Algorithm Design and Analysis (ADA)
242-535, Semester 1 2014-2015
5. Quicksort
• Objective
o describe the quicksort algorithm, it's partition
function, and analyse its running time under
different data conditions
242-535 ADA: 5. Quicksort
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Overview
1. Quicksort
2. Partitioning Function
3. Analysis of Quicksort
4. Quicksort in Practice
242-535 ADA: 5. Quicksort
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1. Quicksort
• Proposed by Tony Hoare in 1962.
• Voted one of top 10 algorithms of 20th century in
science and engineering
o http://www.siam.org/pdf/news/637.pdf
• A divide-and-conquer algorithm.
• Sorts “in place” -- rearranges elements using only
the array, as in insertion sort, but unlike merge sort
which uses extra storage.
• Very practical (after some code tuning).
242-535 ADA: 5. Quicksort
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Divide and conquer
Quicksort an n-element array:
1. Divide: Partition the array into two subarrays
around a pivot x such that elements in lower
subarray ≤ x ≤ elements in upper subarray.
2. Conquer: Recursively sort the two subarrays.
3. Combine: Nothing to do.
Key: implementing a linear-time partitioning function
242-535 ADA: 5. Quicksort
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Pseudocode
quicksort(int[] A, int left, int right)
if (left < right) // If the array has 2 or more items
pivot = partition(A, left, right)
// recursively sort elements smaller than the pivot
quicksort(A, left, pivot-1)
// recursively sort elements bigger than the pivot
quicksort(A, pivot+1, right)
242-535 ADA: 5. Quicksort
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Quicksort
Diagram
pivot
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Fine Tuning the Code
• quicksort will stop when the subarray is 0 or 1
element big.
• When the subarray gets to a small size, switch over
to dedicated sorting code rather than relying on
recursion.
• quicksort is tail-recursive, a recursive behaviour
which can be optimized.
242-535 ADA: 5. Quicksort
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Tail-Call Optimization
• Tail-call optimization avoids allocating a new stack
frame for a called function.
o It isn't necesary because the calling function only returns
the value that it gets from the called function.
• The most common use of this technique is for
optimizing tail-recursion
o the recursive function can be rewritten to use a constant
amount of stack space (instead of linear)
242-535 ADA: 5. Quicksort
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Tail-Call Graphically
• Before applying tail-call optimization:
• After applying it:
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Pseudocode
• Before:
int foo(int n) {
if (n == 0)
return A();
else {
int x = B(n);
return foo(x);
}
}
• After:
int foo(int n) {
if (n == 0)
return A();
else {
int x = B(n);
goto start of foo() code
with x as argument value
}
}
2. Partitioning Function
PARTITION(A, p, q)
x ← A[p]
// pivot = A[p]
i←p
// index
// A[p . . q]
Running time
= O(n) for n
elements.
for j ← p + 1 to q
if A[ j] ≤ x then
i←i+1
// move the i boundary
exchange A[i] ↔ A[ j] // switch big and small
exchange A[p] ↔ A[i]
return i
// return index of pivot
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Example of partitioning
scan right until find something
less than the pivot
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Example of partitioning
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Example of partitioning
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Example of partitioning
swap 10 and 5
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Example of partitioning
resume scan right until find
something less than the pivot
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Example of partitioning
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Example of partitioning
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Example of partitioning
swap 13 and 3
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Example of partitioning
swap 10 and 2
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Example of partitioning
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Example of partitioning
j runs to the end
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Example of partitioning
swap pivot and 2
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so in the middle
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3. Analysis of Quicksort
• The analysis is quite tricky.
• Assume all the input elements are distinct
o no duplicate values makes this code faster!
o there are better partitioning algorithms when duplicate
input elements exist (e.g. Hoare's original code)
• Let T(n) = worst-case running time on an array of n
elements.
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3.1. Worst-case of quicksort
• QUICKSORT runs very slowly when its input array is
already sorted (or is reverse sorted).
o almost sorted data is quite common in the real-world
• This is caused by the partition using the min (or max)
element which means that one side of the partition
will have has no elements. Therefore:
T(n) = T(0) +T(n-1) + Θ(n)
= Θ(1) +T(n-1) + Θ(n)
= T(n-1) + Θ(n)
= Θ(n2) (arithmetic series)
no elements
n-1 elements
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Worst-case recursion tree
T(n) = T(0) +T(n-1) + cn
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Worst-case recursion tree
T(n) = T(0) +T(n-1) + cn
T(n)
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Worst-case recursion tree
T(n) = T(0) +T(n-1) + cn
cn
T(0)
T(n-1)
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Worst-case recursion tree
T(n) = T(0) +T(n-1) + cn
cn
T(0)
c(n-1)
T(0)
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T(n-2)
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Worst-case recursion tree
T(n) = T(0) +T(n-1) + cn
cn
T(0)
c(n-1)
T(0)
T(n-2)
T(0)
Θ(1)
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Worst-case recursion tree
T(n) = T(0) +T(n-1) + cn
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Quicksort isn't Quick?
• In the worst case, quicksort isn't any quicker than
insertion sort.
• So why bother with quicksort?
• It's average case running time is very good, as we'll
see.
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3.2. Best-case Analysis
If we’re lucky, PARTITION splits the
Case 2 of the
array evenly:
Master Method
T(n) = 2T(n/2) + Θ(n)
= Θ(n log n)
(same as merge sort)
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3.3. Almost Best-case
What if the split is always 1/10 : 9/10?
T(n) = T(1/10n) + T(9/10n) + Θ(n)
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Analysis of “almost-best” case
T(n)
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Analysis of “almost-best” case
cn
T(1/10n)
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T(9/10n)
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Analysis of “almost-best” case
cn
T(1/10n)
T(1/100n ) T(9/100n)
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T(9/10n)
T(9/100n) T(81/100n)
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Analysis of “almost-best” case
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Analysis of “almost-best” case
short
path
long
path
cn * short path
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cn * long path
all leaves
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Short and Long Path Heights
• Short path node value:
n  (1/10)n  (1/10)2n  ...  1
n(1/10)sp
• 
=1
•  n = 10sp
•  log10n = sp
sp steps
// take logs
• Long path node value:
n  (9/10)n  (9/10)2n  ...  1
n(9/10)lp
• 
=1
•  n = (10/9)lp
• 242-535
 log
n = lp
ADA:10/9
5. Quicksort
lp steps
// take logs
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3.4. Good and Bad
Suppose we alternate good, bad, good, bad, good,
partitions ….
G(n) = 2B(n/2) + Θ(n)
good
B(n) = L(n – 1) + Θ(n)
bad
Solving:
G(n) = 2( G(n/2 – 1) + Θ(n/2) ) + Θ(n)
= 2G(n/2 – 1) + Θ(n)
= Θ(n log n)
Good!
How can we make sure we choose good partitions?
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Randomized Quicksort
IDEA: Partition around a random element.
• Running time is then independent of the input
order.
• No assumptions need to be made about the
input distribution.
• No specific input leads to the worst-case
behavior.
• The worst case is determined only by the output
of a random-number generator.
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4. Quicksort in Practice
• Quicksort is a great general-purpose sorting
algorithm.
o especially with a randomized pivot
o Quicksort can benefit substantially from code tuning
o Quicksort can be over twice as fast as merge sort
• Quicksort behaves well even with caching
and virtual memory.
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Timing Comparisons
• Running time estimates:
• Home PC executes 108 compares/second.
• Supercomputer executes 1012 compares/second
Lesson 1. Good algorithms are better than supercomputers.
Lesson 2. Great algorithms are better than good ones.
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