168 Chapter 4 Loops
38. Why is there a statement
cout << endl
in the outer loop but not in the inner loop?
39. How would you change the program so that all powers from x0 to x5 are
displayed?
40. If you make the change in Self Check 39, how many values are displayed?
41. What do the following nested loops display?
for (int i = 0; i < 3; i++)
{
for (int j = 0; j < 4; j++)
{
cout << i + j;
}
cout << endl;
}
42. Write nested loops that make the following pattern of brackets:
[][][][]
[][][][]
[][][][]
Practice It Now you can try these exercises at the end of the chapter: R4.23, P4.21, P4.22.
4.9 Random Numbers and Simulations
A simulation program uses the computer to simulate an activity in the real world (or
an imaginary one). Simulations are commonly used for predicting climate change,
analyzing traffic, picking stocks, and many other applications in science and busi-
ness. In the following sections, you will learn how to imple ment simulations that
model phenomena with a degree of randomness.
4.9.1 Generating Random Numbers
Many events in the real world are difficult to predict with absolute precision, yet we
can sometimes know the average behavior quite well. For example, a store may know
from experience that a customer arrives every five minutes. Of course, that is an aver-
age—customers don’t arrive in five minute intervals. To accurately model customer
traffic, you want to take that random fluctuation into account. Now, how can you
run such a simulation in the computer?
The C++ library has a random number generator, which produces numbers that
appear to be com pletely random. Calling
rand()
yields a random integer between
0 and
RAND_MAX
(which is an implementa tion-dependent constant, typically, but not
always, the largest valid
int
value). Call
rand()
again, and you get a different number.
The
rand
function is declared in the
<cstdlib>
header.
The following program calls the
rand
function ten times.
ch04/random.cpp
1
#include <iostream>
2
#include <cstdlib>
3
4
using namespace std;
SELF CHECK
In a simulation, you
use the computer to
simulate an activity.
You can introduce
randomness by
calling the random
number generator.
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