Problem 16: "Fall Out Pick Up"

The video game "Fallout 3", and its sequel, "Fallout: New Vegas", are set in a post-nuclear-war world where most civilization has been destroyed and the few people remaining alive must scavenge for supplies to stay alive. Each piece of loot that you can pick up has a weight and value. There is a limit to how much weight you can carry, and when you get to a town you can sell your loot for bottle caps (the game's currency) based on its value. The problem of maximizing the total value of items you can pick up while staying within your weight capacity is known as the "knapsack problem" and is a well-known NP-complete problem.

In the game, however, there is an additional complication. You only observe each item one at a time, and because of the dangers and rival scavengers it is impossible to go back and pick up an item that you dropped or left behind. So you have to make a decision about whether to pick up each item, as well as which items to drop in order to make room, as you see each item.

(a) Is there an algorithm that gives the optimal result in all situations? (There is no limit on the running time of the algorithm.)

(b) The "competitive ratio" of an algorithm is the worst-case ratio of the value of the solution found by the algorithm to the optimal solution. For instance, if an algorithm always gives you a set of items with value at least equal to half the value of the optimal solution, then its competitive ratio is 0.5. Does there exist an algorithm for this problem with competitive ratio greater than 0? If so, give one; if not, prove it is impossible.

(Hint: Imagine there was an "adversary" controlling the capacity and sequence of items, and it gets to observe which items the algorithm selects before deciding which new items to present.)

(c) Suppose that the capacity is C, and the highest weight of any one item is M. Give an algorithm for this problem with competitive ratio 1-(M/C).

The solution is here.

## Thursday, February 24, 2011

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