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This chapter is from the book

Constants

Like a variable, a constant is a data storage location used by your program. Unlike a variable, the value stored in a constant can't be changed during program execution. C has two types of constants, each with its own specific uses:

  • Literal Constants

  • Symbolic Constants

Literal Constants

A literal constant is a value that is typed directly into the source code wherever it is needed. Here are two examples:

int count = 20;
float tax_rate = 0.28;

The 20 and the 0.28 are literal constants. The preceding statements store these values in the variables count and tax_rate. Note that one of these constants contains a decimal point, whereas the other does not. The presence or absence of the decimal point distinguishes floating-point constants from integer constants.

A literal constant written with a decimal point is a floating-point constant and is represented by the C compiler as a double-precision number. Floating-point constants can be written in standard decimal notation, as shown in these examples:

123.456
0.019
100.

Note that the third constant, 100., is written with a decimal point even though it's an integer (that is, it has no fractional part). The decimal point causes the C compiler to treat the constant as a double-precision value. Without the decimal point, it is treated as an integer constant.

Floating-point constants also can be written in scientific notation. You might recall from high school math that scientific notation represents a number as a decimal part multiplied by 10 to a positive or negative power. Scientific notation is particularly useful for representing extremely large and extremely small values. In C, scientific notation is written as a decimal number followed immediately by an E or e and the exponent:

1.23E2 1.23 times 10 to the 2nd power, or 123
4.08e6 4.08 times 10 to the 6th power, or 4,080,000
0.85e–4 0.85 times 10 to the –4th power, or 0.000085

A constant written without a decimal point is represented by the compiler as an integer number. Integer constants can be written in three different notations:

  • A constant starting with any digit other than 0 is interpreted as a decimal integer (that is, the standard base-10 number system). Decimal constants can contain the digits 0 through 9 and a leading minus or plus sign. (Without a leading minus or plus, a constant is assumed to be positive.)

  • A constant starting with the digit 0 is interpreted as an octal integer (the base-8 number system). Octal constants can contain the digits 0 through 7 and a leading minus or plus sign.

  • A constant starting with 0x or 0X is interpreted as a hexadecimal constant (the base-16 number system). Hexadecimal constants can contain the digits 0 through 9, the letters A through F, and a leading minus or plus sign.

NOTE

See Appendix C, "Working with Binary and Hexadecimal Numbers," for a more complete explanation of decimal and hexadecimal notation.

Symbolic Constants

A symbolic constant is a constant that is represented by a name (symbol) in your program. Like a literal constant, a symbolic constant can't change. Whenever you need the constant's value in your program, you use its name as you would use a variable name. The actual value of the symbolic constant needs to be entered only once, when it is first defined.

Symbolic constants have two significant advantages over literal constants, as the following example shows. Suppose that you're writing a program that performs a variety of geometrical calculations. The program frequently needs the value ? (3.14) for its calculations. (You might recall from geometry class that ? is the ratio of a circle's circumference to its diameter.) For example, to calculate the circumference and area of a circle with a known radius, you could write

circumference = 3.14 * (2 * radius);
area = 3.14 * (radius)*(radius);

The asterisk (*) is C's multiplication operator and is covered on Day 4. Thus, the first of these statements means "Multiply 2 times the value stored in the variable radius, and then multiply the result by 3.14. Finally, assign the result to the variable named circumference."

If, however, you define a symbolic constant with the name PI and the value 3.14, you could write

circumference = PI * (2 * radius);
area = PI * (radius)*(radius);

The resulting code is clearer. Rather than puzzling over what the value 3.14 is for, you can see immediately that the constant PI is being used.

The second advantage of symbolic constants becomes apparent when you need to change a constant. Continuing with the preceding example, you might decide that for greater accuracy your program needs to use a value of PI with more decimal places: 3.14159 rather than 3.14. If you had used literal constants for PI, you would have to go through your source code and change each occurrence of the value from 3.14 to 3.14159. With a symbolic constant, you need to make a change only in the place where the constant is defined. The rest of your code would not need to be changed.

Defining Symbolic Constants

C has two methods for defining a symbolic constant: the #define directive and the const keyword. The #define directive is used as follows:

#define CONSTNAME literal

This creates a constant named CONSTNAME with the value of literal. literal represents a literal constant, as described earlier. CONSTNAME follows the same rules described earlier for variable names. By convention, the names of symbolic constants are uppercase. This makes them easy to distinguish from variable names, which by convention are lowercase. For the previous example, the required #define directive for a constant called PI would be

#define PI 3.14159

Note that #define lines don't end with a semicolon (;). #defines can be placed anywhere in your source code, but the defined constant is in effect only for the portions of the source code that follow the #define directive. Most commonly, programmers group all #defines together, near the beginning of the file and before the start of the main() function.

How a #define Works

The precise action of the #define directive is to instruct the compiler as follows: "In the source code, replace CONSTNAME with literal." The effect is exactly the same as if you had used your editor to go through the source code and make the changes manually. Note that #define doesn't replace instances of its target that occur as parts of longer names, within double quotes, or as part of a program comment. For example, in the following code, the instances of PI in the second and third lines would not get changed:

#define PI 3.14159
/* You have defined a constant for PI. */
#define PIPETTE 100

NOTE

The #define directive is one of C's preprocessor directives, and it is discussed more fully on Day 21, "Advanced Compiler Use."

Defining Constants with the const Keyword

The second way to define a symbolic constant is with the const keyword. const is a modifier that can be applied to any variable declaration. A variable declared to be const can't be modified when the program is executed. A value is initialized at the time of declaration and is then prohibited from being changed. Here are some examples:

const int count = 100;
const float pi = 3.14159;
const long debt = 12000000, float tax_rate = 0.21;

const affects all variables on the declaration line. In the last line, debt and tax_rate are symbolic constants. As a side note, you should notice that in this example, debt was declared as a long and tax_rate was declared as a float.

If your program tries to modify a const variable, the compiler generates an error message. The following code would generate an error:

const int count = 100;
count = 200;    /* Does not compile! Cannot reassign or alter */
          /* the value of a constant. */

What are the practical differences between symbolic constants created with the #define directive and those created with the const keyword? The differences have to do with pointers and variable scope. Pointers and variable scope are two very important aspects of C programming, and you will learn about them on Day 9, "Understanding Pointers," and Day 12.

Now take a look at a program that demonstrates variable declarations and the use of literal and symbolic constants. Listing 3.2 prompts the you to enter your weight and year of birth. It then calculates and displays the your weight in grams and your age in the year 2010. You can enter, compile, and run this program using the procedures explained on Day 1, "Getting Started with C."

NOTE

Most C programmers today use const instead of #define when declaring constants.

Listing 3.2 const.c—A program that demonstrates the use of variables and constants

1:  /* Demonstrates variables and constants */
2:  #include <stdio.h>
3:
4:  /* Define a constant to convert from pounds to grams */
5:  #define GRAMS_PER_POUND 454
6:
7:  /* Define a constant for the start of the next century */
8:  const int TARGET_YEAR = 2010;
9:
10:  /* Declare the needed variables */
11:  long weight_in_grams, weight_in_pounds;
12  int year_of_birth, age_in_2010;
13:
14:  int main( void )
15:  {
16:    /* Input data from user */
17:
18:    printf("Enter your weight in pounds: ");
19:    scanf("%d", &weight_in_pounds);
20:    printf("Enter your year of birth: ");
21:    scanf("%d", &year_of_birth);
22:
23:    /* Perform conversions */
24:
25:    weight_in_grams = weight_in_pounds * GRAMS_PER_POUND;
26:    age_in_2010 = TARGET_YEAR - year_of_birth;
27:
28:    /* Display results on the screen */
29:
30:    printf("\nYour weight in grams = %ld", weight_in_grams);
31:    printf("\nIn 2010 you will be %d years old\n", age_in_2010);
32:
33:    return 0;
34:  }
Enter your weight in pounds: 175
Enter your year of birth: 1965

Your weight in grams = 79450
In 2010 you will be 45 years old

This program declares the two types of symbolic constants in lines 5 and 8. In line 5, a constant is used to make the value 454 more understandable. Because it uses GRAMS_PER_POUND, line 25 is easy to understand. Lines 11 and 12 declare the variables used in the program. Notice the use of descriptive names such as weight_in_grams. You can tell what this variable is used for. Lines 18 and 20 print prompts on-screen. The printf() function is covered in greater detail later. To allow the user to respond to the prompts, lines 19 and 21 use another library function, scanf(), which is covered later. scanf() gets information from the screen. For now, accept that this works as shown in the listing. Later, you will learn exactly how it works. Lines 25 and 26 calculate the user's weight in grams and his or her age in the year 2010. These statements and others are covered in detail in tomorrow's chapter. To finish the program, lines 30 and 31 display the results for the user.

DO use constants to make your programs easier to read.

DON'T try to assign a value to a constant after it has already been initialized.

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