CSC151 2007S, Class 10: Why LISP?

Admin:
* Please make the effort to be in class on time!
* HW 3 returned.  We'll spend a bit of time discussing it.
* Which of the following C-related concepts are you comfortable with: Header files, Makefiles, Macros?
* Are there questions on homework 4?
* Don't forget Friday's lunch with Atul Gupta.

Overview:
* Why lists?
* Graham's differences.
* Other issues.

/HW3/

About 1/3 of you wrote incorrect exponentiation procedures.
* Analysis would help
* The efficient version used a helper, y
  Original X and N
  At each iteration, you'd change y, x, and/or n.
  At the end, n is 0 and we return y
    y * x^n = X^N
  Also works as a loop invariant

  // Invariant: y * x^n = X^N
  while (n > 0) {
    // Suppose n is odd, 
    if ((n % 2) == 1) {
      // make it even
      n = n-1
      // Whoops y*x^n has now been divided by x.  Restore the invt
      y = y*x;
    }
    else { // n is even
      n = n/2;
      // How do we restore the invariant?  
      // (1) Multiply y by x^(n/2)  NOPE
      // (2) We need x_new^n_new  = x_old^n_old
      //     where n_new = n_old/2
      //     For what q is q^(n/2) = x^n?
      //     q = x_new = x^2
      x = x*x;
    }

/NEW STUFF/

	while (true) {
		if (foo) {
	bar;
		baz;
	   bubble;
	}

Q: What if copying and pasting makes my code ugly?
A: Use an attachemnt
}

/Programming *well* in C/

In Quicksort, it would make sense to have at least four .c files
	quicksort.c
		Provides void quicksort(int[] a, int n)
	stack.c
		Provides stack operations
	quicksort_unit_test.c
		Provides main for unit testing
	qs.c
		Provides main for interactive testing
	int_array_utils.c
		Provides the array operations you need
Why separate main from quicksort?
* It's more effort to take it out later than it is to never leave
  it in.
* You may want multiple mains

Why have separate files for stack.c and array_utils.c?
* Simplifies error checking
* Makes it easier for other people to test
* Code reuse
* Code update

Part of the principle of refactoring
* Idea: When you write nearly identical code twice (or when you
  find that you're programming by copy-and-paste), make a common
  routine for the common code.

/Given this separation, how do I build qs?/
* quicksort.c stack.c ia_utils.c qs.c
* A strategy that makes Sam scream (DO NOT USE)
	/* qs.c - A test of quicksort */
	#include "quicksort.c"
	#include "stack.c"
	#include "ia_utils.c"
	int 
	main(int argc, int **argv)
	{
		...
	}
* Simple, but ugly, strategy
	cc -o qs quicksort.c stack.c ia_utils.c qs.c
* Use a Makefile

qs: quicksort.o stack.o ia_utils.o qs.o
	cc -o qs quicksort.o stack.o ia_utils.o qs.o
	
/HW4/