Lec1 Algorthm

Introduction to Algorithms
6.046J/18.401J
LECTURE 1
Analysis of Algorithms
• Insertion sort
• Asymptotic analysis
• Merge sort
• Recurrences

Prof. Charles E. Leiserson
Copyright © 2001-5 Erik D. Demaine and Charles E. Leiserson

Course information

1. Staff 8. Course website
2. Distance learning 9. Extra help
3. Prerequisites 10. Registration
4. Lectures 11. Problem sets
5. Recitations 12. Describing algorithms
6. Handouts 13. Grading policy
7. Textbook 14. Collaboration policy

September 7, 2005 Introduction to Algorithms L1.2

Analysis of algorithms
The theoretical study of computer-program
performance and resource usage.
What’s more important than performance?
• modularity • user-friendliness
• correctness • programmer time
• maintainability • simplicity
• functionality • extensibility
• robustness • reliability


Why study algorithms and
performance?
• Algorithms help us to understand scalability.
• Performance often draws the line between what
is feasible and what is impossible.
• Algorithmic mathematics provides a language
for talking about program behavior.
• Performance is the currency of computing.
• The lessons of program performance generalize
to other computing resources.
• Speed is fun!

The problem of sorting

Input: sequence 〈a1, a2, …, an〉 of numbers.

Output: permutation 〈a'1, a'2, …, a'n〉 such
that a'1 ≤ a'2 ≤ … ≤ a'n .

Example:
Input: 8 2 4 9 3 6
Output: 2 3 4 6 8 9

Insertion sort
INSERTION-SORT (A, n) ⊳ A[1 . . n]
for j ← 2 to n
do key ← A[ j]
i←j–1
“pseudocode” while i > 0 and A[i] > key
do A[i+1] ← A[i]
i←i–1
A[i+1] = key


Insertion sort
INSERTION-SORT (A, n) ⊳ A[1 . . n]
for j ← 2 to n
do key ← A[ j]
i←j–1
“pseudocode” while i > 0 and A[i] > key
do A[i+1] ← A[i]
i←i–1
A[i+1] = key
1 i j n
A:
key
sorted

Example of insertion sort
8 2 4 9 3 6


8 2 4 9 3 6


8 2 4 9 3 6
2 8 4 9 3 6


8 2 4 9 3 6
2 8 4 9 3 6
2 4 8 9 3 6


8 2 4 9 3 6
2 8 4 9 3 6
2 4 8 9 3 6
2 4 8 9 3 6


8 2 4 9 3 6
2 8 4 9 3 6
2 4 8 9 3 6
2 4 8 9 3 6
2 3 4 8 9 6


8 2 4 9 3 6
2 8 4 9 3 6
2 4 8 9 3 6
2 4 8 9 3 6
2 3 4 8 9 6
2 3 4 6 8 9 done


Running time

• The running time depends on the input: an
already sorted sequence is easier to sort.
• Parameterize the running time by the size of
the input, since short sequences are easier to
sort than long ones.
• Generally, we seek upper bounds on the
running time, because everybody likes a
guarantee.


Kinds of analyses
Worst-case: (usually)
• T(n) = maximum time of algorithm
on any input of size n.
Average-case: (sometimes)
• T(n) = expected time of algorithm
over all inputs of size n.
• Need assumption of statistical
distribution of inputs.
Best-case: (bogus)
• Cheat with a slow algorithm that
works fast on some input.

Machine-independent time

What is insertion sort’s worst-case time?
• It depends on the speed of our computer:
• relative speed (on the same machine),
• absolute speed (on different machines).
BIG IDEA:
• Ignore machine-dependent constants.
• Look at growth of T(n) as n → ∞ .
“Asymptotic Analysis”

Θ-notation

Math:
Θ(g(n)) = { f (n) : there exist positive constants c1, c2, and
n0 such that 0 ≤ c1 g(n) ≤ f (n) ≤ c2 g(n)
for all n ≥ n0 }
Engineering:
• Drop low-order terms; ignore leading constants.
• Example: 3n3 + 90n2 – 5n + 6046 = Θ(n3)


Asymptotic performance
When n gets large enough, a Θ(n2) algorithm
always beats a Θ(n3) algorithm.
• We shouldn’t ignore
asymptotically slower
algorithms, however.
• Real-world design
situations often call for a
T(n) careful balancing of
engineering objectives.
• Asymptotic analysis is a
useful tool to help to
n n0 structure our thinking.

Insertion sort analysis
Worst case: Input reverse sorted.
n
T ( n) = ∑ Θ( j ) = Θ(n 2 ) [arithmetic series]
j =2
Average case: All permutations equally likely.
n
T ( n) = ∑ Θ( j / 2) = Θ(n 2 )
j =2
Is insertion sort a fast sorting algorithm?
• Moderately so, for small n.
• Not at all, for large n.

Merge sort

MERGE-SORT A[1 . . n]
1. If n = 1, done.
2. Recursively sort A[ 1 . . ⎡n/2⎤ ]
and A[ ⎡n/2⎤+1 . . n ] .
3. “Merge” the 2 sorted lists.

Key subroutine: MERGE


Merging two sorted arrays
20 12
13 11
7 9
2 1


20 12
13 11
7 9
2 1

1


20 12 20 12
13 11 13 11
7 9 7 9
2 1 2

1


20 12 20 12
13 11 13 11
7 9 7 9
2 1 2

1 2


20 12 20 12 20 12
13 11 13 11 13 11
7 9 7 9 7 9
2 1 2

1 2


20 12 20 12 20 12
13 11 13 11 13 11
7 9 7 9 7 9
2 1 2

1 2 7


20 12 20 12 20 12 20 12
13 11 13 11 13 11 13 11
7 9 7 9 7 9 9
2 1 2

1 2 7


20 12 20 12 20 12 20 12
13 11 13 11 13 11 13 11
7 9 7 9 7 9 9
2 1 2

1 2 7 9


20 12 20 12 20 12 20 12 20 12
13 11 13 11 13 11 13 11 13 11
7 9 7 9 7 9 9
2 1 2

1 2 7 9


20 12 20 12 20 12 20 12 20 12
13 11 13 11 13 11 13 11 13 11
7 9 7 9 7 9 9
2 1 2

1 2 7 9 11


20 12 20 12 20 12 20 12 20 12 20 12
13 11 13 11 13 11 13 11 13 11 13
7 9 7 9 7 9 9
2 1 2

1 2 7 9 11


20 12 20 12 20 12 20 12 20 12 20 12
13 11 13 11 13 11 13 11 13 11 13
7 9 7 9 7 9 9
2 1 2

1 2 7 9 11 12


20 12 20 12 20 12 20 12 20 12 20 12
13 11 13 11 13 11 13 11 13 11 13
7 9 7 9 7 9 9
2 1 2

1 2 7 9 11 12

Time = Θ(n) to merge a total
of n elements (linear time).

Analyzing merge sort

T(n) MERGE-SORT A[1 . . n]
Θ(1) 1. If n = 1, done.
2T(n/2) 2. Recursively sort A[ 1 . . ⎡n/2⎤ ]
Abuse and A[ ⎡n/2⎤+1 . . n ] .
Θ(n) 3. “Merge” the 2 sorted lists
Sloppiness: Should be T( ⎡n/2⎤ ) + T( ⎣n/2⎦ ) ,
but it turns out not to matter asymptotically.


Recurrence for merge sort
Θ(1) if n = 1;
T(n) =
2T(n/2) + Θ(n) if n > 1.
• We shall usually omit stating the base
case when T(n) = Θ(1) for sufficiently
small n, but only when it has no effect on
the asymptotic solution to the recurrence.
• CLRS and Lecture 2 provide several ways
to find a good upper bound on T(n).


Recursion tree
Solve T(n) = 2T(n/2) + cn, where c > 0 is constant.


Recursion tree
T(n)


Recursion tree
cn
T(n/2) T(n/2)


Recursion tree
cn
cn/2 cn/2

T(n/4) T(n/4) T(n/4) T(n/4)


Recursion tree
cn
cn/2 cn/2

cn/4 cn/4 cn/4 cn/4
…

Θ(1)


Recursion tree
cn
cn/2 cn/2
h = lg n cn/4 cn/4 cn/4 cn/4
…

Θ(1)


Recursion tree
cn cn
cn/2 cn/2
…

Θ(1)


Recursion tree
cn cn
cn/2 cn/2 cn
…

Θ(1)


Recursion tree
cn cn
cn/2 cn/2 cn
h = lg n cn/4 cn/4 cn/4 cn/4 cn
…

…
Θ(1)


Recursion tree
cn cn
cn/2 cn/2 cn
…

…
Θ(1) #leaves = n Θ(n)


Recursion tree
cn cn
cn/2 cn/2 cn
…

…
Θ(1) #leaves = n Θ(n)
Total = Θ(n lg n)

Conclusions

• Θ(n lg n) grows more slowly than Θ(n2).
• Therefore, merge sort asymptotically
beats insertion sort in the worst case.
• In practice, merge sort beats insertion
sort for n > 30 or so.
• Go test it out for yourself!


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