the d tells it to print the variable as a decimal (base 10) integer. The printf() function allows several choices for the format of printed variables, including hexadecimal (base 16) integers and numbers with decimal points. Indeed, the f in printf() is a reminder that this is a formatting print function.

Return Statement

return 0;

The int in int main(void) means that the main() function is supposed to return an integer. The ANSI C standard requires that main() behave that way. C functions that return values do so with a return statement, which consists of the keyword return followed by the returned value followed by a semicolon. If you leave out the return statement for main() , most compilers will chide you for the omission, but still compile the program. At this point, you can regard the return statement in main() as something required for logical consistency, but it has a practical use with some operating systems, including DOS and UNIX. I'll return to that topic in Chapter 11, "Character Strings and String Functions."