Post on 25-May-2015
Knapsack Problem
Knapsack Problem• In a knapsack problem or rucksack
problem, we are given a set of items, where each item is specified by a size and a value . We are also given a size bound , the size of our knapsack.
Item # Size Value1 1 82 3 63 5 5
Knapsack ProblemThere are two versions of the problem:
1. 0-1 Knapsack Problem2. Fractional Knapsack Problem
i. Bounded Knapsack Problemii. Unbounded Knapsack Problem
Knapsack ProblemSample Problem
A B C D E F Gvalue 7 9 5 12 14 6 12time 3 4 2 6 7 3 5
Solutions to Knapsack Problems
Brute-Force Approach – solve the problem with a straightforward algorithm
Solutions to Knapsack Problems
{}{1, 2, 3, 4,
5}
{}{2, 3, 4, 5}
{}{3, 4, 5}
{2}{3, 4, 5}
{1}{2, 3, 4, 5}
{1}{3, 4, 5}
{1, 2}{3, 4, 5}
Solutions to Knapsack Problems
Greedy Algorithm – keep taking most valuable items until maximum weight is reached or taking the largest value of each item by calculating
Dynamic Programming – solve each sub problem once and store their solutions in an array
ExampleGiven: = 4 (# of elements) = 5 pounds (maximum size)Elements (size, value) = { (1, 200), (3, 240), (2, 140), (5, 150) }
Greedy Algorithm1. Calculate for 2. Sort the items by decreasing 3. Find j, such that
Greedy AlgorithmSample Problem
A B C D
cost200
240
140
150
weight 1 3 2 5
value200
80 70 30
Greedy Algorithm The optimal solution to the
fractional knapsack Not an optimal solution to the 0-1
knapsack
Dynamic ProgrammingRecursive formula for sub problems:
Dynamic Programmingfor = 0 to
V[ 0, ] = 0for = 1 to
V[, 0 ] = 0
Dynamic Programmingfor = 1 to if
if
else
else
ExampleGiven: = 4 (# of elements) = 5 (maximum size)Elements (size, value) = { (2, 3), (3, 4), (4, 5), (5, 6) }
Examplei\s 0 1 2 3 4 5
0
1
2
3
4
Examplefor = 0 to
V[ 0, ] = 0
i\s 0 1 2 3 4 5
0 0 0 0 0 0 0
1
2
3
4
Examplefor = 0 to
V[, 0 ] = 0
i\s 0 1 2 3 4 5
0 0 0 0 0 0 0
1 0
2 0
3 0
4 0
Exampleif if
else
else i\s 0 1 2 3 4 5
0 0 0 0 0 0 0
1 0 0
2 0
3 0
4 0
Items (, )1: (2, 3)2: (3, 4)3: (4, 5)4: (5, 6)
Exampleif
if
else
else
i\s 0 1 2 3 4 5
0 0 0 0 0 0 0
1 0 0 3
2 0
3 0
4 0
Items (, )1: (2, 3)2: (3, 4)3: (4, 5)4: (5, 6)
Exampleif
if
else
else
i\s 0 1 2 3 4 5
0 0 0 0 0 0 0
1 0 0 3 3
2 0
3 0
4 0
Items (, )1: (2, 3)2: (3, 4)3: (4, 5)4: (5, 6)
Exampleif
if
else
else
i\s 0 1 2 3 4 5
0 0 0 0 0 0 0
1 0 0 3 3 3
2 0
3 0
4 0
Items (, )1: (2, 3)2: (3, 4)3: (4, 5)4: (5, 6)
Exampleif if
else
else
i\s 0 1 2 3 4 5
0 0 0 0 0 0 0
1 0 0 3 3 3 3
2 0
3 0
4 0
Items (, )1: (2, 3)2: (3, 4)3: (4, 5)4: (5, 6)
Exampleif if
else
else i\s 0 1 2 3 4 5
0 0 0 0 0 0 0
1 0 0 3 3 3 3
2 0 0
3 0
4 0
Items (, )1: (2, 3)2: (3, 4)3: (4, 5)4: (5, 6)
Exampleif if
else
else i\s 0 1 2 3 4 5
0 0 0 0 0 0 0
1 0 0 3 3 3 3
2 0 0 3
3 0
4 0
Items (, )1: (2, 3)2: (3, 4)3: (4, 5)4: (5, 6)
Exampleif if
else
else
i\s 0 1 2 3 4 5
0 0 0 0 0 0 0
1 0 0 3 3 3 3
2 0 0 3 4
3 0
4 0
Items (, )1: (2, 3)2: (3, 4)3: (4, 5)4: (5, 6)
Example
i\s 0 1 2 3 4 5
0 0 0 0 0 0 0
1 0 0 3 3 3 3
2 0 0 3 4 4
3 0
4 0
if if
else
else
Items (, )1: (2, 3)2: (3, 4)3: (4, 5)4: (5, 6)
Exampleif if
else
else
i\s 0 1 2 3 4 5
0 0 0 0 0 0 0
1 0 0 3 3 3 3
2 0 0 3 4 4 7
3 0
4 0
Items (, )1: (2, 3)2: (3, 4)3: (4, 5)4: (5, 6)
Exampleif if
else
else i\s 0 1 2 3 4 5
0 0 0 0 0 0 0
1 0 0 3 3 3 3
2 0 0 3 4 4 7
3 0 0 3 4
4 0
Items (, )1: (2, 3)2: (3, 4)3: (4, 5)4: (5, 6)
Exampleif if
else
else
i\s 0 1 2 3 4 5
0 0 0 0 0 0 0
1 0 0 3 3 3 3
2 0 0 3 4 4 7
3 0 0 3 4 5
4 0
Items (, )1: (2, 3)2: (3, 4)3: (4, 5)4: (5, 6)
Exampleif if
else
else
i\s 0 1 2 3 4 5
0 0 0 0 0 0 0
1 0 0 3 3 3 3
2 0 0 3 4 4 7
3 0 0 3 4 5 7
4 0
Items (, )1: (2, 3)2: (3, 4)3: (4, 5)4: (5, 6)
Exampleif if
else
else i\s 0 1 2 3 4 5
0 0 0 0 0 0 0
1 0 0 3 3 3 3
2 0 0 3 4 4 7
3 0 0 3 4 5 7
4 0 0 3 4 5
Items (, )1: (2, 3)2: (3, 4)3: (4, 5)4: (5, 6)
Example
i\s 0 1 2 3 4 5
0 0 0 0 0 0 0
1 0 0 3 3 3 3
2 0 0 3 4 4 7
3 0 0 3 4 5 7
4 0 0 3 4 5 7
if if
else
else
Items (, )1: (2, 3)2: (3, 4)3: (4, 5)4: (5, 6)
Dynamic ProgrammingLet and
if
else
Example
i\s 0 1 2 3 4 5
0 0 0 0 0 0 0
1 0 0 3 3 3 3
2 0 0 3 4 4 7
3 0 0 3 4 5 7
4 0 0 3 4 5 7
while if
else
Items (, )1: (2, 3)2: (3, 4)3: (4, 5)4: (5, 6)
Example
i\s 0 1 2 3 4 5
0 0 0 0 0 0 0
1 0 0 3 3 3 3
2 0 0 3 4 4 7
3 0 0 3 4 5 7
4 0 0 3 4 5 7
while if
else
Items (, )1: (2, 3)2: (3, 4)3: (4, 5)4: (5, 6)
Example
i\s 0 1 2 3 4 5
0 0 0 0 0 0 0
1 0 0 3 3 3 3
2 0 0 3 4 4 7
3 0 0 3 4 5 7
4 0 0 3 4 5 7
while if
else
Items (, )1: (2, 3)2: (3, 4)3: (4, 5)4: (5, 6)
Example
i\s 0 1 2 3 4 5
0 0 0 0 0 0 0
1 0 0 3 3 3 3
2 0 0 3 4 4 7
3 0 0 3 4 5 7
4 0 0 3 4 5 7
while if
else
Items (, )1: (2, 3)2: (3, 4)3: (4, 5)4: (5, 6)
Example
i\s 0 1 2 3 4 5
0 0 0 0 0 0 0
1 0 0 3 3 3 3
2 0 0 3 4 4 7
3 0 0 3 4 5 7
4 0 0 3 4 5 7
while if
else
Items (, )1: (2, 3)2: (3, 4)3: (4, 5)4: (5, 6)
Example
i\s 0 1 2 3 4 5
0 0 0 0 0 0 0
1 0 0 3 3 3 3
2 0 0 3 4 4 7
3 0 0 3 4 5 7
4 0 0 3 4 5 7
while if
else
Items (, )1: (2, 3)2: (3, 4)3: (4, 5)4: (5, 6)
Example
i\s 0 1 2 3 4 5
0 0 0 0 0 0 0
1 0 0 3 3 3 3
2 0 0 3 4 4 7
3 0 0 3 4 5 7
4 0 0 3 4 5 7
The optimal knapsack should contain {1,2} = 7
Items (, )1: (2, 3)2: (3, 4)3: (4, 5)4: (5, 6)
Knapsack ProblemCommon Applications• Resource allocation with financial
constraints• Construction and scoring of
heterogeneous test• Selection of capital investments