C++ Primer Plus

This is just an average, but there will be considerable variations from trial to trial. For example, 1,000 trials of attempting to travel 50 feet in 2-foot steps yielded an average of 636 steps (close to the theoretical value of 625) to travel that far, but the range was from 91 to 3,951. Similarly, 1,000 trials of traveling 50 feet in 1-foot steps averaged 2,557 steps (close to the theoretical value of 2,500), with a range of 345 to 10,882. So if you find yourself walking randomly, be confident and take long steps. You still won’t have any control over the direction you wind up going, but at least you’ll get farther.

Program Notes

First, let’s note how painless it was to use the VECTOR namespace in Listing 11.15. The following using declaration places the name of the Vector class in scope:

using VECTOR::Vector;

Because all the Vector class methods have class scope, importing the class name also makes the Vector methods available, without the need for any further using declarations.

Next, let’s talk about random numbers. The standard ANSI C library, which also comes with C++, includes a rand() function that returns a random integer in the range from 0 to some implementation-dependent value. The random walk program uses the modulus operator to get an angle value in the range 0 to 359. The rand() function works by applying an algorithm to an initial seed value to get a random value. That value is used as the seed for the next function call, and so on. The numbers are really pseudorandom because 10 consecutive calls normally produce the same set of 10 random numbers. (The exact values depend on the implementation.) However, the srand() function lets you override the default seed value and initiate a different sequence of random numbers. This program uses the return value of time(0) to set the seed. The time(0) function returns the current calendar time, often implemented as the number of seconds since some specific date. (More generally, time() takes the address of a type time_t variable and puts the time into that variable and also returns it. Using 0 for the address argument obviates the need for an otherwise unneeded time_t variable.) Thus, the following statement sets a different seed each time you run the program, making the random output appear even more random:

srand(time(0));

The cstdlib header file (formerly stdlib.h) contains the prototypes for srand() and rand(), whereas ctime (formerly time.h) contains the time() prototype. (C++11 provides more extensive random number support with functions supported by the random header file.)

The program uses the result vector to keep track of the walker’s progress. On each cycle of the inner loop, the program sets the step vector to a new direction and adds it to the current result vector. When the magnitude of result exceeds the target distance, the loop terminates.

By setting the vector mode, the program displays the final position in rectangular terms and in polar terms.

Incidentally, the following statement has the effect of placing result in the RECT mode, regardless of the initial modes of result and step:

result = result + step;

Here’s why. First, the addition operator function creates and returns a new vector that holds the sum of the two arguments. The function creates that vector by using the default constructor, which creates vectors in the RECT mode. Thus, the vector being assigned to result is in the RECT mode. By default, assignment assigns each member variable individually, so RECT is assigned to result.mode. If you would prefer some other behavior, such as result retaining its original mode, you can override default assignment by defining an assignment operator for the class. Chapter 12 shows examples of this.

By the way, it’s a simple matter to save successive positions in a file. First, you include , declare an ofstream object, and associate the object with a file:

#include

...

ofstream fout;

fout.open("thewalk.txt");

Then, in the loop that calculates the result, you insert something like this:

fout << result << endl;

This invokes the friend function call operator<<(fout, result), causing the os reference parameter to refer to fout, thus sending output to the file. You could also use fout to write other information to the file, such as the summary information currently displayed by cout.

Automatic Conversions and Type Casts for Classes