Advanced C 1992

Data Types, Constants,CVariables,Cand Arrays CC C C

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Data Types, Constants,

Variables, and Arrays

The C language offers a number of data types, which can be used for constants, variables, and arrays. This chapter helps you become more familiar with data objects and how to use them.

Data Types

The C language supports a number of data types, all of which are necessary in writing programs. Because most CPUs generally support these data types directly, it is unnecessary for the compiler to convert the data types into the types the CPU understands. In addition to the standard types, new data types are needed, which are often unique to a given application, and C provides the mechanisms to create and use types of data created by the programmer.

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Part I • Honing Your C Skills

The basic data types as they are defined by the ANSI standard are listed in Table 2.1. They are all that are needed when simpler applications are created (and are generally adequate for many of the more complex programs).

Table 2.1. C’s data types.

In addition to these data types, some of them may be used with a modifier that affects the characteristics of the data object. These modifiers are listed in Table 2.2.

Table 2.2. C’s data type modifiers.

Each of the data types (and their modifiers) has a minimum and maximum value (see Table 2.3). Check your compiler documentation because some compilers extend

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these values. Be careful not to assume that a variable created as int is either 16 bits or 32 bits. Different compilers, on different computers, may default the size of an int variable to either size, depending on the CPU’s default integer size. If you must know the size of the variable, be sure you specify either long or short when you create it.

When you are entering constants, determining the value to use can be difficult. For instance, if the following line is in your program, the results probably are not going to be what you expected:

#define INT_MAX 0x8000 /* Really not a good idea! */

In this example, you expect INT_MAX to contain the value (–32768); the compiler promotes the constant to unsigned, however, and the value of INT_MAX, 32,768, is probably not what you expect.

A much easier solution exists. A number of useful identifiers are defined in the limits.h header file in ANSI C (see Table 2.3). Use limits.h so that predefined identifiers can define the limits for the integer data types. The values shown in Tables 2.3 through 2.5 represent the ANSI limits, although many compilers exceed the values shown.

Table 2.3. C’s int limits identifiers, from limits.h.

continues

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Part I • Honing Your C Skills

Table 2.3. continued

Three different-size variables can be defined for floating-point variables (see Table 2.4). The identifiers for floating-point numbers are subdivided into three parts. The first three letters indicate the size of the floating-point object: DBL_ for a double, FLT_ for a float, and LDBL_ for a long double.

Table 2.4. C’s floating-point limits identifiers, from float.h.

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continues

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Part I • Honing Your C Skills

Table 2.4. continued

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Other identifiers generally are defined in float.h; however, they usually are either CPUor compiler-dependent. Refer to your compiler manual for a description of these other identifiers, or print float.h to see whether comments in the file help you understand the purpose of the identifiers.

Rather than code constants for these values into your program, you should use one of the predefined identifiers shown in Tables 2.3 and 2.4. These identifiers allow for better portability and make the meaning of your program clear.

Constants

All homes are buildings, but not all buildings are homes. All literals are constants, but not all constants are literals. Maybe this example is not clear, but with the const modifier applied to a variable, it becomes nonmodifiable—a constant. Let’s look at a few constants. Constants can come in any data type that the C compiler supports. A special constant, the string, can be used to either initialize a character array or be substituted for one. Table 2.5 shows a number of constants.

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Part I • Honing Your C Skills

Table 2.5. Constants in C.

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1.23double—floating- Always a decimal point; both leading

The suffixes shown in Table 2.5 can be in either upperor lowercase. I prefer uppercase because a lowercase l is difficult to distinguish from the number 1. If a number that does not fit in the default size is presented to the compiler, it either is changed to an unsigned type or its size is increased. As an example, when the value 45000 is encountered, the compiler assumes that is an unsigned value; 500000, which is too large for either a signed or unsigned 16-bit value, is promoted to a 32-bit long value.

String constants present several unique situations. First, unlike numeric constants, it’s possible to obtain the address of a string constant. This capability is necessary because string functions use addresses (see Listing 2.1).

Listing 2.1. BADSTR.C.

/* BADSTR, written 12 May 1992 by Peter D. Hipson */ /* An example of changing a string constant. */

#include <stdio.h> // Make includes first part of file #include <string.h>

int main(void); // Declare main() and the fact that this program doesn’t // use any passed parameters.

continues

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Part I • Honing Your C Skills

Listing 2.1. continued

int main()

{

char szMyName[] = “John Q. Public”; char szYourName[50];

szYourName[0] = ‘\0’;

strcpy(szYourName, szMyName); // szYourName is now the same as // szMyName.

printf(“MyName ‘%s’ YourName ‘%s’ \n”, szMyName,

szYourName);

strcpy(szMyName, “My New Name”); // strcpy() actually receives the // address of the constant

// “My New Name”

printf(“MyName ‘%s’ YourName ‘%s’ \n”, szMyName,

szYourName);

printf(“Before: MyName ‘%s’ Constant ‘%s’ \n”, szMyName,

“My New Name”);

strcpy(“My New Name”, // strcpy() actually receives the address szYourName); // of the constant “My New Name”

// This will fail and destroy the constant!

printf(“After: MyName ‘%s’ Constant ‘%s’ \n”, szMyName,

“My New Name”);

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