Algorithim lec1.pptx

Design and Analysis of Algorithms
LECTURE 1
Analysis of Algorithms
• Insertion sort
• Asymptotic analysis
• Merge sort
• Recurrences

Analysis of algorithms
The theoretical study of computer-program
performance and resource usage.
What’s more important than performance?
• modularity
• correctness
• maintainability
• functionality
• robustness
• user-friendliness
• programmer time
• simplicity
• extensibility
• 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 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
⊳A[1 . . n]
INSERTION-SORT (A, n)
for j ← 2 to n
do key ← A[ j]
i ← j – 1
while i > 0 and A[i] > key
do A[i+1] ← A[i]
i ← i – 1
A[i+1] = key
“pseudocode”

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

Example of insertion sort
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”

-notation
Math:
(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 = (n3)

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

Insertion sort analysis
Worst case: Input reverse sorted.
n
T(n)  ( j)  n2
n
[arithmetic series]
j2
Average case: All permutations equally likely.
T(n)  ( j /2)  n2
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

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
11

1 2 7 9
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
11

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 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
2 7 9 11
Time = (n) to merge a total
of n elements (linear time).

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

Recurrence for merge sort
T(n) =
(1) if n = 1;
2T(n/2) + (n) if n > 1.
• We shall usually omit stating the base
case when T(n) = (1) for sufficiently
small n, but only when it has no effect on
the asymptotic solution to the recurrence.

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

Recursion tree
T(n)

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

Recursion tree
cn
T(n/4) T(n/4) T(n/4) T(n/4)
cn/2 cn/2

Recursion tree
cn
cn/4 cn/4 cn/4 cn/4
cn/2 cn/2
(1)

Recursion tree
cn
cn/4 cn/4 cn/4 cn/4
cn/2 cn/2
(1)
h = lg n

Recursion tree
cn/4 cn/4 cn/4 cn/4
cn/2 cn/2
(1)
h = lg n
cn cn

Recursion tree
cn/4 cn/4 cn/4 cn/4
cn/2
(1)
h = lg n
cn cn
cn/2 cn

Recursion tree
cn/4 cn/4
cn/2
(1)
h = lg n
cn cn
cn/2 cn
cn/4 cn/4 cn
…

Recursion tree
cn/4 cn/4
cn/2
(1)
h = lg n
cn cn
cn/2 cn
cn/4 cn/4 cn
#leaves = n (n)
…

Recursion tree
cn/4 cn/4
cn/2
(1)
h = lg n
cn cn
cn/2 cn
cn/4 cn/4 cn
#leaves = n (n)
…
Total  (n lg n)

Conclusions
• (n lg n) grows more slowly than (n2).
• Therefore, merge sort asymptotically
beats insertion sort in the worst case.
• In practice, merge sort beats insertion
sort for n > 30 or so.

Algorithim lec1.pptx

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