Home > Articles

This chapter is from the book


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:


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.


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


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."


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>
4:  /* Define a constant to convert from pounds to grams */
5:  #define GRAMS_PER_POUND 454
7:  /* Define a constant for the start of the next century */
8:  const int TARGET_YEAR = 2010;
10:  /* Declare the needed variables */
11:  long weight_in_grams, weight_in_pounds;
12  int year_of_birth, age_in_2010;
14:  int main( void )
15:  {
16:    /* Input data from user */
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);
23:    /* Perform conversions */
25:    weight_in_grams = weight_in_pounds * GRAMS_PER_POUND;
26:    age_in_2010 = TARGET_YEAR - year_of_birth;
28:    /* Display results on the screen */
30:    printf("\nYour weight in grams = %ld", weight_in_grams);
31:    printf("\nIn 2010 you will be %d years old\n", age_in_2010);
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.

InformIT Promotional Mailings & Special Offers

I would like to receive exclusive offers and hear about products from InformIT and its family of brands. I can unsubscribe at any time.


Pearson Education, Inc., 221 River Street, Hoboken, New Jersey 07030, (Pearson) presents this site to provide information about products and services that can be purchased through this site.

This privacy notice provides an overview of our commitment to privacy and describes how we collect, protect, use and share personal information collected through this site. Please note that other Pearson websites and online products and services have their own separate privacy policies.

Collection and Use of Information

To conduct business and deliver products and services, Pearson collects and uses personal information in several ways in connection with this site, including:

Questions and Inquiries

For inquiries and questions, we collect the inquiry or question, together with name, contact details (email address, phone number and mailing address) and any other additional information voluntarily submitted to us through a Contact Us form or an email. We use this information to address the inquiry and respond to the question.

Online Store

For orders and purchases placed through our online store on this site, we collect order details, name, institution name and address (if applicable), email address, phone number, shipping and billing addresses, credit/debit card information, shipping options and any instructions. We use this information to complete transactions, fulfill orders, communicate with individuals placing orders or visiting the online store, and for related purposes.


Pearson may offer opportunities to provide feedback or participate in surveys, including surveys evaluating Pearson products, services or sites. Participation is voluntary. Pearson collects information requested in the survey questions and uses the information to evaluate, support, maintain and improve products, services or sites, develop new products and services, conduct educational research and for other purposes specified in the survey.

Contests and Drawings

Occasionally, we may sponsor a contest or drawing. Participation is optional. Pearson collects name, contact information and other information specified on the entry form for the contest or drawing to conduct the contest or drawing. Pearson may collect additional personal information from the winners of a contest or drawing in order to award the prize and for tax reporting purposes, as required by law.


If you have elected to receive email newsletters or promotional mailings and special offers but want to unsubscribe, simply email information@informit.com.

Service Announcements

On rare occasions it is necessary to send out a strictly service related announcement. For instance, if our service is temporarily suspended for maintenance we might send users an email. Generally, users may not opt-out of these communications, though they can deactivate their account information. However, these communications are not promotional in nature.

Customer Service

We communicate with users on a regular basis to provide requested services and in regard to issues relating to their account we reply via email or phone in accordance with the users' wishes when a user submits their information through our Contact Us form.

Other Collection and Use of Information

Application and System Logs

Pearson automatically collects log data to help ensure the delivery, availability and security of this site. Log data may include technical information about how a user or visitor connected to this site, such as browser type, type of computer/device, operating system, internet service provider and IP address. We use this information for support purposes and to monitor the health of the site, identify problems, improve service, detect unauthorized access and fraudulent activity, prevent and respond to security incidents and appropriately scale computing resources.

Web Analytics

Pearson may use third party web trend analytical services, including Google Analytics, to collect visitor information, such as IP addresses, browser types, referring pages, pages visited and time spent on a particular site. While these analytical services collect and report information on an anonymous basis, they may use cookies to gather web trend information. The information gathered may enable Pearson (but not the third party web trend services) to link information with application and system log data. Pearson uses this information for system administration and to identify problems, improve service, detect unauthorized access and fraudulent activity, prevent and respond to security incidents, appropriately scale computing resources and otherwise support and deliver this site and its services.

Cookies and Related Technologies

This site uses cookies and similar technologies to personalize content, measure traffic patterns, control security, track use and access of information on this site, and provide interest-based messages and advertising. Users can manage and block the use of cookies through their browser. Disabling or blocking certain cookies may limit the functionality of this site.

Do Not Track

This site currently does not respond to Do Not Track signals.


Pearson uses appropriate physical, administrative and technical security measures to protect personal information from unauthorized access, use and disclosure.


This site is not directed to children under the age of 13.


Pearson may send or direct marketing communications to users, provided that

  • Pearson will not use personal information collected or processed as a K-12 school service provider for the purpose of directed or targeted advertising.
  • Such marketing is consistent with applicable law and Pearson's legal obligations.
  • Pearson will not knowingly direct or send marketing communications to an individual who has expressed a preference not to receive marketing.
  • Where required by applicable law, express or implied consent to marketing exists and has not been withdrawn.

Pearson may provide personal information to a third party service provider on a restricted basis to provide marketing solely on behalf of Pearson or an affiliate or customer for whom Pearson is a service provider. Marketing preferences may be changed at any time.

Correcting/Updating Personal Information

If a user's personally identifiable information changes (such as your postal address or email address), we provide a way to correct or update that user's personal data provided to us. This can be done on the Account page. If a user no longer desires our service and desires to delete his or her account, please contact us at customer-service@informit.com and we will process the deletion of a user's account.


Users can always make an informed choice as to whether they should proceed with certain services offered by InformIT. If you choose to remove yourself from our mailing list(s) simply visit the following page and uncheck any communication you no longer want to receive: www.informit.com/u.aspx.

Sale of Personal Information

Pearson does not rent or sell personal information in exchange for any payment of money.

While Pearson does not sell personal information, as defined in Nevada law, Nevada residents may email a request for no sale of their personal information to NevadaDesignatedRequest@pearson.com.

Supplemental Privacy Statement for California Residents

California residents should read our Supplemental privacy statement for California residents in conjunction with this Privacy Notice. The Supplemental privacy statement for California residents explains Pearson's commitment to comply with California law and applies to personal information of California residents collected in connection with this site and the Services.

Sharing and Disclosure

Pearson may disclose personal information, as follows:

  • As required by law.
  • With the consent of the individual (or their parent, if the individual is a minor)
  • In response to a subpoena, court order or legal process, to the extent permitted or required by law
  • To protect the security and safety of individuals, data, assets and systems, consistent with applicable law
  • In connection the sale, joint venture or other transfer of some or all of its company or assets, subject to the provisions of this Privacy Notice
  • To investigate or address actual or suspected fraud or other illegal activities
  • To exercise its legal rights, including enforcement of the Terms of Use for this site or another contract
  • To affiliated Pearson companies and other companies and organizations who perform work for Pearson and are obligated to protect the privacy of personal information consistent with this Privacy Notice
  • To a school, organization, company or government agency, where Pearson collects or processes the personal information in a school setting or on behalf of such organization, company or government agency.


This web site contains links to other sites. Please be aware that we are not responsible for the privacy practices of such other sites. We encourage our users to be aware when they leave our site and to read the privacy statements of each and every web site that collects Personal Information. This privacy statement applies solely to information collected by this web site.

Requests and Contact

Please contact us about this Privacy Notice or if you have any requests or questions relating to the privacy of your personal information.

Changes to this Privacy Notice

We may revise this Privacy Notice through an updated posting. We will identify the effective date of the revision in the posting. Often, updates are made to provide greater clarity or to comply with changes in regulatory requirements. If the updates involve material changes to the collection, protection, use or disclosure of Personal Information, Pearson will provide notice of the change through a conspicuous notice on this site or other appropriate way. Continued use of the site after the effective date of a posted revision evidences acceptance. Please contact us if you have questions or concerns about the Privacy Notice or any objection to any revisions.

Last Update: November 17, 2020