Lecture Notes: 02-06 Values and References

In C, we have a bunch of integer types and two ways to combine them into larger data structures:

• Arrays (a sequence of the same type, fixed length at allocation but unknown length on use).
• Structs (a fixed sequence of potentially different things)

Structs

#

• Make a goat
• Allocate a goat on the stack
• Return a goat from a function
• Allocate a reference goat with malloc
• Talk about stars and arrows.
• Array of goats.
• Herd struct.
  • Fixed size herd.
  • Variable sized herd.
• Can’t pass an array to a function.
• Can pass a herd to a function, either fixed or variable sized.

Linked Lists

#

If we just had structs and no arrays we could still program.

Linked lists are sometimes simpler since we can add an item in actual O(1) time.

Stuff to cover:

• Handling a linked list with a for loop.
• Header files, include guards, modules.
• Include quotes vs angle brackets.
• The concept of “ownership”, explicitly.
• Passing a value by reference vs. by value.
• Arrow vs dot in structs.
• The “java” pattern (all refs).
• Memory layouts of different structures.

#include <stdio.h>
#include <stdlib.h>

typedef struct cons_cell {
  int head;
  struct cons_cell* tail;
} cons_cell;

cons_cell* empty = (cons_cell*) 0;

cons_cell*
cons(int xx, cons_cell* ys)
{
  cons_cell* cell = malloc(sizeof(cons_cell));
  cell->head = xx;
  cell->tail = ys;
  return cell;
}

void
list_free(cons_cell* xs)
{
  if (xs) {
    list_free(xs->tail);
    free(xs);
  }
}

int
main(int argc, char* argv[])
{
  cons_cell* xs = empty;
  for (int ii = 0; ii < 10; ++ii) {
    xs = cons(ii, xs);
  }

  cons_cell* ys = xs;
  for (int ii = 0; ii < 5; ++ii) {
    ys = ys->tail;
  }

  ys = cons(35, ys);

  printf("\nfirst list:\n");
  for (cons_cell* it = xs; it; it = it->tail) {
    printf("%d\n", it->head);
  }

  printf("\n2nd list:\n");
  for (cons_cell* it = ys; it; it = it->tail) {
    printf("%d\n", it->head);
  }

  list_free(xs);
  free(ys);

  return 0;
}

Games example.

typedef struct game {
    char* team1_name;
    int team1_score;
    char* team2_name;
    int team2_score;
} game;

typedef struct season {
    game games[3];
} season;

• Plymouth Panthers
• Fitchburg Falcons
• Massachusetts Maritime Buccaneers
• Castleton Spartans