Resources for Project Evolution

You will need the following files to produce graphical output out of Chapter 17's Project Evolution as explained in the book.

	world-to-eps.h    world-to-eps.c

Notes and errata

• On page 168, Listing 17.2, line 10, we have

  } animal;
  

  Change that to

  };
  

• On page 177, Listing 17.12, lines 31 and 32, "unsigned int n" should be "unsigned long int n" to be consistent with the declarations of n and f in main() on page 178, Listing 17.13, lines 3 and 4, and also for agreement with Listing 17.17 on page 189.
• In Remark 17.2 on page 177 we have:

  All you need are the simplified versions of lines 5, 6, 26, 38, and 43.

  You need line 8 as well.
• In subsection 17.11.5 on page 182 we have:

  The function dead_or_alive() … returns true or false … depending on whether the animal is alive or dead, respectively.

  The last part of that sentence should be

  … whether the animal is dead or alive, respectively.

  In retrospect, since the function dead_or_alive() returns a Boolean, it would have made better sense to name it as a question, such as “is_dead()” or “r_u_dead()”. You may want to replace dead_or_alive() with one such alternative name in your code for clarity.
• If you try out Experiment #1 of Section 17.12, you will find out that our animal starves to death after around 200 timesteps. To help the animal survive in this and the subsequent experiments, increase its initial energy from 200 to 250, and change its starting point to the center of the world. Thus, in Listing 17.1 of page 166, change line 8 to

  ( 50  50)  0  250  |  5     5     5     5     5     5     5     5  |
  

  Actually, the specific outcome depends on your system's pseudorandom number generator and its seed. Experiment a bit if it does not work for you for the first time.

An alternative path to Project Evolution

The book's Project Evolution depends on Chapter 7's Project Fetchline. It is possible to bypass that prerequisite if you are willing to sacrifice some of the program's flexibility. Specifically, the book's version of evolution reads its input data from a World Description File (WDF) which is a plain text file. To change the input data, we edit the WDF; the program itself is not modified. The Fetchline module is needed to read the WDF.

The alternative which I outline below has no WDF file, therefore it has no need for the Fetchline module because the simulation's data is built into the C program. This reduces the program's complexity. The price we pay is that whenever we change the data we have to recompile the program.

To produce this alternative version, we remove the files read.[ch] and fetch-line.[ch] from the program's directory. The new directory looks like this:

$ cd evolution
$ ls -F
Makefile     evolution.h     linked-list.h@  world-to-eps.c  write.h
array.h@     interlude.c     world-spec.c    world-to-eps.h  xmalloc.c@
evolution.c  linked-list.c@  world-spec.h    write.c         xmalloc.h@

The files world-spec.[ch] are new. This is where the user enters the simulation's data, and compiles it along with the rest of the files. I will describe their contents here.

The file world-spec.c

Here is an outline of the file world-spec.c:

#include "array.h"
#include "linked-list.h"
#include "world-spec.h"

void free_herd(conscell *herd)
{
    ...
}

void free_world(struct world *world)
{
	...
}

struct world *make_world(void)
{
        static struct world World = {    // C99-style initialization!
                .world_h        = 100,
                .world_w        = 100,
                .eden_h         = 10,
                .eden_w         = 10,
                .plant_energy   = 80,
                .reproduction_threshold = 300,
                .plants         = NULL,
                .herd           = NULL,
        };
        struct world *world = &World;
        struct animal *animal;

        // Allocate a matrix to hold the plants and zeros all its entries.
        // Thus, we begin with no plants at all.
        make_matrix(world->plants, world->world_h, world->world_w);
        for (int i = 0; i < world->world_h; i++)
                for (int j = 0; j < world->world_w; j++)
                        world->plants[i][j] = 0;

        animal = xmalloc(sizeof *animal);
        animal->i = 50;
        animal->j = 50;
        animal->d = 0;
        animal->e = 250;
        for (int i = 0; i < 8; i++)
                animal->genes[i] = 5;
        world->herd = ll_push(world->herd, animal);

        /* repeat the stanza above any number of times
           if you wish to add more animals */

        return world;
}

The functions free_herd() is described on the book's page 182. The book's version places it in the file evolution.c. In this alternative version we move it to world-spec.c because that is where it more naturally belongs.

The function free_world() is new. It is called at the end of the program, just before it exits, to free the allocated memory resources, which are

1. the herd;
2. the matrix of plants.

You should be able to supply the details.

The function make_world() is shown in its entirety. I defines a structure to hold the world specification, with the plants and herd members set to NULL. Subsequently it allocates a matrix of plants according to the world's dimensions and zeros all its entries, indicating that the world starts off with no food supply.

The file world-spec.h

You will supply a header file world-spec.h which constitutes an interface to the implementation in world-spec.c.

Changes in evolution.c

The changes to evolution.c are minimal. In Listing 17.12 on page 177:

1. Replace read.h on line 5 with world-spec.h;
2. Delete line 15 since free_herd() has moved to the file world-spec.c (see above);
3. Delete line 20; the role of initialize_plants() function now is subsumed in the for-loop within the body of the function make_world() show above;

In Listing 17.13 on page 178:

1. Replace lines 5–6 with

   struct world *world = NULL;
   

2. Delete lines 8–9.
3. Replace lines 26–29 with

   world = make_world();
   

4. Replace lines 42–43 with

   free_world(world);
   

Finally, the Usage message, shown in the lower half of page 167 should be adjusted appropriately since we no longer read from a WDF.

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Programming Projects in C