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Example of Matrix Chain Multiplication

Example: We are given the sequence {4, 10, 3, 12, 20, and 7}. The matrices have size 4 x 10, 10 x 3, 3 x 12, 12 x 20, 20 x 7. We need to compute M [i,j], 0 ≤ i, j≤ 5. We know M [i, i] = 0 for all i.

Example of Matrix Chain Multiplication

Let us proceed with working away from the diagonal. We compute the optimal solution for the product of 2 matrices.

Example of Matrix Chain Multiplication

Here P0 to P5 are Position and M1 to M5 are matrix of size (pi to pi-1)

On the basis of sequence, we make a formula

Example of Matrix Chain Multiplication

In Dynamic Programming, initialization of every method done by '0'.So we initialize it by '0'.It will sort out diagonally.

We have to sort out all the combination but the minimum output combination is taken into consideration.

Calculation of Product of 2 matrices:
1. m (1,2) = m1  x m2
           = 4 x 10 x  10 x 3
           = 4 x 10 x 3 = 120
		   
2. m (2, 3) = m2 x m3
            = 10 x 3  x  3 x 12
            = 10 x 3 x 12 = 360
			
3. m (3, 4) = m3 x m4 
            = 3 x 12  x  12 x 20
            = 3 x 12 x 20 = 720
			
4. m (4,5) = m4 x m5
           = 12 x 20  x  20 x 7
           = 12 x 20 x 7 = 1680
Example of Matrix Chain Multiplication
  • We initialize the diagonal element with equal i,j value with '0'.
  • After that second diagonal is sorted out and we get all the values corresponded to it

Now the third diagonal will be solved out in the same way.

Now product of 3 matrices:

M [1, 3] = M1 M2 M3
  1. There are two cases by which we can solve this multiplication: ( M1 x M2) + M3, M1+ (M2x M3)
  2. After solving both cases we choose the case in which minimum output is there.
Example of Matrix Chain Multiplication

M [1, 3] =264

As Comparing both output 264 is minimum in both cases so we insert 264 in table and ( M1 x M2) + M3 this combination is chosen for the output making.

M [2, 4] = M2 M3 M4
  1. There are two cases by which we can solve this multiplication: (M2x M3)+M4, M2+(M3 x M4)
  2. After solving both cases we choose the case in which minimum output is there.
DAA Example of Matrix Chain Multiplication

M [2, 4] = 1320

As Comparing both output 1320 is minimum in both cases so we insert 1320 in table and M2+(M3 x M4) this combination is chosen for the output making.

M [3, 5] = M3  M4  M5
  1. There are two cases by which we can solve this multiplication: ( M3 x M4) + M5, M3+ ( M4xM5)
  2. After solving both cases we choose the case in which minimum output is there.
Example of Matrix Chain Multiplication
M [3, 5] = 1140

As Comparing both output 1140 is minimum in both cases so we insert 1140 in table and ( M3 x M4) + M5this combination is chosen for the output making.

Example of Matrix Chain Multiplication

Now Product of 4 matrices:

M [1, 4] = M1  M2 M3 M4

There are three cases by which we can solve this multiplication:

  1. ( M1 x M2 x M3) M4
  2. M1 x(M2 x M3 x M4)
  3. (M1 xM2) x ( M3 x M4)

After solving these cases we choose the case in which minimum output is there

Example of Matrix Chain Multiplication

M [1, 4] =1080

As comparing the output of different cases then '1080' is minimum output, so we insert 1080 in the table and (M1 xM2) x (M3 x M4) combination is taken out in output making,

M [2, 5] = M2 M3 M4 M5

There are three cases by which we can solve this multiplication:

  1. (M2 x M3 x M4)x M5
  2. M2 x( M3 x M4 x M5)
  3. (M2 x M3)x ( M4 x M5)

After solving these cases we choose the case in which minimum output is there

DAA Example of Matrix Chain Multiplication
M [2, 5] = 1350

As comparing the output of different cases then '1350' is minimum output, so we insert 1350 in the table and M2 x( M3 x M4 xM5)combination is taken out in output making.

DAA Example of Matrix Chain Multiplication

Now Product of 5 matrices:

M [1, 5] = M1  M2 M3 M4 M5

There are five cases by which we can solve this multiplication:

  1. (M1 x M2 xM3 x M4 )x M5
  2. M1 x( M2 xM3 x M4 xM5)
  3. (M1 x M2 xM3)x M4 xM5
  4. M1 x M2x(M3 x M4 xM5)

After solving these cases we choose the case in which minimum output is there

DAA Example of Matrix Chain Multiplication
M [1, 5] = 1344

As comparing the output of different cases then '1344' is minimum output, so we insert 1344 in the table and M1 x M2 x(M3 x M4 x M5)combination is taken out in output making.

Final Output is:

DAA Example of Matrix Chain Multiplication

Step 3: Computing Optimal Costs: let us assume that matrix Ai has dimension pi-1x pi for i=1, 2, 3....n. The input is a sequence (p0,p1,......pn) where length [p] = n+1. The procedure uses an auxiliary table m [1....n, 1.....n] for storing m [i, j] costs an auxiliary table s [1.....n, 1.....n] that record which index of k achieved the optimal costs in computing m [i, j].

The algorithm first computes m [i, j] ← 0 for i=1, 2, 3.....n, the minimum costs for the chain of length 1.





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