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# Inventing A Really Bad Sort: It Seemed Like A Good Idea At The Time - PowerPoint PPT Presentation

Inventing A Really Bad Sort: It Seemed Like A Good Idea At The Time. Jim Huggins Kettering University [email protected] http://www.kettering.edu/~jhuggins. Bless me, Father Knuth, for I have sinned …. The Set-Up.

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### Inventing A Really Bad Sort:It Seemed Like A Good Idea At The Time

Jim Huggins

Kettering University

http://www.kettering.edu/~jhuggins

Writing questions for take-home exam for Advanced Algorithms course (sophomore)

Desired: an algorithm which:

• does something useful

• is simple to analyze

• hasn’t been done before on the web

public void StoogeSort(int[] arr, int start, int stop) {

if (arr[start] > arr[stop]) {

int swap = arr[start];

arr[start] = arr[stop];

arr[stop] = swap;

}

if (start+1 >= stop)

return;

int third = (stop - start + 1) / 3;

StoogeSort(arr, start, stop-third); // First two-thirds

StoogeSort(arr, start+third, stop); // Last two-thirds

StoogeSort(arr, start, stop-third); // First two-thirds

}

Comparison count:

T(n) = 3T(⅔n) + 1; T(0)=0, T(1)=0, T(2) = 1

T(n) = Θ(nlog3/2 3) ≈ Θ(n2.7)

public void goofySort (int[] array, int start, int stop) {

if (start>=stop) return;

goofySort(array, start, stop-1); // first n-1 items

if (array[stop-1] > array[stop]) {

int swap = array[stop-1]; // swap last item

array[stop-1] = array[stop];

array[stop] = swap;

goofySort(array, start, stop-1); // first n-1 items

} // again

}

An added bonus: different behavior in best case, worst case. (More questions!)

public void goofySort (int[] array, int start, int stop) {

if (start>=stop) return;

goofySort(array, start, stop-1);

if (array[stop-1] > array[stop]) { // best case: false

int swap = array[stop-1];

array[stop-1] = array[stop];

array[stop] = swap;

goofySort(array, start, stop-1);

}

}

Comparison count:

T(n) = T(n-1) + 1, T(1) = 0

T(n) = O(n)

public void goofySort (int[] array, int start, int stop) {

if (start>=stop) return;

goofySort(array, start, stop-1);

if (array[stop-1] > array[stop]) { // worst case: true

int swap = array[stop-1];

array[stop-1] = array[stop];

array[stop] = swap;

goofySort(array, start, stop-1);

}

}

Comparison count:

T(n) = 2T(n-1) + 1, T(1) = 0

T(n) = O(2n)

And so, to avoid embarrassment … correct, not tried it.”

• Coded the algorithm in Java

• Tested with a variety of random inputs

• Tested with a variety of list sizes 20, 30, 40, …

• And it all works! Great! (What could possibly go wrong?)

Actual Student Answers: correct, not tried it.”

• T(n) = O(n3)

• T(n) = T(n-1) + O(n2) = O(n3)

• T(n) = T(n-1) + Σ1n i = ?

• T(n) = O(2n)

• One bright student, at least!

Preparing to hand them back… correct, not tried it.”

• Preparing my rant …

• “you completely missed the point”

• “we did this in class … ”

• “I even tested this on lots of inputs ...”

• And then I remember:

• If this is really exponential time, how did I run it on an input of size 40?

• @#@!. What if I’m wrong and they’re right?

Bentley: Three Beautiful Quicksorts correct, not tried it.”

Paraphrasing:

“If you double the input size, and the instruction count quadruples, you’ve got a quadratic algorithm.”

(Watch the Google TechTalk … it’s neat.)

Racing to the computer correct, not tried it.”

Take the average over 100 random runs …

@#\$!. It looks like it’s cubic!

How could this be cubic? correct, not tried it.”

public void goofySort (int[] array, int start, int stop) {

if (start>=stop) return;

goofySort(array, start, stop-1);

if (array[stop-1] > array[stop]) {

int swap = array[stop-1];

array[stop-1] = array[stop];

array[stop] = swap;

goofySort(array, start, stop-1);

}

}

The first recursive call is always bad;the array could be completely unordered

The second recursive call is always good;all but the last item are ordered

How badly cubic? correct, not tried it.”

• What’s the worst case input?

• Reverse sorted, right?

• At this point, I don’t trust myself, so …

• Generate all permutations on a list of size n

• We just covered this in class! (Lucky this is an algorithms course!)

• Verified: worst case happens in reverse order

So, what’s the worst-case time? correct, not tried it.”

• Do a bunch of input sizes …

• … and putz around with a calculator …

• The closed-form formula appears to be: T(n) = n(n-1)(n-2)/6 + (n-1)

• So this does appear to be cubic after all. (Now, how do I prove it?)

A quick overview of the proof correct, not tried it.”

• T(n) = 1 + T(n-1) + GT(n-1); T(1) = 0

• GT(n) = (n-1) + GT(n-1); GT(1) = 0 …GT(n) = n(n-1)/2

• T(n) = 1 + T(n-1) + (n-1)(n-2)/2; T(1) = 0 …T(n) = (n-1) + n(n-1)(n-2)/6

(see me for the full proof … it’s not that bad)

Aftermath correct, not tried it.”

• Deep apologies to the students

• They were gracious

• Q: “How did y’all know it was cubic?”A: “We ran it and it looked cubic.”

• They had no idea how to proceed …so they did the empirical analysis first to find the “right” answer!

“Beware of bugs in the above correct, not tried it.”analysis; I have only proved it correct, not verified it empirically.”