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CSE 417, Wi '11: FAQ |
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A variety of common questions:
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 417 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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