MORE EXAMPLE PROGRAMS
Example program ------> e_c33_p1.ada
Occasionally when writing a computer program, you will have a need to use a random number generator. Very few compilers have a random number generator as a part of the system, so it is necessary for you to either write one yourself or find one that has been written and debugged by someone else. The example package named e_c33_p1.ada is presented to you for several reasons. The first reason is to provide you with a useful example of a complete generic package to serve as an illustration of how to write one. Secondly, it is a useful package that you can use as a part of your own programs when you have a need to use a random number generator. Finally, it is an illustration of good formatting style and it illustrates the inclusion of enough comments in the package specification to completely define the method used, and how to utilize this package as part of another program.
When Ada 83 was upgraded to Ada 95, a random number generator was defined as part of the standard library making it a required component of every Ada 95 compilation system. The random number generator in e_c33_p1.ada was not removed from the tutorial because it is a good example of a well commented Ada utility package. No attempt has been made to prove that it generates truly random numbers, so no claim can be made for it. It would be of benefit to you to study it, then use the random number generator supplied with your compiler for any production work you do,
No other comments need to be given about the operation of this package, so you will be left on your own to study the listing then compile this package in preparation for use with the next example program.
TESTING THE RANDOM NUMBERS
Example program ------> e_c33_p2.ada
Examine the program named e_c33_p2.ada which was written solely to test the random number generator in the package named Random. It instantiates a copy of the generic package using the type FLOAT in line 7, then declares a few objects and an array type. In the executable part of the program the random number generator is initialized with the Set_Seed procedure in line 21, and 12 random numbers are read and printed to see that they do cover the range of 0.0 to 1.0 as defined in the header of the package named Random.
The real test of the random number generator is in the loop beginning in line 35 where ten thousand random numbers are generated and converted into integer type values by multiplying by 100. The integer values will therefore cover a range of 1 to 100, and they are counted in the array named Events. The count in each element of the array should be about 100 since there are ten thousand cases distributed over 100 elements. Execution of the program will reveal that the count in each array element is about 100 as expected, so we declare the random number generator to be at least reasonably random. A mathematician may decide that this method is too crude to be called a good random number generator, but for our purposes it is good enough.
Compile and execute this program, and you will find that each time you run it, you should get different results because it uses the system clock to set the seed, resulting in a new starting seed for each execution.
A NEW DYNAMIC STRING PACKAGE
Example program ------> e_c33_p3.ada
Examine the program named e_c16_p3.ada, which is included in part 2 of this tutorial for a better dynamic string package. You will recall that when we studied the dynamic string package in chapter 16 of part 1 of this tutorial, we found a problem when using string constants in the Copy procedure calls. This was because the system found ambiguous procedures. It could not tell if the string constant was of type STRING or of our own declared type which we named DYNAMIC_STRING. Since we had not studied the discriminated record at that time, we could not properly fix the problem. The new DynStrng package, using a discriminated record, is offered as a better package for the problem of using dynamic strings.
The DYNAMIC_STRING type is declared in lines 33 through 37 and is declared as a record this time so there is no confusion as to whether it is a string or a record, and the overloading ambiguity problem is gone. The package specification is essentially unchanged from the last dynamic string package, except for the type of course, but the body is changed considerably to reflect the new data structure. You will be left on your own to compare the bodies of these two packages if you so desire.
THE STRING CONSTANT PROBLEM IS FIXED
Example program ------> e_c33_p4.ada
The program named e_c33_p4.ada is designed to test the new package with a few string constants to prove that it really does work as advertised. You can compile and execute this file to see that it really does work with string constants in a Copy procedure call.
Example program ------> e_c16_p4.ada
You can return to the program named e_c16_p4.ada from chapter 16 to prove that the new package still works with this old program. You will find that a couple of changes must be made to reflect the different data type. Lines 11 and 12 must be modified to reflect only the upper limit on the static length of the dynamic string variables. They will read as follows;
Name : DYNAMIC_STRING(15); Stuff : DYNAMIC_STRING(35);In addition, because the type is changed, lines 21 and 22 must also be modified as follows;
Name.Dynamic_Length := 3; Stuff.Dynamic_Length := 7;After making these two changes, this program should execute exactly as it did when it used the old dynamic string package.
HOW OLD ARE YOU IN DAYS
Example program ------> e_c33_p5.ada
This program is a repeat of the program given in chapter 16, but it is improved somewhat here. Since we now know how to use the Calendar package, we can use it to get today's date for us, and we do this in the new program named e_c33_p5.ada. Notice especially the way the data is read in and checked for validity before continuing on. If the data were read into the corresponding variables, an invalid entry would cause an exception, but since the data is read into an INTEGER type variable with a wide range, it can be checked for validity before being assigned to the correct variable with a much smaller range. The program should be very simple for you to understand, but it would be good for you to spend a little time studying it before compiling and executing it.
THE DINING PHILOSOPHERS
Example program ------> e_c33_p6.ada
Most books and articles on tasking or concurrency at least mention the problem of the dining philosophers, so it would not be good to leave this tutorial without a little discussion of this problem. In fact, the program named e_c33_p6.ada is a program you can study and execute to see this problem illustrated.
The problem is stated that five philosophers sit down to eat. They like to eat for awhile then think for awhile and repeat the pattern forever. In order to eat, they require a fork in both their left and their right hands, and the table is set with a fork on each side of their plates. The problem occurs when we state that there is only one fork between each adjacent philosopher, and he is therefore required to share each fork with his adjacent colleague.
Each philosopher sits down, waits a random length of time, then picks up the fork on his left, waits another random length of time, and picks up the fork on his right. He then proceeds to eat for a random length of time, then returns both forks to the table and thinks for a random length of time. Once he picks up the left fork, he stubbornly hangs on to it until he gets the right fork. If we ever reach the condition where each philosopher has his left fork, then none will ever return it and none can ever therefore pick up his right fork. The entire system is said to be deadlocked because nothing else will ever be accomplished. All five of the uncooperative philosophers will eventually starve since none can eat.
THE PROGRAM ILLUSTRATES DEADLOCK
The program uses a package to define a task type for one philosopher with the required delays and the logic to acquire each fork, eat, then return the forks to the table. Following our study of tasking, you should have no problem understanding the logic presented here.
The main program named Philos, which starts in line 93 of the file simply declares the five philosophers, starts them through their cycles in lines 108 through 112, then loops while it is watching for deadlock. When deadlock is detected, a message is output to the monitor, and the entire system is aborted.
Compile and execute this program, so you can observe deadlock occurring and the system aborting operation. If you run it several times, you will see that quite often deadlock occurs immediately, but at other times it will run for several seconds before deadlock is detected.
This is an interesting problem, but the more interesting point is the fact that this program, which begins in line 93 uses Ada.Text_IO and One_Man, and One_Man in turn uses Ada.Text_IO, Ada.Calendar, and Random. This program uses quite a bit of the resources available in Ada, and uses several packages to accomplish its intended mission. Since we are using Random, which was developed for an earlier project, we are actually illustrating the reusability of Ada software.
THE GENERIC STACK
Example program ------> e_c33_p7.ada
We promised we would include a generic stack when we studied the character stack in chapter 16, and e_c33_p7.ada is a generic stack to fulfill that promise. It is really only a copy of e_c16_p1.ada from chapter 16 made into a generic package according to the rules studied in this tutorial.
Example program ------> e_c33_p8.ada
The program e_c16_p2.ada is nearly identical to the program of the same name from chapter 16, the only difference being the instantiation of the generic package so it can be used in the main program.
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