What's wrong with my print_random_sprites function? - c

Question

I'm writing some code in C that randomly displays sprites on an LCD screen on a microprocessor. Currently when I run this code it produces 8 lines running from the top to bottom. So it's printing something in random order but not the sprite. Why is this? Could anyone help me? (NOTE: rand is seeded in a separate function which works fine, the problem is just within this code.)
void zombies() {
Sprite zombie_sprite;
Sprite * zombie_sprite_pointer = &zombie_sprite;
byte zombie_bitmap [] = {
BYTE( 11100000 ),
BYTE( 01000000 ),
BYTE( 11100000 )
};
for (int i = 0; i < 8; i++) {
Sprite * zombie_sprites = &zombie_sprites[i];
init_sprite(zombie_sprites, rand()%76, rand()%42, 3, 3, zombie_bitmap);
}
create_zombies();
}
void create_zombies(Sprite * zombie_sprites) {
while(1) {
clear();
draw_sprite( &zombie_sprites );
refresh();
}
return 0;
}

Answer 1

The main problem is that Sprite zombie_sprite is only one object. Make this an array of objects and you can start looking at other problems. Next there is a fair bit of confusion over pointers to the Sprite objects. To simplify things a bit we can tweak the variables in the zombies function, along with some 'tidying up' for best practices.
// Start by using a compile-time constant to define the number of zombies.
// This could be changed to a vriable in the, once the simple case is working.
#define NUMBER_OF_ZOMBIES 8
void zombies()
{
// Eight zombies are required, so define an array of eight sprites.
Sprite zombie_sprites[NUMBER_OF_ZOMBIES];
byte zombie_bitmap [] =
{
BYTE( 11100000 ),
BYTE( 01000000 ),
BYTE( 11100000 )
};
// Continued below...
This makes the rest of the function to initialise the sprites easier. This time, it is possible to get the pointer to the ith element in the array. Also, you will see that the create_zombies function requires an argument: the address of a Sprite object, so pass it the address of the first sprite in the same array that has just been initialised.
Again, with a bit of housekeeping, the rest of the function would look like this:
// ...continued from above
for (int i = 0; i < NUMBER_OF_ZOMBIES; i++)
{
// Initialise the ith sprite using the address of its element
// in the zombie_sprites array.
init_sprite(&zombie_sprites[i], rand()%76, rand()%42,
3, 3, zombie_bitmap);
}
// Animate the zombies in the array.
create_zombies(&zombie_sprites[0]);
}
Finally, the create_zombies function itself needs a minor change to loop through all of the sprites in the array passed as its parameter. Also, being of type void it does not have a return statement.
void create_zombies(Sprite * zombie_sprites)
{
while(1)
{
clear();
// loop round drawing all of the sprites.
for(int zombie_index = 0; zombie_index < NUMBER_OF_ZOMBIES; zombie_index++)
{
draw_sprite( &zombie_sprites[zombie_index] );
}
refresh();
}
}
Future enhancements might include:
Changing NUMBER_OF_ZOMBIES to be a variable.
Replacing the static array with a dynamically allocated array using malloc and free.
Replacing the array with a more complex abstract data type such as a list or doubly linked list, so that zombies can be added or removed at run-time.
Renaming the functions and restructuring where each gets called.