Dynamic Programming An algorithm design paradigm like divide-and-conquer “Programming”: A...

Dynamic Programming

• An algorithm design paradigm like divide-and-conquer• “Programming”: A tabular method (not writing computer

code)• Divide-and-Conquer (DAC): subproblems are independent• Dynamic Programming (DP): subproblems are not

independent• Overlapping subproblems: subproblems share sub-

subproblems• In solving problems with overlapping subproblems

• A DAC algorithm does redundant work– Repeatedly solves common subproblems

• A DP algorithm solves each problem just once– Saves its result in a table

BIL741: Advanced Analysis of Algorithms I (İleri Algoritma Çözümleme I) 1

Optimization Problems

• DP typically applied to optimization problems• In an optimization problem

– There are many possible solutions (feasible solutions)– Each solution has a value– Want to find an optimal solution to the problem

• A solution with the optimal value (min or max value)

– Wrong to say “the” optimal solution to the problem• There may be several solutions with the same optimal value

Development of a DP Algorithm

1. Characterize the structure of an optimal solution

2. Recursively define the value of an optimal solution

3. Compute the value of an optimal solution in a bottom-up fashion

4. Construct an optimal solution from the information computed in Step 3

Example: Matrix-chain Multiplication

Matrix-chain Multiplication: An ExampleParenthesization

Cost of Multiplying two Matrices

Matrix-chain Multiplication Problem

Counting the Number of Parenthesizations

Number of Parenthesizations

The Structure of an Optimal Parenthesization

Step 1: Characterize the structure of an optimal solution

•Ai..j : matrix that results from evaluating the product Ai Ai+1 Ai+2 Aj

•An optimal parenthesization of the product A1A2An

– Splits the product between Ak and Ak1, for some 1≤k<n

(A1A2A3 Ak) · (Ak+1Ak+2 ... An)

– i.e., first compute A1..k and Ak+1..n and then multiply these two

•The cost of this optimal parenthesizationCost of computing A1..k

+ Cost of computing Ak+1..n

+ Cost of multiplying A1..k · Ak+1..n

Step 1: Characterize the Structure of an Optimal Solution

The Structure of an Optimal Parenthesization

Step 2: Define Value of an Optimal Sol. Recursively(mij =?)

Step 2: Recursive Equation for mij

Step 2: mij = MIN{mik + mk+1, j +pi-1pk pj}

Computing the Optimal Cost (Matrix-Chain Multiplication)

Algorithm for Computing the Optimal Costs

l 2for i 1 to n - 1m[i, i1] compute m[i, i1]for k i to i do {m[1, 2], m[2, 3], …, m[n-1, n]}

. (n-1) valuesl 3for i 1 to n - 2m[i, i2] compute m[i, i2]for k i to i1 do {m[1, 3], m[2, 4], …, m[n-2, n]}

. (n-2) valuesl 4for i 1 to n - 3m[i, i3] compute m[i, i3]for k i to i2 do {m[1, 4], m[2, 5], …, m[n-3, n]}

. (n-3) values

Constructing an Optimal Solution

Example: Recursive Construction of an Optimal Solution

return A6

Elements of Dynamic Programming

• When should we look for a DP solution to an optimization problem?

• Two key ingredients for the problem

– Optimal substructure

– Overlapping subproblems

Optimal Substructure

Overlapping Subproblems

Recursive Matrix-chain Order

Running Time of RMC

Memoized Recursive Algorithm

• Offers the efficiency of the usual DP approach while maintaining top-down strategy

• Maintains an entry in a table for the soln to each subproblem

• Each table entry contains a special value to indicate that the entry has yet to be filled in

• When the subproblem is first encountered its solution is computed and then stored in the table

• Each subsequent time that the subproblem encountered the value stored in the table is simply looked up and returned

• The approach assumes that– The set of all possible subproblem parameters are known– The relation between the table positions and subproblems is

establishedBIL741: Advanced Analysis of Algorithms I (İleri Algoritma Çözümleme I) 45

Memoized Recursive Algorithm

Elements of Dynamic Programming: Summary

Longest Common Subsequence

Characterizing a Longest Common Subsequence

Longest Common Subsequence Algorithm

A Recursive Solution to Subproblems

Computing the Length of an LCS

BIL741: Advanced Analysis of Algorithms I (İleri Algoritma Çözümleme I)57

Constructing an LCS

This improvement works if we only need the length of an LCS

Dynamic Programming An algorithm design paradigm like divide-and-conquer “Programming”: A...

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