Lec38

Dynamic Programming
• Steps.
View the problem solution as the result of a sequence
of decisions.
Obtain a formulation for the problem state.
Verify that the principle of optimality holds.
Set up the dynamic programming recurrence
equations.
Solve these equations for the value of the optimal
solution.
 Perform a traceback to determine the optimal
solution.

Dynamic Programming
• When solving the dynamic programming
recurrence recursively, be sure to avoid the
recomputation of the optimal value for the
same problem state.
• To minimize run time overheads, and hence
to reduce actual run time, dynamic
programming recurrences are almost always
solved iteratively (no recursion).

0/1 Knapsack Recurrence
• If wn <= y, f(n,y) = pn.
• If wn > y, f(n,y) = 0.
• When i < n
 f(i,y) = f(i+1,y) whenever y < wi.
 f(i,y) = max{f(i+1,y), f(i+1,y-wi) + pi}, y >= wi.
• Assume the weights and capacity are integers.
• Only f(i,y)s with 1 <= i <= n and 0 <= y <= c
are of interest.

Iterative Solution Example
• n = 5, c = 8, w = [4,3,5,6,2], p = [9,7,10,9,3]
f[i][y] 0 1 2 3 4 5 6 7 8
y
5
4
3
2
1
i

Compute f[5][*]
• n = 5, c = 8, w = [4,3,5,6,2], p = [9,7,10,9,3]
f[i][y] 0 1 2 3 4 5 6 7 8
y
5
4
3
2
1
i
0 0 3 3 3 3 3 3 3

Compute f[4][*]
• n = 5, c = 8, w = [4,3,5,6,2], p = [9,8,10,9,3]
f[i][y] 0 1 2 3 4 5 6 7 8
y
5
4
3
2
1
i
0 0 3 3 3 3 3 3 3
0 0 3 3 3 3 9 9 12
f(i,y) = max{f(i+1,y), f(i+1,y-wi) + pi}, y >= wi

Compute f[3][*]
• n = 5, c = 8, w = [4,3,5,6,2], p = [9,8,10,9,3]
f[i][y] 0 1 2 3 4 5 6 7 8
y
5
4
3
2
1
i
0 0 3 3 3 3 3 3 3
0 0 3 3 3 3 9 9 12
0 0 3 3 3 10 10 13 13

Compute f[2][*]
• n = 5, c = 8, w = [4,3,5,6,2], p = [9,8,10,9,3]
f[i][y] 0 1 2 3 4 5 6 7 8
y
5
4
3
2
1
i
0 0 3 3 3 3 3 3 3
0 0 3 3 3 3 9 9 12
0 0 3 3 3 10 10 13 13
0 0 3 8 8 11 11 13 18

Compute f[1][c]
• n = 5, c = 8, w = [4,3,5,6,2], p = [9,8,10,9,3]
f[i][y] 0 1 2 3 4 5 6 7 8
y
5
4
3
2
1
i
0 0 3 3 3 3 3 3 3
0 0 3 3 3 3 9 9 12
0 0 3 3 3 10 10 13 13
0 0 3 8 8 11 11 13 18
18

Traceback
• n = 5, c = 8, w = [4,3,5,6,2], p = [9,8,10,9,3]
f[i][y] 0 1 2 3 4 5 6 7 8
y
5
4
3
2
1
i
0 0 3 3 3 3 3 3 3
0 0 3 3 3 3 9 9 12
0 0 3 3 3 10 10 13 13
0 0 3 8 8 11 11 13 18
18
f[1][8] = f[2][8] => x1 = 0

Traceback
• n = 5, c = 8, w = [4,3,5,6,2], p = [9,8,10,9,3]
f[i][y] 0 1 2 3 4 5 6 7 8
y
5
4
3
2
1
i
0 0 3 3 3 3 3 3 3
0 0 3 3 3 3 9 9 12
0 0 3 3 3 10
10 13 13
0 0 3 8 8 11 11 13 18
18
f[2][8] != f[3][8] => x2 = 1

Traceback
• n = 5, c = 8, w = [4,3,5,6,2], p = [9,8,10,9,3]
f[i][y] 0 1 2 3 4 5 6 7 8
y
5
4
3
2
1
i
0 0 3 3 3 3 3 3 3
0
0 3 3 3 3 9 9 12
0 0 3 3 3 10 10 13 13
0 0 3 8 8 11 11 13 18
18
f[3][5] != f[4][5] => x3 = 1

Traceback
• n = 5, c = 8, w = [4,3,5,6,2], p = [9,8,10,9,3]
f[i][y] 0 1 2 3 4 5 6 7 8
y
5
4
3
2
1
i
0 3 3 3 3 3 3 3
0
0 0 3 3 3 3 9 9 12
0 0 3 3 3 10 10 13 13
0 0 3 8 8 11 11 13 18
18
f[4][0] = f[5][0] => x4 = 0

Traceback
• n = 5, c = 8, w = [4,3,5,6,2], p = [9,8,10,9,3]
f[i][y] 0 1 2 3 4 5 6 7 8
y
5
4
3
2
1
i
0 0 3 3 3 3 3 3 3
0 0 3 3 3 3 9 9 12
0 0 3 3 3 10 10 13 13
0 0 3 8 8 11 11 13 18
18
f[5][0] = 0 => x5 = 0

Complexity Of Traceback
• O(n)

Matrix Multiplication Chains
• Multiply an m x n matrix A and an n x p matrix
B to get an m x p matrix C.
n
C(i,j) = A(i,k) * B(k,j)
k = 1
• We shall use the number of multiplications as
our complexity measure.
• n multiplications are needed to compute one
C(i,j).
• mnp multiplicatons are needed to compute all
mp terms of C.

• Suppose that we are to compute the product X*Y*Z of
three matrices X, Y and Z.
• The matrix dimensions are:
 X:(100 x 1), Y:(1 x 100), Z:(100 x 1)
• Multiply X and Y to get a 100 x 100 matrix T.
 100 * 1 * 100 = 10,000 multiplications.
• Multiply T and Z to get the 100 x 1 answer.
 100 * 100 * 1 = 10,000 multiplications.
• Total cost is 20,000 multiplications.
• 10,000 units of space are needed for T.

• The matrix dimensions are:
 X:(100 x 1)
 Y:(1 x 100)
 Z:(100 x 1)
• Multiply Y and Z to get a 1 x 1 matrix T.
 1 * 100 * 1 = 100 multiplications.
• Multiply X and T to get the 100 x 1 answer.
 100 * 1 * 1 = 100 multiplications.
• Total cost is 200 multiplications.
• 1 unit of space is needed for T.

Product Of 5 Matrices
• Some of the ways in which the product of 5 matrices
may be computed.
 A*(B*(C*(D*E))) right to left
 (((A*B)*C)*D)*E left to right
 (A*B)*((C*D)*E)
 (A*B)*(C*(D*E))
 (A*(B*C))*(D*E)
 ((A*B)*C)*(D*E)

Find Best Multiplication Order
• Number of ways to compute the product of q
matrices is O(4q/q1.5).
• Evaluating all ways to compute the product
takes O(4q/q0.5) time.

An Application
• Registration of pre- and post-operative 3D brain
MRI images to determine volume of removed
tumor.

3D Registration
• Each image has 256 x 256 x 256 voxels.
• In each iteration of the registration algorithm, the
product of three matrices is computed at each
voxel … (12 x 3) * (3 x 3) * (3 x 1)
• Left to right computation => 12 * 3 * 3 + 12 *
3*1 = 144 multiplications per voxel per iteration.
• 100 iterations to converge.

3D Registration
• Total number of multiplications is about 2.4 *
1011.
• Right to left computation => 3 * 3*1 + 12 * 3 * 1
= 45 multiplications per voxel per iteration.
• Total number of multiplications is about 7.5 *
1010.
• With 108 multiplications per second, time is 40
min vs 12.5 min.

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