Home > Articles

Dealing with Data

This chapter is from the book

This chapter is from the book

Floating-Point Numbers

Now that you have seen the complete line of C++ integer types, let's look at the floating-point types, which compose the second major group of fundamental C++ types. These numbers let you represent numbers with fractional parts, such as the gas mileage of an M1 tank (0.56 MPG). They also provide a much greater range in values. If a number is too large to be represented as type long—for example, the number of stars in our galaxy (an estimated 400,000,000,000)—you can use one of the floating-point types.

With floating-point types, you can represent numbers such as 2.5 and 3.14159 and 122442.32—that is, numbers with fractional parts. A computer stores such values in two parts. One part represents a value, and the other part scales that value up or down. Here's an analogy. Consider the two numbers 34.1245 and 34124.5. They're identical except for scale. You can represent the first one as 0.341245 (the base value) and 100 (the scaling factor). You can represent the second as 0.341245 (the same base value) and 100,000 (a bigger scaling factor). The scaling factor serves to move the decimal point, hence the term floating-point. C++ uses a similar method to represent floating-point numbers internally, except it's based on binary numbers, so the scaling is by factors of 2 instead of by factors of 10. Fortunately, you don't have to know much about the internal representation. The main points are that floating-point numbers let you represent fractional, very large, and very small values, and they have internal representations much different from those of integers.

Writing Floating-Point Numbers

C++ has two ways of writing floating-point numbers. The first is to use the standard decimal-point notation you've been using much of your life:

12.34       // floating-point
939001.32   // floating-point
0.00023     // floating-point
8.0         // still floating-point

Even if the fractional part is 0, as in 8.0, the decimal point ensures that the number is represented in floating-point format and not as an integer. (The C++ Standard does allow for implementations to represent different locales—for example, providing a mechanism for using the European method of using a comma instead of a period for the decimal point. However, these choices govern how the numbers can appear in input and output, not in code.)

The second method for representing floating-point values is called E notation, and it looks like this: 3.45E6. This means that the value 3.45 is multiplied by 1,000,000; the E6 means 10 to the 6th power, which is 1 followed by 6 zeros. Thus 3.45E6 means 3,450,000. The 6 is called an exponent, and the 3.45 is termed the mantissa. Here are more examples:

2.52e+8       // can use E or e, + is optional
8.33E-4       // exponent can be negative
7E5           // same as 7.0E+05
-18.32e13     // can have + or - sign in front
7.123e12      // U.S. public debt, early 2004
5.98E24       // mass of earth in kilograms
9.11e-31      // mass of an electron in kilograms

As you might have noticed, E notation is most useful for very large and very small numbers.

E notation guarantees that a number is stored in floating-point format, even if no decimal point is used. Note that you can use either E or e, and the exponent can have a positive or negative sign. (See Figure 3.3.) However, you can't have spaces in the number, so, for example, 7.2 E6 is invalid.

Figure 3.3Figure 3.3 E notation.

To use a negative exponent means to divide by a power of 10 instead of to multiply by a power of 10. So 8.33E-4 means 8.33 / 104, or 0.000833. Similarly, the electron mass 9.11e-31 kg means 0.000000000000000000000000000000911 kg. Take your choice. (Incidentally, note that 911 is the usual emergency telephone number in the United States and that telephone messages are carried by electrons. Coincidence or scientific conspiracy? You be the judge.) Note that –8.33E4 means –83300. A sign in front applies to the number value, and a sign in the exponent applies to the scaling.


The form d.dddE+n means move the decimal point n places to the right, and the form d.dddE-n means move the decimal point n places to the left.

Floating-Point Types

Like ANSI C, C++ has three floating-point types: float, double, and long double. These types are described in terms of the number of significant figures they can represent and the minimum allowable range of exponents. Significant figures are the meaningful digits in a number. For example, writing the height of Mt. Shasta in California as 14,162 feet uses five significant figures, for it specifies the height to the nearest foot. But writing the height of Mt. Shasta as about 14,000 feet tall uses two significant figures, for the result is rounded to the nearest thousand feet; in this case, the remaining three digits are just placeholders. The number of significant figures doesn't depend on the location of the decimal point. For example, you can write the height as 14.162 thousand feet. Again, this uses five significant digits because the value is accurate to the fifth digit.

In effect, the C and C++ requirements for significant digits amount to float being at least 32 bits, double being at least 48 bits and certainly no smaller than float, and long double being at least as big as double. All three can be the same size. Typically, however, float is 32 bits, double is 64 bits, and long double is 80, 96, or 128 bits. Also, the range in exponents for all three types is at least –37 to +37. You can look in the cfloat or float.h header files to find the limits for your system. (cfloat is the C++ version of the C float.h file.) Here, for example, are some annotated entries from the float.h file for Borland C++ Builder:

// the following are the minimum number of significant digits
#define DBL_DIG 15     // double
#define FLT_DIG 6      // float
#define LDBL_DIG 18    // long double

// the following are the number of bits used to represent the mantissa
#define DBL_MANT_DIG   53
#define FLT_MANT_DIG   24
#define LDBL_MANT_DIG  64

// the following are the maximum and minimum exponent values
#define DBL_MAX_10_EXP  +308
#define FLT_MAX_10_EXP  +38
#define LDBL_MAX_10_EXP +4932

#define DBL_MIN_10_EXP  -307
#define FLT_MIN_10_EXP  -37
#define LDBL_MIN_10_EXP -4931

Compatibility Note

Some C++ implementations have not yet added the cfloat header file, and some C++ implementations based on pre-ANSI C compilers don't provide a float.h header file.

Listing 3.8 examines types float and double and how they can differ in the precision to which they represent numbers (that's the significant figure aspect). The program previews an ostream method called setf() from Chapter 17, "Input, Output, and Files." This particular call forces output to stay in fixed-point notation so that you can better see the precision. It prevents the program from switching to E notation for large values and causes the program to display six digits to the right of the decimal. The arguments ios_base::fixed and ios_base::floatfield are constants provided by including iostream.

Listing 3.8 floatnum.cpp

// floatnum.cpp -- floating-point types
#include <iostream>
int main()
  using namespace std; 
  cout.setf(ios_base::fixed, ios_base::floatfield); // fixed-point
  float tub = 10.0 / 3.0;    // good to about 6 places
  double mint = 10.0 / 3.0;  // good to about 15 places
  const float million = 1.0e6;

  cout << "tub = " << tub;
  cout << ", a million tubs = " << million * tub;
  cout << ",\nand ten million tubs = ";
  cout << 10 * million * tub << endl;

  cout << "mint = " << mint << " and a million mints = ";
  cout << million * mint << endl;
  return 0;

Here is the output from the program in Listing 3.8:

tub = 3.333333, a million tubs = 3333333.250000,
and ten million tubs = 33333332.000000
mint = 3.333333 and a million mints = 3333333.333333

Compatibility Note

The C++ Standard has replaced ios::fixed with ios_base::fixed and ios::floatfield with ios_base::floatfield. If your compiler does not accept the ios_base forms, try using ios instead; that is, substitute ios::fixed for ios_base::fixed, etc. By default, older versions of C++, when they display floating-point values, display six digits to the right of the decimal, as in 2345.831541. Standard C++, by default, displays a total of six digits (2345.83), switching to E notation when values reach a million or greater (2.34583E+06). However, the nondefault display modes, such as fixed in the preceding example, display six digits to the right of the decimal in both old and new versions.

The default setting also suppresses trailing zeros, displaying 23.4500 as 23.45. Implementations differ in how they respond to using the setf() statement to override the default settings. Older versions, such as Borland C++ 3.1 for DOS, suppress trailing zeros in this mode as well. Versions conforming to the standard, such as Microsoft Visual C++ 7.0, Metrowerks CodeWarrior 9, Gnu GCC 3.3, and Borland C++ 5.5, display the zeros, as shown in Listing 3.8.

Program Notes

Normally cout drops trailing zeros. For example, it would display 3333333.250000 as 3333333.25. The call to cout.setf() overrides that behavior, at least in new implementations. The main thing to note in Listing 3.8 is how float has less precision than double. Both tub and mint are initialized to 10.0 / 3.0. That should evaluate to 3.33333333333333333...(etc.). Because cout prints six figures to the right of the decimal, you can see that both tub and mint are accurate that far. But after the program multiplies each number by a million, you see that tub diverges from the proper value after the 7th three. tub is good to 7 significant figures. (This system guarantees 6 significant figures for float, but that's the worst-case scenario.) The type double variable, however, shows 13 threes, so it's good to at least 13 significant figures. Because the system guarantees 15, this shouldn't surprise you. Also, note that multiplying a million tubs by 10 doesn't quite result in the correct answer; this again points out the limitations of float precision.

The ostream class to which cout belongs has class member functions that give you precise control over how the output is formatted—field widths, places to the right of the decimal point, decimal form or E form, and so on. Chapter 17 outlines those choices. This book's examples keep it simple and usually just use the << operator. Occasionally, this practice displays more digits than necessary, but that causes only esthetic harm. If you do mind, you can skim Chapter 17 to see how to use the formatting methods. Don't, however, expect to fully follow the explanations at this point.

Real-World Note: Reading Include Files

The include directives found at the top of C++ source files often take on the air of a magical incantation; novice C++ programmers learn, through reading and experience, which header files add particular functionalities, and they include them solely to make their programs work. Don't rely on the include files only as a source of mystic and arcane knowledge; feel free to open them up and read them. They are text files, so you can read them easily. All the files you include in your programs exist on your computer, or in a place where your computer can use them. Find the includes you use and see what they contain. You'll quickly see that the source and header files that you use are an excellent source of knowledge and information—in some cases, the best documentation available. Later, as you progress into more complex inclusions and begin to use other, nonstandard libraries in your applications, this habit will serve you well.

Floating-Point Constants

When you write a floating-point constant in a program, in which floating-point type does the program store it? By default, floating-point constants such as 8.24 and 2.4E8 are type double. If you want a constant to be type float, you use an f or F suffix. For type long double, you use an l or L suffix. (Because the lowercase l looks a lot like the digit 1, the uppercase L is a better choice.) Here are some samples:

1.234f       // a float constant
2.45E20F     // a float constant
2.345324E28  // a double constant
2.2L         // a long double constant

Advantages and Disadvantages of Floating-Point Numbers

Floating-point numbers have two advantages over integers. First, they can represent values between integers. Second, because of the scaling factor, they can represent a much greater range of values. On the other hand, floating-point operations are slower than integer operations, at least on computers without math coprocessors, and you can lose precision. Listing 3.9 illustrates the last point.

Listing 3.9 fltadd.cpp

// fltadd.cpp -- precision problems with float
#include <iostream>
int main()
  using namespace std;
  float a = 2.34E+22f;
  float b = a + 1.0f;

  cout << "a = " << a << endl;
  cout << "b - a = " << b - a << endl;
  return 0; 

Compatibility Note

Some ancient C++ implementations based on pre-ANSI C compilers don't support the f suffix for indicating type float constants. If you find yourself facing this problem, you can replace 2.34E+22f with 2.34E+22 and replace 1.0f with (float) 1.0.

The program in Listing 3.9 takes a number, adds 1, and then subtracts the original number. That should result in a value of 1. Does it? Here is the output from the program in Listing 3.9 for one system:

a = 2.34e+022
b - a = 0

The problem is that 2.34E+22 represents a number with 23 digits to the left of the decimal. By adding 1, you are attempting to add 1 to the 23rd digit in that number. But type float can represent only the first 6 or 7 digits in a number, so trying to change the 23rd digit has no effect on the value .

Classifying Data Types

C++ brings some order to its basic types by classifying them into families. Types signed char, short, int, and long are termed signed integer types. The unsigned versions are termed unsigned integer types. The bool, char, wchar_t, signed integer, and unsigned integer types together are termed integral types or integer types. The float, double, and long double types are termed floating-point types. Integer and floating-point types are collectively termed arithmetic types.

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