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CSE 421, Su '14: The Course FAQ |
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Common questions:
My evil advisor is forcing me to go to a conference in some horrible place called ``Hawaii'' next week, so I won't be able to come to class to turn in my homework. Can I do it electronically? --- S. N. Seattle
A.
Dear Sunless,
Yes. Assignments may be turned in electronically via the Catalyst dropbox, if you find that
more convenient. (Avoid email if possible, even for late submissions; the Catalyst dropbox is strongly preferred,
and much more reliable.)
x_{i,j}^{2}
Or is just giving the algorithm enough? --- P. Optimist
A.
Dear Perpetual,
In general, I want at least a sketch of correctness with all your algorithms, unless I
say otherwise. Likewise, an analysis of your algorithm's run time complexity is also expected.
E.g., if I want the vertex that has the lowest number, may I write?:
Vertex := Min (VertexA, VertexB);
Or should I be more detailed?:
temp := vertexA;
if(vertexB < vertexA) temp:=vertexA;
--- N. T. Know.
A.
Dear Need,
I don't want code, I want a clear, understandable description of the method. A program is a communication between
you and a machine; an algorithm is a communication between you and another human about a program.
E.g., "Min (VertexA, VertexB)" is fine, but I'd be just as happy with English: "between vertices A and B, choose the one with the smaller vertex number", or "keep an array with an entry for each vertex holding the count of the number of times that vertex is reached during process X and find the vertex with the min count." If you need a balanced binary search tree, say "using an WhizBlat tree from chapter 4 or my data structures class, we can ..."; definitely don't give me WhizBlat code.
The basic issue is that you should break the problem down into steps that are clear enough that everyone else in the class should be able to:
You've asked us to measure the time taken by our algorithms. How do I to this? Should I buy that $3.99 Rolex I keep getting emails about? --- T. Treasure
A. Dear Timeless,
No, save the Rolex to impress that Certain Someone. Computers generally have built-in clocks. Accesssing them
depends on the language. For example, in C/C++, you do:
#include <time.h>
int main(void) {
clock_t start_tick, end_tick;
double elapsed;
start_tick = clock();
my_really_fast_algorithm(42);
end_tick = clock();
elapsed = (end_tick - start_tick) / (double)CLOCKS_PER_SEC;
printf("My Really Fast Algorithm took %f seconds!\n", elapsed);
return 0;
}
CLOCKS_PER_SEC, defined in time.h, tells how to scale the system-dependent
clock() results to seconds. (It is often 1000, but that does not tell you the resolution of the
timer, which is often 10 or .001 milliseconds. Oldtimers remember 16.7 millisecond clocks, i.e., 60 hertz powerline
frequency...)
Similarly, in Java public static long currentTimeMillis() returns the current time in millisecond units
(but not necessarily millisecond accuracy).
I did what you outlined above, but I always get 0 seconds?!? ---S. LaCode
A.
Dear Swifty,
Congratulations! Your algorithm is a blazing marvel of speed. But just to be on the safe side, I
always do this, too: Since the system clock may tick only once every few milliseconds, and my computer executes
about a billion operations per second, a fair bit of work gets done between clock ticks. To time an algorithm more
accurately for small n, repeat it r times, for some number r ≥ 1 judiciously chosen so that
the elapsed time for all repetitions is, say, 500 milliseconds or greater, then report the elapsed time divided
by r. It's OK to use a different r for different input sizes, just so each chunk you time is bigger
than 500 ms or so. A big-O estimate of running time might suggest how r should be scaled with n.
Specific systems may have additional features. These will be less portable, but may be more convenient. E.g., I
learned about the following Windows functions from a former student:
"Here are some links to high performance timer functions for windows. The first is
QueryPerformanceFrequency, which returns a Boolean indicating whether the system supports HPCs (and
everything NT4+ does, maybe much earlier), and takes a pointer to a 64 bit integer in which to store the clock
frequency, in ticks per second. The second is
QueryPerformanceCounter; it also returns a Boolean indicating support, and takes a pointer to a 64
bit integer in which to store the current clock."
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