analysis & design of algorithms (csce 321)

Prof. Amr Goneid, AUC 1

Analysis & Design of Analysis & Design of AlgorithmsAlgorithms(CSCE 321)(CSCE 321)

Prof. Amr GoneidDepartment of Computer Science, AUC

Part 8. Greedy Algorithms

Greedy AlgorithmsGreedy Algorithms

Microsoft Interview

From: http://www.cs.pitt.edu/~kirk/cs1510/

Greedy Algorithms The General Method Continuous Knapsack Problem Optimal Merge Patterns

1. Greedy Algorithms1. Greedy Algorithms

Methodology: Start with a solution to a small sub-

problem Build up to the whole problem Make choices that look good in the

short term but not necessarily in the long term

Disadvantages: They do not always work. Short term choices may be disastrous on the

long term. Correctness is hard to proveAdvantages: When they work, they work fast Simple and easy to implement

2. The General method2. The General methodLet a[ ] be an array of elements that may contribute to a solution. Let S be a solution,

Greedy (a[ ],n) {

S = empty;for each element (i) from a[ ], i = 1:n {

x = Select (a,i);if (Feasible(S,x)) S = Union(S,x);

}return S;

The General method (continued)The General method (continued) Select:

Selects an element from a[ ] and removes it.Selection is optimized to satisfy an objective function.

Feasible:True if selected value can be included in the solution vector, False otherwise.

Union:Combines value with solution and updates objective function.

3. Continuous Knapsack Problem3. Continuous Knapsack Problem

Continuous Knapsack ProblemContinuous Knapsack Problem

Environment Object (i):

Total Weight wi

Total Profit pi

Fraction of object (i) is continuous (0 =< xi <= 1)

A Number of Objects

1 =< i <= n A knapsack

Capacity m

The problemThe problem Problem Statement:

For n objects with weights wi and profits pi, obtain the set of fractions of objects x i which will maximize the total profit without exceeding a total weight m.

Formally:

Obtain the set X = (x1 , x2 , … , xn) that will maximize 1 i n pi xi subject to the constraints:

1 i n wi xi m , 0 xi 1 , 1 i n

Optimal SolutionOptimal Solution

Feasible Solution:

by satisfying constraints. Optimal Solution:

Feasible solution and maximizing profit. Lemma 1:

If 1 i n wi = m then xi = 1 is optimal.

Lemma 2:

An optimal solution will give 1 i n wi xi = m

Greedy AlgorithmGreedy Algorithm

To maximize profit, choose highest p first.

Also choose highest x , i.e., smallest w first.

In other words, let us define the “value” of an object (i) to be the ratio vi = pi/wi and so we choose first the object with the highest vi value.

AlgorithmAlgorithm

GreedyKnapsack ( p[ ] , w[ ] , m , n ,x[ ] ){

insert indices (i) of items in a maximum heap on value vi = pi / wi ;

Zero the vector x; Rem = m ;For k = 1..n{ remove top of heap to get index (i); if (w[i] > Rem) then break; x[i] = 1.0 ; Rem = Rem – w[i] ;}if (k < = n ) x[i] = Rem / w[i] ;

}// T(n) = O(n log n)

ExampleExample n = 3 objects, m = 20 P = (25 , 24 , 15) , W = (18 , 15 , 10),

V = (1.39 , 1.6 ,1.5) Objects in decreasing order of V are {2 , 3 , 1} Set X = {0 ,0 ,0} and Rem = m = 20 K = 1, Choose object i = 2:

w2 < Rem, Set x2 = 1, w2 x2 = 15 , Rem = 5 K = 2, Choose object i = 3:

w3 > Rem, break; K < n , x3 = Rem / w3 = 0.5 Optimal solution is X = (0 , 1.0 , 0.5) , Total profit is 1 i n pi xi = 31.5 Total weight is 1 i n wi xi = m = 20

4. Optimal Merge Patterns4. Optimal Merge Patterns(a) Definitions(a) Definitions Binary Merge Tree:

A binary tree with external nodes representing entities and internal nodes representing merges of these entities.

Optimal Binary Merge Tree:

The sum of paths from root to external nodes is optimal (e.g. minimum). Assuming that the node (i) contributes to the cost by pi and the path from root to such node has length Li, then optimality requires a pattern that minimizes

Optimal Binary Merge TreeOptimal Binary Merge Tree

If the items {A,B,C} contribute to the merge cost by PA , PB , PC, respectively, then the following 3 different patterns will cost:

P1= 2(PA+PB)+PC P2 = PA+2(PB+PC) P3 = 2PA+PB+2PC

Which of these merge patterns is optimal?

(b) Optimal Merging of Lists(b) Optimal Merging of Lists

Lists {A,B,C} have lengths 30,25,10, respectively. The cost of merging two lists of lengths n,m is n+m. The following 3 different merge patterns will cost:

P1= 2(30+25)+10 = 120 P2 = 30+2(25+10) = 100 P3 = 25+2(30+10) = 105

P2 is optimal so that the merge order is {{B,C},A}.

The Greedy MethodThe Greedy Method

Insert lists and their lengths in a minimum heap of lengths. Repeat

Remove the two lowest length lists (pi ,pj) from heap. Merge lists with lengths (pi,pj) to form a new list with length pij = pi+ pj

Insert pij and its symbols into the heap

until all symbols are merged into one final list

B 25 A 30

A 30 BC 35 BCA 65

Notice that both Lists (B : 25 elements) and (C : 10 elements) have been merged (moved) twice

List (A : 30 elements) has been merged (moved) only once.

Hence the total number of element moves is 100. This is optimal among the other merge patterns.

(c) Huffman Coding(c) Huffman CodingTerminologyTerminology Symbol:

A one-to-one representation of a single entity. Alphabet:

A finite set of symbols. Message:

A sequence of symbols. Encoding:

Translating symbols to a string of bits. Decoding:

The reverse.

Encoding:a 00b 01c 10d 11

Decoding:0110001100b c a d a

This is fixed length coding

Example: Coding Tree for 4-Symbol Example: Coding Tree for 4-Symbol Alphabet (a,b,c,d)Alphabet (a,b,c,d)

a b c d

Coding Efficiency & RedundancyCoding Efficiency & Redundancy

Li =Length of path from root to symbol (i) = no. of bits representing that symbol.

Pi = probability of occurrence of symbol (i) in message.

n = size of alphabet. < L > = Average Symbol Length = 1 i n Pi Li

bits/symbol (bps) For fixed length coding, Li = L = constant, < L > = L

(bps) Is this optimal (minimum) ? Not necessarily.

The absolute minimum < L > in a message is called the Entropy.

The concept of entropy as a measure of the average content of information in a message has been introduced by Claude Shannon (1948).

Shannon's entropy represents an absolute limit on the best possible lossless compression of any communication. It is computed as:

loglog11

Coding Efficiency: = H / < L > 0 1 Coding Redundancy: R = 1 - 0 R 1

Actual <L>

Optimal <L>

Perfect <L>

Example: Fixed Length CodingExample: Fixed Length Coding

4- Symbol Alphabet (a,b,c,d). All symbols have the same length L = 2 bits

Message : abbcaada

< L > = 2 (bps)

Symbol (i) pi -log pi -pi log pi code Li

a 0.5 1 0.5 00 2

b 0.25 2 0.5 01 2

c 0.125 3 0.375 10 2

d 0.125 3 0.375 11 2

H = 1.75

ExampleExample Entropy

H = 0.5 + 0.5 + 0.375 + 0.375 = 1.75 (bps),

Coding Efficiency

= H / < L > = 1.75 / 2 = 0.875, Coding Redundancy

R = 1 – 0.875 = 0.125 This is not optimal

ResultResult Fixed length coding is optimal (perfect) only when all

symbol probabilities are equal.

To prove this:

With n = 2m symbols, L = m bits and <L> = m (bps).

If all probabilities are equal,

log,21

HHence

Variable Length CodingVariable Length Coding ((Huffman Coding)Huffman Coding)

The problem: Given a set of symbols and their

probabilities Find a set of binary codewords

that minimize the average length of the symbols

Variable Length CodingVariable Length Coding ((Huffman Coding)Huffman Coding)Formally: Input: A message M(A,P) with

a symbol alphabet A = {a1,a2,…,an} of size (n)

a set of probabilities for the symbols P = {p1,p2,….pn} Output: A set of binary codewords C = {c1,c2,….cn}

with bit lengths L = {L1,L2,….Ln} Condition:

Minimize

Variable Length CodingVariable Length Coding ((Huffman Coding)Huffman Coding) To achieve optimality, we use optimal

binary merge trees to code symbols of unequal probabilities.

Huffman Coding: More frequent symbols occur nearer to the root ( shorter code lengths), less frequent symbols occur at deeper levels (longer code lengths).

Store each symbol in a parentless node of a binary tree. Insert symbols and their probabilities in a minimum heapof probabilities. Repeat

Remove lowest two probabilities (pi ,pj) from heap. Merge symbols with (pi,pj) to form a new symbol (aiaj) with

probability pij = pi+ pj

Store symbol (aiaj) in a parentless node with two children ai and aj

Insert pij and its symbols into the heap

until all symbols are merged into one final alphabet (root) Trace path from root to each leaf (symbol) to form the bit string for

that symbol. Concatenate “0” for a left branch, and “1” for a right branch.

Example (1):Example (1):

4- Symbol Alphabet A = {a, b, c, d} of size (4). Message M(A,P) : abbcaada, P = {0.5, 0.25, 0.125, 0.125} H = 1.75

Symbol (i) pi -log pi -pi log pi

a 0.5 1 0.5

b 0.25 2 0.5

c 0.125 3 0.375

d 0.125 3 0.375

Building The Optimal Merge TableBuilding The Optimal Merge Table

si pi si pi si pi si pi

d 0.125

c 0.125 cd 0.25

b 0.25 b 0.25 bcd 0.5

a 0.5 a 0.5 a 0.5 abcd 1.0

Optimal Merge Tree for Example(1)Optimal Merge Tree for Example(1)Example:

a (50%), b (25%), c (12.5%), d (12.5%)

a b c d

a (50%), b (25%), c (12.5%), d (12.5%)

a b c d

a (50%), b (25%), c (12.5%), d (12.5%)

ai ci Li

(bits)

b 10 2

c 110 3

d 111 3

Coding Efficiency for Example(1)Coding Efficiency for Example(1)

< L > = ( 1* 0.5 + 2 * 0.25 + 3 * 0.125 + 3 * 0.125) = 1.75 (bps)

H = 0.5 + 0.5 + 0.375 + 0.375 = 1.75 (bps), = H / < L > = 1.75 / 1.75 = 1.00 , R = 0.0

Notice that:

Symbols exist at leaves, i.e., no symbol code is the prefix of another symbol code.

This is why the method is also called

“prefix coding”

AnalysisAnalysis

The cost of insertion in a minimum heap is O(n logn)

The repeat loop is done (n-1) times.

In each iteration, the worst case removal of the least two elements is 2 logn and the insertion of the merged element is logn

Hence, the complexity of the Huffman algorithm is

O(n logn)

Example (2):Example (2):

4- Symbol Alphabet A = {a, b, c, d} of size (4). P = {0.4, 0.25, 0.18, 0.17} H = 1.909

Symbol (i) pi -log pi -pi log pi

a 0.40 1.322 0.5288

b 0.25 2 0.5

c 0.18 2.474 0.4453

d 0.17 2.556 0.4345

Example(2): Merge TableExample(2): Merge Table

si pi si pi si pi si pi

d 0.17

c 0.18 b 0.25

b 0.25 cd 0.35 a 0.40

a 0.40 a 0.40 cdb 0.60 cdba 1.0

Optimal Merge Tree for Example(2)Optimal Merge Tree for Example(2)

ai ci Li

(bits)

b 01 2

c 001 3

d 000 3

Coding Efficiency for Example(2)Coding Efficiency for Example(2)

a (40%), b (25%), c (18%), d (17%)<L> = 1.95 bps (Optimal)H = 1.909 = 97.9 %R = 2.1 %Coding is optimal (97.9%) but not perfect

Important Result:Perfect coding ( = 100 %) can be achieved only forprobability values of the form 2- m (1/2, ¼, 1/8,…etc

File CompressionFile Compression

Variable Length Codes can be used to compress files. Symbols are initially coded using ASCII (8-bit) fixed length codes.

Steps:

1. Determine Probabilities of symbols in file.

2. Build Merge Tree (or Table)

3. Assign variable length codes to symbols.

4. Encode symbols using new codes.

5. Save coded symbols in another file together with the symbol code table.

The Compression Ratio = < L > / 8

Huffman Coding AnimationsHuffman Coding Animations

For examples of animations of Huffman coding, see:

http://www.cs.pitt.edu/~kirk/cs1501/animations

Huffman.html

http://peter.bittner.it/tugraz/huffmancoding.html

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