CSC161 2010F, Class 43: Structures

Overview:
* Questions on Assignment 8.
* Structured types in C.
* Lab.

Admin:
* Reading for Wednesday: K&R 6.5 (redo) and 6.6.
* Soda question.
  Dr. Pepper    3
  Diet Dr. Pepper       0
  Seven Up      6
  Diet Seven Up 1
  A&W Rootbeer  6
  Diet A&W Rootbeer     0
  Sunkist Orange        6
  Diet Sunkist Orange   1
  Diet Rite     0
* EC for Thursday extra (some grad student from somewhere).
  Science 3821 at 4:30, food at 4:15
* EC for Rosenfield Corn Symposium activites.

Questions on Assignment 8

* Can we use -w ### instead of -w###?
  * Yes.

* Do we need to support both?
  * No.

* How do I ask whether the first two characters of a string are "-w".
  if (strncmp (str, "-w", 2) == 0) 
    ...

* How do I say "skip the first two letters of str and turn the rest into
  an integer"?
  atoi (str+2)

* Can you grade HW7 first, or at least give a sample solution?
  I'll try

* Does "continue" in a for loop still do the increment?
  Yes, I believe so.

* How can we compare alphabetically?
  strcmp

* How might I do X?
  See K&R

* How can I deal with the issue that I might have user-specified
  string sizes (-w) and numbers of lines (-l)?
  Write a helper function of the form

  void
  sort_helper (int stringsize, int lines, ...)
  {
    char strings[lines][stringsize];
    ...
  }

* Boy, that wastes a lot of space.  Is there anything better?
  void
  sort_helper (int stringsize, int lines, ...)
  {
    char *strings[lines];
    ...

    strings[i] = alloc(size);
    ...
  }

* Where do I get the alloc function?
  K&R!

Structs (Structured Types) in C

* Design problem: We have values that naturally "go together"
  When we're talking about a point, the x and y coordinates form a
    natural pair.
  When we're talking about a student, the first name, last name,
    userid, studentid, and GPA form a natural grouping
* Not all groupings are homogeneous
  * We could treat a point as a two-element array of ints (or doubles)
  * But students are heterogeneous: Strings plus numbers
  * And named points are also heterogeneous
* Even when groupings are homogeneous, arrays can be confusing

In C, and in many other languages, we have "structured types" that
address these issues
* User gets to define them
* Group together data
* Name the parts (fields)

struct NAME { TYPE FIELDNAME; TYPE FIELDNAME; TYPE FIELDNAME; };

E.g.

struct point { int x; int y; };
* The thing above defines the type "struct point"

How do we declare variables of this type?

  struct point start;
  * start is a variable of type "struct point"

or 

  struct point { int x; int y; } thesis;
  * struct point is a type with two integer fields, x, and y
  * thesis is a variable of type "struct point"

Once we've declared a variable of that type, we can access the component
fields.

  thesis.x = 5;
  thesis.y = 23;

  distance = sqrt (square(start.x - thesis.x) + square(start.y - thesis.y));


Issue: Some people find "struct foo" to be too much to write (and too
hard to modify).

typedef struct point { int x; int y; } pt;

* "pt" is another name for the type "struct point"

typedef TYPE NAME

typedef int boolean;

Questions one might ask about structured types:

* Can I assign them?
* Can I pass them as parameters?
* Can I coerce them into each other?
  * And what happens if I can?

* How much space do they require?
* How are they laid out in memory?