WHAT IS THE FUNCTION OF THE INT ARGUMENT TO THE VOID FUNCTION EXIT

4. What is the function of the int argument to the void function exit?

■ A RANDOM NUMBER GENERATOR

A random number generator is a function that returns a “randomly chosen” number. It

is unlike the functions we have seen so far in that the value returned is not determined

by the arguments (of which there are usually none) but rather by some global condi-

tions. Since you can think of the value returned as being a random number, you can

use a random number generator to simulate random events, such as the result of throw-

ing dice or flipping a coin. In addition to simulating games of chance, random number

generators can be used to simulate things that strictly speaking may not be random but

that appear to us to be random, such as the amount of time between the arrival of cars

at a toll booth.

100 Function Basics

The C++ library with header file

<cstdlib>

contains a random number function

named

rand

. This function has no arguments. When your program invokes

rand

, the

rand

function returns an integer in the range

0

to

RAND_MAX

, inclusive. (The number gener-

ated might be equal to

0

or

RAND_MAX

.)

RAND_MAX

is a defined integer constant whose

RAND_MAX

definition is also in the library with header file

<cstdlib>

. The exact value of

RAND_MAX

is system-dependent but will always be at least

32767

(the maximum two-byte positive

integer). For example, the following outputs a list of ten “random” numbers in the

range

0

to

RAND_MAX

:

int i;for (i = 0; i < 10; i++) cout << rand( ) << endl;

You are more likely to want a random number in some smaller range, such as the

range 0 to 10. To ensure that the value is in the range 0 to 10 (including the end

points), you can use

rand( ) % 11

This is called scaling. The following outputs ten “random” integers in the range 0 to

scaling

10 (inclusive):

cout << (rand( ) % 11) << endl;

Random number generators, such as the function

^rand

, do not generate truly ran-

dom numbers. (That’s the reason for all the quotes around “random.”) A sequence of

calls to the function

^rand

(or almost any random number generator) will produce a

sequence of numbers (the values returned by

^rand

) that appear to be random. How-

ever, if you could return the computer to the state it was in when the sequence of calls

to

^rand

began, you would get the same sequence of “random numbers.” Numbers that

pseudorandom number

appear to be random but really are not, such as a sequence of numbers generated by

calls to

^rand

, are called pseudorandom numbers.

A sequence of pseudorandom numbers is usually determined by one number known

as the seed. If you start the random number generator with the same seed, over and

seed

over, then each time it will produce the same (random-looking) sequence of numbers.

You can use the function

srand

to set the seed for the function

rand

. The

void

function

srandsrand

takes one (positive) integer argument, which is the seed. For example, the follow-

ing will output two identical sequences of ten pseudorandom numbers:

srand(99);

Predefined Functions 101

There is nothing special about the number

99

, other than the fact that we used the

same number for both calls to

srand

.

Note that the sequence of pseudorandom numbers produced for a given seed might

be system-dependent. If rerun on a different system with the same seed, the sequence

of pseudorandom numbers might be different on that system. However, as long as you

are on the same system using the same implementation of C++, the same seed will pro-

duce the same sequence of pseudorandom numbers.

P

SEUDORANDOM

N

^UMBERSThe function rand takes no arguments and returns a pseudorandom integer in the range 0 to RAND_MAX (inclusive). The void function srand takes one argument, which is the seed for the random number generator rand. The argument to srand is of type unsignedint, so the argu-ment must be nonnegative. The functions rand and srand, as well as the defined constant RAND_MAX, are defined in the library cstdlib, so programs that use them must contain the following directives:#include <cstdlib>using namespace std;

These pseudorandom numbers are close enough to true random numbers for most

applications. In fact, they are often preferable to true random numbers. A pseudoran-

dom number generator has one big advantage over a true random number generator:

The sequence of numbers it produces is repeatable. If run twice with the same seed

value, it will produce the same sequence of numbers. This can be very handy for a

number of purposes. When an error is discovered and fixed, the program can be rerun

with the same sequence of pseudorandom numbers as those that exposed the error.

Similarly, a particularly interesting run of the program can be repeated, provided a

pseudorandom number generator is used. With a true random number generator every

run of the program is likely to be different.

Display 3.4 shows a program that uses the random number generator

^rand

to “pre-

dict” the weather. In this case the prediction is random, but some people think that is

about as good as weather prediction gets. (Weather prediction can actually be very

accurate, but this program is just a game to illustrate pseudorandom numbers.)

Note that in Display 3.4, the seed value used for the argument of

^srand

is the

month times the day. That way if the program is rerun and the same date is entered, the

same prediction will be made. (Of course, this program is still pretty simple. The pre-

diction for the day after the 14th may or may not be the same as the 15th, but this pro-

gram will do as a simple example.)

floating-point

Probabilities are usually expressed as a floating-point number in the range 0.0 to

random