Programming in C (Fourth Edition) (2015)

The first statement of Program 3.1 declares the variable integerVar to be an integer variable and also assigns to it an initial value of 100, as if the following two statements had been used instead:

int integerVar;
integerVar = 100;

In the second line of the program’s output, notice that the value of 331.79, which is assigned to floatingVar, is actually displayed as 331.790009. In fact, the actual value displayed is dependent on the particular computer system you are using. The reason for this inaccuracy is the particular way in which numbers are internally represented inside the computer. You have probably come across the same type of inaccuracy when dealing with numbers on your pocket calculator. If you divide 1 by 3 on your calculator, you get the result .33333333, with perhaps some additional 3s tacked on at the end. The string of 3s is the calculator’s approximation to one third. Theoretically, there should be an infinite number of 3s. But the calculator can hold only so many digits, thus the inherent inaccuracy of the machine. The same type of inaccuracy applies here: Certain floating-point values cannot be exactly represented inside the computer’s memory.

When displaying the values of float or double variables, you have the choice of three different formats. The %f characters are used to display values in a standard manner. Unless told otherwise, printf() always displays a float or double value to six decimal places rounded. You see later in this chapter how to select the number of decimal places that are displayed.

The %e characters are used to display the value of a float or double variable in scientific notation. Once again, six decimal places are automatically displayed by the system.

With the %g characters, printf() chooses between %f and %e and also automatically removes from the display any trailing zeroes. If no digits follow the decimal point, it doesn’t display that either.

In the next-to-last printf() statement, the %c characters are used to display the single character 'W' that you assigned to charVar when the variable was declared. Remember that whereas a character string (such as the first argument to printf()) is enclosed within a pair of double quotes, a character constant must always be enclosed within a pair of single quotes.

The last printf() shows that a _Bool variable can have its value displayed using the integer format characters %i.

Type Specifiers: long, long long, short, unsigned, and signed

If the specifier long is placed directly before the int declaration, the declared integer variable is of extended range on some computer systems. An example of a long int declaration might be

long int factorial;

This declares the variable factorial to be a long integer variable. As with floats and doubles, the particular accuracy of a long variable depends on your particular computer system. On many systems, an int and a long int have the same range and either can be used to store integer values up to 32-bits wide (2³¹ − 1, or 2,147,483,647).

A constant value of type long int is formed by optionally appending the letter L (upper- or lowercase) onto the end of an integer constant. No spaces are permitted between the number and the L. So, the declaration

long int numberOfPoints = 131071100L;

declares the variable numberOfPoints to be of type long int with an initial value of 131,071,100.

To display the value of a long int using printf(), the letter l is used as a modifier before the integer format characters i, o, and x. This means that the format characters %li can be used to display the value of a long int in decimal format, the characters %lo can display the value in octal format, and the characters %lx can display the value in hexadecimal format.

There is also a long long integer data type, so

long long int maxAllowedStorage;

declares the indicated variable to be of the specified extended accuracy, which is guaranteed to be at least 64 bits wide. Instead of a single letter l, two ls are used in the printf string to display long long integers, as in "%lli".

The long specifier is also allowed in front of a double declaration, as follows:

long double US_deficit_2004;

A long double constant is written as a floating constant with the letter l or L immediately following, such as

1.234e+7L

To display a long double, the L modifier is used. So, %Lf displays a long double value in floating-point notation, %Le displays the same value in scientific notation, and %Lg tells printf() to choose between %Lf and %Le.

The specifier short, when placed in front of the int declaration, tells the C compiler that the particular variable being declared is used to store fairly small integer values. The motivation for using short variables is primarily one of conserving memory space, which can be an issue in situations in which the program needs a lot of memory and the amount of available memory is limited.

On some machines, a short int takes up half the amount of storage as a regular int variable does. In any case, you are guaranteed that the amount of space allocated for a short int will not be less than 16 bits.

There is no way to explicitly write a constant of type short int in C. To display a short int variable, place the letter h in front of any of the normal integer conversion characters: %hi, %ho, or %hx. Alternatively, you can also use any of the integer conversion characters to displayshort ints, due to the way they can be converted into integers when they are passed as arguments to the printf() routine.

The final specifier that can be placed in front of an int variable is used when an integer variable will be used to store only positive numbers. The declaration

unsigned int counter;

declares to the compiler that the variable counter is used to contain only positive values. By restricting the use of an integer variable to the exclusive storage of positive integers, the accuracy of the integer variable is extended.

An unsigned int constant is formed by placing the letter u (or U) after the constant, as follows:

0x00ffU

You can combine the letters u (or U) and l (or L) when writing an integer constant, so

20000UL

tells the compiler to treat the constant 20000 as an unsigned long.

An integer constant that’s not followed by any of the letters u, U, l, or L and that is too large to fit into a normal-sized int is treated as an unsigned int by the compiler. If it’s too small to fit into an unsigned int, the compiler treats it as a long int. If it still can’t fit inside along int, the compiler makes it an unsigned long int. If it doesn’t fit there, the compiler treats it as a long long int if it fits, and as an unsigned long long int otherwise.

When declaring variables to be of type long long int, long int, short int, or unsigned int, you can omit the keyword int. Therefore, the unsigned variable counter could have been equivalently declared as follows:

unsigned counter;

You can also declare char variables to be unsigned.

The signed qualifier can be used to explicitly tell the compiler that a particular variable is a signed quantity. Its use is primarily in front of the char declaration, and further discussion is deferred until Chapter 13, “More on Data Types.”

Don’t worry if the discussions of these specifiers seem a bit esoteric to you at this point. In later sections of this book, many of these different types are illustrated with actual program examples. Chapter 13 goes into more detail about data types and conversions.

Table 3.1 summarizes the basic data types and qualifiers.

Table 3.1 Basic Data Types

Working with Variables

Early computer programmers had the onerous task of having to write their programs in the binary language of the machine they were programming. This meant that computer instructions had to be hand-coded into binary numbers by the programmer before they could be entered into the machine. Furthermore, the programmer had to explicitly assign and reference any storage locations inside the computer’s memory by a specific number or memory address.

Today’s programming languages allow you to concentrate more on solving the particular problem at hand than worrying about specific machine codes or memory locations. They enable you to assign symbolic names, known as variable names, for storing program computations and results. A variable name can be chosen by you in a meaningful way to reflect the type of value that is to be stored in that variable.

In Chapter 2, “Compiling and Running Your First Program,” you used several variables to store integer values. For example, you used the variable sum in Program 2.4 to store the result of the addition of the two integers 50 and 25.

The C language allows data types other than just integers to be stored in variables as well, provided the proper declaration for the variable is made before it is used in the program. Variables can be used to store floating-point numbers, characters, and even pointers to locations inside the computer’s memory.

The rules for forming variable names are quite simple: They must begin with a letter or underscore ( _ ) and can be followed by any combination of letters (upper- or lowercase), underscores, or the digits 0–9. The following is a list of valid variable names.

sum
pieceFlag
i
J5x7
Number_of_moves
_sysflag

On the other hand, the following variable names are not valid for the stated reasons:

int cannot be used as a variable name because its use has a special meaning to the C compiler. This use is known as a reserved name or reserved word. In general, any name that has special significance to the C compiler cannot be used as a variable name. Appendix A provides a complete list of such reserved names.

You should always remember that upper- and lowercase letters are distinct in C. Therefore, the variable names sum, Sum, and SUM each refer to a different variable.

Your variable names can be as long as you want, although only the first 63 characters might be significant, and in some special cases (as described in Appendix A), only the first 31 characters might be significant. It’s typically not practical to use variable names that are too long—just because of all the extra typing you have to do. For example, although the following line is valid

theAmountOfMoneyWeMadeThisYear = theAmountOfMoneyLeftAttheEndOfTheYear –
theAmountOfMoneyAtTheStartOfTheYear;

this line

moneyMadeThisYear = moneyAtEnd – moneyAtStart;

conveys almost as much information in much less space.

When deciding on the choice of a variable name, keep one recommendation in mind—don’t be lazy. Pick names that reflect the intended use of the variable. The reasons are obvious. Just as with comments, meaningful variable names can dramatically increase the readability of a program and pay off in the debug and documentation phases. In fact, the documentation task is probably greatly reduced because the program is more self-explanatory.

Working with Arithmetic Expressions

In C, just as in virtually all programming languages, the plus sign (+) is used to add two values, the minus sign (−) is used to subtract two values, the asterisk (*) is used to multiply two values, and the slash (/) is used to divide two values. These operators are known as binary arithmetic operators because they operate on two values or terms.

You have seen how a simple operation such as addition can be performed in C. Program 3.2 further illustrates the operations of subtraction, multiplication, and division. The last two operations performed in the program introduce the notion that one operator can have a higher priority, orprecedence, over another operator. In fact, each operator in C has a precedence associated with it. This precedence is used to determine how an expression that has more than one operator is evaluated: The operator with the higher precedence is evaluated first. Expressions containing operators of the same precedence are evaluated either from left to right or from right to left, depending on the operator. This is known as the associative property of an operator. Appendix A provides a complete list of operator precedences and their rules of association.

Program 3.2 Using the Arithmetic Operators