CSC161 2010F, Class 21: Functions and Parameters

Overview:
* Basics of functions in C.
* Questions and answers.
* Lab!

Admin:
* Candy
* CS Table today: Is Google Evil? And other tales of the Web.
* There is no assignment for Wednesday.  
  Please use the time to catch up on labs.  
  (Use Tutors, email, and my office hours to get help if you can't 
   complete a problem.)
* For Monday, read 4.3-4.6.
* Given that I'm giving you time to catch up on labs, I may drop my 
  promise not to talk, particularly given that there are some 
  interesting questions.
* EC for soccer on Saturday and Sunday (1:30 or so)
* EC for Ren Fair 2-5 on Saturday
* EC for Singers at 2 on Saturday in S-L
* EC for A Cappella at 4 or 5 on Saturday in Herrick
* EC for Free Web somewhere.  Ask Asa's little brother

Basics of Functions

* Three key aspects to a function: Definition, Use, and Declaration
* What does a function definition look like in Scheme?
  Hmmm Um
  ()()(((((()))()((NAME)((((LAMBDA))))))) DAMN!)  That's a big negative
  You all retroactively fail 151
  (define NAME (lambda (PARAMS) BODY)) - DB was close
* What does a function definition look like in C?
RETURN TYPE
NAME (PARAMETERS)
{
  CODE
}
* What are the differences between the ways we define the parameters in C and Scheme?
  C requires types, Scheme does not
  * They also differ in how they use local variables, but that's not the question I asked, and I DISCOURAGE INITIATIVE!  
* Where do we put the definition?  
int like_squared(int awesome);
int 
main ()
{
  printf ("%d", like_squared (5));
}
int
like_squared (int awesome)
{
  return awesome * awesome;
} // like_squared


* We explicitly use "return" in C, not in Scheme
* What does return do?
  * It doesn't print it or anything like that, but it says "This is the
    output of my function" which another function can use.a
  * Return also exits the function immediately
* How do we call a function
  * In Scheme? (NAME PARAMETERS), separate params with spaces
  * In C? NAME(PARAMETERS) separate params with commas
* How do we declare a function
  * In Scheme?
  * In C?  Before you can call a function, you should either define it or
    declare it
    Like a definition, but without the body.
* Your questions on declaratiosn
  * Do we have to write the parameters in the definition if there's a declaration? Yes
  * Do the parameter names have to match?  Can I use fred in the declaration and x in the defintiion?  Yes
  * Do we really need to give parameters in the declaration?  No, but the C compiler is nicer if you do.
  * Can we just give the types in the declaration, and avoid the names altogether? Yes
  * Why bother?  
    * May help with organization.  We can group similar functions together w/o worrying about calling order.
    * Sometimes we have mutually recursive functions
* Why not in Scheme?
  * Multiple possible return types, so we would expect the language to return values
  * Scheme is a bit more casual than C.

Questions:

* So, external variables are "globally accessible"
  + What does globally accessible mean?
    - It's accessible anywhere

  + How exactly does this work?  Do the function and the declared variable have to be in the same code, or directory, or put in a library that has to be included?  
    * In the same final program, so from anywhere that knows about it.
      If a library provides function foo() or variable bar, than
      any code that uses the library gets access to it.
  * There's a benefit to being able to keep the variable permanent
    * In the RPN calculator, the array/stack could be global and we
      could recurse through
      * static will help with the same sisue, and ewe'll come back to it
* I'm having a little trouble understanding why the atoi code on pg. 73 works. 
        int atoi (char s[])
        {
          double atof (char s[]);
        
          return (int) atof (s);
        } // atoi

  We've already defined atof (or at least K&R have defined atof).
  The (int) "operation " converts a double into an int
  We call this conversion "casting"
* I don't entirely understand how getch and ungetch store and deal with chars. (x2)
  We'll cover this when we do file I/O.
* I could not understand the for loop in the function strindex on page 69. How does it work when there are two variables in the for condition?
     for (i = 0; s[i] != '\0'; i++)
       {
         for (j = i, k = 0; t[k] != '\0' && s[j] == t[k]; j++, k++)
           ;
         if (k > 0 && t[k] == '\0')
           return i;
       }
* How do void functions work?
  We'll talk about it later