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Algorithm Design and Analysis Introduction

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Algorithm Design and Analysis Introduction

http://ada.miulab.tw

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Outline

• Terminology

• Problem (問題)

• Problem instance (個例)

• Computation model (計算模型)

• Algorithm (演算法)

• The hardness of a problem (難度)

• Algorithm Design & Analysis Process

• Review: Asymptotic Analysis

• Algorithm Complexity

(3)

Efficiency Measurement = Speed

• Why we care?

• Computers may be fast, but they are not infinitely fast

• Memory may be inexpensive, but it is not free

(4)

Terminology

Textbook Ch. 1 – The Role of Algorithms in Computing

(5)

Problem (問題)

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Problem Instance (個例)

• An instance of the champion problem

7 4 2 9 8

A[1] A[2] A[3] A[4] A[5]

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Computation Model (計算模型)

• Each problem must have its rule (遊戲規則)

• Computation model (計算模型) = rule (遊戲規則)

• The problems with different rules have different hardness levels

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Hardness (難易程度)

• How difficult to solve a problem

• Example: how hard is the champion problem?

• Following the comparison-based rule

What does “solve (解)” mean?

What does “difficult (難)” mean?

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Problem Solving (解題)

• Definition of “solving” a problem

• Giving an algorithm (演算法) that produces a correct output for any instance of the problem.

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Algorithm (演算法)

• Algorithm: a detailed step-by-step instruction

• Must follow the game rules

• Like a step-by-step recipe

• Programming language doesn’t matter

→ problem-solving recipe (technology)

• If an algorithm produces a correct output for any instance of the problem

→ this algorithm “solves” the problem

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Hardness (難度)

• Hardness of the problem

• How much effort the best algorithm needs to solve any problem instance

• 防禦力

• 看看最厲害的賽亞人要花多少攻擊力才能打贏對手

防禦力 100000 攻擊力 50000

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Algorithm Design &

Analysis Process

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Algorithm Design & Analysis Process

2 3 4 1

Analyze running time/space requirement Prove the correctness

Develop an algorithm Formulate a problem

Design Step

Analysis

Step

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1. Problem Formulation

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2. Algorithm Design

• Create a detailed recipe for solving the problem

• Follow the comparison-based rule

• 不准偷看信封的內容

• 請別人幫忙「比大小」

• Algorithm: 擂台法

1. int i, j;

2. j = 1;

3. for (i = 2; i <= n; i++)

4. if (A[i] > A[j])

5. j = i;

6. return j;

Q1: Is this a comparison-based algorithm?

Q2: Does it solve the champion problem?

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3. Correctness of the Algorithm

• Prove by contradiction (反證法)

1. int i, j;

2. j = 1;

3. for (i = 2; i <= n; i++)

4. if (A[i] > A[j])

5. j = i;

6. return j;

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Hardness of The Champion Problem

• How much effort the best algorithm needs to solve any problem instance

• Follow the comparison-based rule

• 不准偷看信封的內容

• 請別人幫忙「比大小」

• Effort: we first use the times of comparison for measurement

1. int i, j;

2. j = 1;

3. for (i = 2; i <= n;

i++)

4. if (A[i] > A[j])

5. j = i;

6. return j;

(n - 1) comparisons

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Hardness of The Champion Problem

• The hardness of the champion problem is (n - 1) comparisons

a) There is an algorithm that can solve the problem using at most (n – 1) comparisons

• This can be proved by 擂臺法, which uses (n – 1) comparisons for any problem instance

b) For any algorithm, there exists a problem instance that requires (n - 1) comparisons

• Why?

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Hardness of The Champion Problem

• Q: Is there an algorithm that only needs 𝑛 − 2 comparisons?

• A: Impossible!

• Reason

• A single comparison only decides a loser

• If there are only 𝑛 − 2 comparisons, the most number of losers is 𝑛 − 2

• There exists a least 2 integers that did not lose

→ any algorithm cannot tell who the champion is

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Finding Hardness

• Use the upper bound and the lower bound

• When they meet each other, we know the hardness of the problem

Upper bound

Upper bound

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Hardness of The Champion Problem

• Upper bound

• how many comparisons are sufficient to solve the champion problem

• Each algorithm provides an upper bound

• The smarter algorithm provides tighter, lower, and better upper bound

• Lower bound

• how many comparisons in the worst case are necessary to solve the

champion problem

• Some arguments provide different lower bounds

• Higher lower bound is better

多此一舉擂臺法 1. int i, j;

2. j = 1;

3. for (i = 2; i <= n; i++)

4. if ((A[i] > A[j]) && (A[j] < A[i]))

5. j = i;

6. return j;

Every integer needs to be in the comparison once

→ (n/2) comparisons

→ (2n - 2) comparisons

When upper bound = lower bound, the problem is solved.

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4. Algorithm Analysis

• The majority of researchers in algorithms studies the time and space required for solving problems in two directions

• Upper bounds: designing and analyzing algorithms

• Lower bounds: providing arguments

• When the upper and lower bounds match, we have an optimal algorithm and the problem is completely resolved

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Asymptotic Analysis

Edmund Landau Donald E. Knuth

教過 我早就會了!

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Motivation

• The hardness of the champion problem is exactly 𝑛 − 1 comparisons

• Different problems may have different 「難度量尺」

• cannot be interchangeable

• Focus on the standard growth of the function to ignore the unit and coefficient effects

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Goal: Finding Hardness

• For a problem P, we want to figure out

• The hardness (complexity) of this problem P is Θ 𝑓 𝑛

• 𝑛 is the instance size of this problem P

• 𝑓 𝑛 is a function

• Θ 𝑓 𝑛 means that “it exactly equals to the growth of the function”

• Then we can argue that under the comparison-based computation model

• The hardness of the champion problem is Θ 𝑛

• The hardness of the sorting problem is Θ 𝑛 log 𝑛

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upper bound is 𝑂 ℎ 𝑛 & lower bound is Ω ℎ 𝑛

→ the problem complexity is exactly Θ ℎ 𝑛

Goal: Finding Hardness

• Use the upper bound and the lower bound

• When they match, we know the hardness of the problem

Upper bound

Upper bound

Lower bound

use 𝑂 𝑓 𝑛 and 𝑜 𝑓 𝑛

(27)

Goal: Finding Hardness

• First learn how to analyze / measure the effort an algorithm needs

• Time complexity

• Space complexity

• Focus on worst-case complexity

• “average-case” analysis requires the assumption about the probability distribution of problem instances

Types Description

Worst Case Maximum running time for any instance of size n

Average Case Expected running time for a random instance of size n Amortized Worse-case running time for a series of operations

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Review of Asymptotic Notation

(Textbook Ch. 3.1)

• 𝑓 𝑛 = time or space of an algorithm for an input of size 𝑛

• Asymptotic analysis: focus on the growth of 𝑓 𝑛 as 𝑛 → ∞

(29)

Review of Asymptotic Notation

(Textbook Ch. 3.1)

• 𝑓 𝑛 = time or space of an algorithm for an input of size 𝑛

• Asymptotic analysis: focus on the growth of 𝑓 𝑛 as 𝑛 → ∞

• Ο, or Big-Oh: upper bounding function

• Ω, or Big-Omega: lower bounding function

• Θ, or Big-Theta: tightly bounding function

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Formal Definition of Big-Oh

(Textbook Ch. 3.1)

• For any two functions 𝑓 𝑛 and 𝑔 𝑛 ,

if there exist positive constants 𝑛0 and 𝑐 s.t.

for all 𝑛 ≥ 𝑛0.

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𝒇 𝒏 = 𝑶 𝒈 𝒏

• Intuitive interpretation

• 𝑓 𝑛 does not grow faster than 𝑔 𝑛

• Comments

1) 𝑓 𝑛 = 𝑂 𝑔 𝑛 roughly means 𝑓 𝑛 ≤ 𝑔 𝑛 in terms of rate of growth 2) “=” is not “equality”, it is like “ϵ (belong to)”

The equality is 𝑓 𝑛 ⊆ 𝑂 𝑔 𝑛 3) We do not write 𝑂 𝑔 𝑛 = 𝑓 𝑛

• Note

• 𝑓 𝑛 and 𝑔 𝑛 can be negative for some integers 𝑛

• In order to compare using asymptotic notation 𝑂, both have to be non-negative for sufficiently large 𝑛

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Review of Asymptotic Notation

(Textbook Ch. 3.1)

• Benefit

• Ignore the low-order terms, units, and coefficients

• Simplify the analysis

• Example: 𝑓 𝑛 = 5𝑛3 + 7𝑛2 − 8

• Upper bound: f(n) = O(n3), f(n) = O(n4), f(n) = O(n3log2n)

• Lower bound: f(n) = Ω(n3), f(n) = Ω(n2), f(n) = Ω(nlog2n)

• Tight bound: f(n) = Θ(n3)

• Q: 𝑓 𝑛 = 𝑂 𝑛3 and 𝑓 𝑛 = 𝑂 𝑛4 , so 𝑂 𝑛3 = 𝑂 𝑛4 ?

• 𝑂 𝑛3 represents a set of functions that are upper bounded by c𝑛3 for some constant

“=” doesn’t mean “equal to”

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Exercise: 𝟏𝟎𝟎𝒏 𝟐 = 𝑶 𝒏 𝟑 − 𝒏 𝟐 ?

• Draft.

• Let 𝑛0 = 2 and 𝑐 = 100

holds for 𝑛 ≥ 2

(34)

Exercise: 𝒏 𝟐 = 𝑶 𝒏 ?

• Disproof.

• Assume for a contradiction that there exist positive constants 𝑐 and 𝑛0 s.t.

holds for any integer 𝑛 with 𝑛 ≥ 𝑛0.

• Assume

and because , it follows that

(35)

Rules

(Textbook Ch. 3.1)

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Other Notations

(Textbook Ch. 3.1)

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Goal: Finding Hardness

• First learn how to analyze / measure the effort an algorithm needs

• Time complexity

• Space complexity

• Focus on worst-case complexity

• “average-case” analysis requires the assumption about the probability distribution of problem instances

Using 𝑂 to give upper bounds on the worst-case time complexity of algorithms

(38)

Algorithm Analysis

• 擂台法

1. int i, j;

2. j = 1;

3. for (i = 2; i <= n; i++)

4. if (A[i] > A[j])

5. j = i;

6. return j;

Adding everything together

→ an upper bound on the

Rule 2 Rule 4

1 = 𝑂 𝑛 & Rule 3 Rule 2

O 1 time O 1 time

O 𝑛 iterations O 1 time

O 1 time O 1 time

• The worst-case time complexity is

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Sorting Problem

(40)

Algorithm Analysis

• Bubble-Sort Algorithm

𝑂 1 time

𝑓 𝑛 iterations 𝑂 1 time

𝑂 𝑛 iterations 𝑂 1 time

𝑂 1 time 𝑂 1 time

1. int i, done;

2. do {

3. done = 1;

4. for (i = 1; i < n; i ++) {

5. if (A[i] > A[i + 1]) {

6. exchange A[i] and A[i + 1];

7. done = 0;

8. }

9. } prove by induction

(41)

Example Illustration

7 3 1 4 6 2 5

7

3 1 4 6 2 5

7 3

1 4 2 5 6

1 3 2 4 5 6 7

1 2 3 4 5 6 7

(42)

Goal: Finding Hardness

• First learn how to analyze / measure the effort an algorithm needs

• Time complexity

• Space complexity

• Focus on worst-case complexity

• “average-case” analysis requires the assumption about the probability distribution of problem instances

Using 𝑂 to give upper bounds on the worst-case time complexity of algorithms Using Ω to give lower bounds on the worst-case time complexity of algorithms

(43)

Algorithm Analysis

• 擂台法

1. int i;

2. int m = A[1];

3. for (i = 2; i <= n; i ++) {

4. if (A[i] > m)

5. m = A[i];

6. }

7. return m;

Adding everything together

→ a lower bound on the worst-case time complexity?

Ω 1 time Ω 1 time

Ω 𝑛 iterations Ω 1 time

Ω 1 time Ω 1 time

(44)

Algorithm Analysis

• 百般無聊擂台法

1. int i;

2. int m = A[1];

3. for (i = 2; i <= n; i ++) {

4. if (A[i] > m)

5. m = A[i];

6. if (i == n)

7. do i++ n times

8. }

9. return m;

Ω 1 time Ω 1 time

Ω 𝑛 iterations Ω 1 time

Ω 1 time Ω 1 time Ω 𝑛 time Ω 1 time

(45)

Algorithm Analysis

• Bubble-Sort Algorithm

𝑓 𝑛 iterations Ω 𝑛 time

1. int i, done;

2. do {

3. done = 1;

4. for (i = 1; i < n; i ++) {

5. if (A[i] > A[i + 1]) {

6. exchange A[i] and A[i + 1];

7. done = 0;

8. }

9. }

10. } while (done == 0)

When A is decreasing, 𝑓 𝑛 = Ω 𝑛 .

Therefore, the worst-case time complexity of Bubble-Sort is

(46)

Example Illustration

7 6 5 4 3 2 1

7

6 5 4 3 2 1

7 6

5 4 3 2 1

5

4 3 2 1 6 7

n iterations

(47)

Algorithm Complexity

In the worst case, what is the growth of function an algorithm takes

(48)

Time Complexity of an Algorithm

• We say that the (worst-case) time complexity of Algorithm A is Θ 𝑓 𝑛 if 1. Algorithm A runs in time 𝑂 𝑓 𝑛 &

2. Algorithm A runs in time Ω 𝑓 𝑛 (in the worst case)

o An input instance 𝐼 𝑛 s.t. Algorithm A runs in Ω 𝑓 𝑛 for each 𝑛

(49)

Tightness of the Complexity

• If we say that the time complexity analysis about 𝑂 𝑓 𝑛 is tight

• = the algorithm runs in time Ω 𝑓 𝑛 in the worst case

• = (worst-case) time complexity of the algorithm is Θ 𝑓 𝑛

• Not over-estimate the worst-case time complexity of the algorithm

• If we say that the time complexity analysis of Bubble-Sort algorithm about 𝑂 𝑛2 is tight

• = Time complexity of Bubble-Sort algorithm is Ω 𝑛2

• = Time complexity of Bubble-Sort algorithm is Θ 𝑛2

(50)

Algorithm Analysis

• 百般無聊擂台法

non-tight analysis

tight analysis

Step 3 takes 𝑂 𝑛 iterations for the for-loop,

where only last iteration takes 𝑂 𝑛 time and the rest take 𝑂 1 time.

The steps 3-8 take time 1. int i;

2. int m = A[1];

3. for (i = 2; i <= n; i ++) {

4. if (A[i] > m)

5. m = A[i];

6. if (i == n)

7. do i++ n times

8. }

9. return m;

𝑂 1 time 𝑂 1 time

𝑂 𝑛 iterations 𝑂 1 time

𝑂 1 time 𝑂 1 time 𝑂 𝑛 time 𝑂 1 time

(51)

Algorithm Comparison

• Q: can we say that Algorithm 1 is a better algorithm than Algorithm 2 if

• Algorithm 1 runs in 𝑂 𝑛 time

• Algorithm 2 runs in 𝑂 𝑛2 time

• A: No! The algorithm with a lower upper bound on its worst-case time does not necessarily have a lower time complexity.

(52)

Comparing A and B

• Algorithm A is no worse than Algorithm B in terms of worst-case time complexity if there exists a positive function 𝑓 𝑛 s.t.

• Algorithm A runs in time 𝑂 𝑓 𝑛 &

• Algorithm B runs in time Ω 𝑓 𝑛 in the worst case

• Algorithm A is (strictly) better than Algorithm B in terms of worst-case time complexity if there exists a positive function 𝑓 𝑛 s.t.

• Algorithm A runs in time 𝑂 𝑓 𝑛 &

• Algorithm B runs in time 𝜔 𝑓 𝑛 in the worst case or

(53)

Problem Complexity

In the worst case, what is the growth of the function the optimal algorithm of the problem takes

(54)

Time Complexity of a Problem

• We say that the (worst-case) time complexity of Problem P is Θ 𝑓 𝑛 if 1. The time complexity of Problem P is 𝑂 𝑓 𝑛 &

o There exists an 𝑂 𝑓 𝑛 -time algorithm that solves Problem P

2. The time complexity of Problem P is Ω 𝑓 𝑛

o Any algorithm that solves Problem P requires Ω 𝑓 𝑛 time

• The time complexity of the champion problem is Θ 𝑛 because 1. The time complexity of the champion problem is 𝑂 𝑛 &

o 「擂臺法」is the 𝑂 𝑛 -time algorithm

(55)

Optimal Algorithm

• If Algorithm A is an optimal algorithm for Problem P in terms of worst-case time complexity

• Algorithm A runs in time 𝑂 𝑓 𝑛 &

• The time complexity of Problem P is Ω 𝑓 𝑛 in the worst case

• Examples (the champion problem)

• 擂台法

• It runs in 𝑂 𝑛 time &

• Any algorithm solving the problem requires time Ω 𝑛 in the worst case

• 百般無聊擂台法

• It runs in 𝑂 𝑛 time &

• Any algorithm solving the problem requires time Ω 𝑛 in the worst case

→ optimal algorithm

→ optimal algorithm

(56)

Comparing P and Q

• Problem P is no harder than Problem Q in terms of (worst-case) time complexity if there exists a function 𝑓 𝑛 s.t.

• The (worst-case) time complexity of Problem P is 𝑂 𝑓 𝑛 &

• The (worst-case) time complexity of Problem Q is Ω 𝑓 𝑛

• Problem P is (strictly) easier than Problem Q in terms of (worst-case) time complexity if there exists a function 𝑓 𝑛 s.t.

• The (worst-case) time complexity of Problem P is 𝑂 𝑓 𝑛 &

• The (worst-case) time complexity of Problem Q is 𝜔 𝑓 𝑛 or

(57)

Concluding Remarks

• Algorithm Design and Analysis Process 1) Formulate a problem

2) Develop an algorithm 3) Prove the correctness

4) Analyze running time/space requirement

• Usually brute force (暴力法) is not very efficient

• Analysis Skills

• Prove by contradiction

• Induction

• Asymptotic analysis

• Problem instance

• Algorithm Complexity

• In the worst case, what is the growth of function an algorithm takes

• Problem Complexity

• In the worst case, what is the growth of the function the optimal algorithm of the problem takes Design Step

Analysis Step

(58)

Reading Assignment

• Textbook Ch. 3 – Growth of Function

(59)

Question?

Important announcement will be sent to

@ntu.edu.tw mailbox & post to the course website

Course Website: http://ada.miulab.tw Email: ada-ta@csie.ntu.edu.tw

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