## The Mastery of Computer Programming: Primary Algorithms - Sykalo Eugene 2023

# Knapsack problem

Dynamic Programming

## Background

Before diving into the Knapsack problem, it is essential to establish a foundation in the fundamental algorithms of computer programming. This includes a thorough understanding of data structures, such as arrays, linked lists, and trees. Sorting algorithms, including quicksort, mergesort, and heapsort, are also essential to know as they are used frequently in many optimization problems. Another critical concept in programming is dynamic programming, which is a technique used to solve many optimization problems, including the Knapsack problem. By understanding these fundamental algorithms, programmers can develop strategies to solve complex problems like the Knapsack problem.

## The Knapsack Problem

The Knapsack problem is a classic optimization problem that involves selecting items to maximize value while fitting within a given capacity. The problem gets its name from the analogy of a thief trying to fill a knapsack with the most valuable items without overloading it.

### 0/1 Knapsack Problem

The 0/1 Knapsack problem is a variation of the Knapsack problem where each item can only be selected once or not at all. In other words, there is only one copy of each item, and the thief must decide whether to take it or leave it. This problem can be solved using dynamic programming techniques.

### Unbounded Knapsack Problem

The Unbounded Knapsack problem is another variation of the Knapsack problem where there are an unlimited number of copies of each item. In other words, the thief can take as many of each item as they want. This problem can also be solved using dynamic programming techniques.

### Applications

The Knapsack problem has numerous applications in real-world scenarios, such as resource allocation, production planning, and portfolio optimization. In resource allocation, the problem can be applied to determine the optimal use of limited resources to maximize profits. In production planning, the problem can be used to determine the optimal selection of products to produce to maximize revenue. In portfolio optimization, the problem can be used to determine the optimal selection of financial assets to invest in to maximize returns.

### Complexity

The Knapsack problem is an NP-hard problem, which means there is no known algorithm that can solve it in polynomial time. However, dynamic programming techniques can be used to solve the problem efficiently for small to medium-sized instances. For larger instances, heuristics and approximation algorithms can be used to find near-optimal solutions.

## Dynamic Programming Techniques

Dynamic programming is a powerful technique that can be used to solve a wide range of optimization problems, including the Knapsack problem. The basic principle of dynamic programming is to break down a complex problem into smaller subproblems and solve each subproblem only once, storing the solutions in a table for future reference. This approach can significantly reduce the number of computations required to solve the problem, making it more efficient.

To apply dynamic programming to the Knapsack problem, we must first define the subproblems. In the case of the 0/1 Knapsack problem, the subproblems can be defined as follows: given the first i items and a knapsack of size j, what is the maximum value that can be obtained? We can then use the solutions to these subproblems to build up the solution to the larger problem.

To solve these subproblems, we can use a table to store the solutions. The table has i+1 rows and j+1 columns, where each cell (i,j) stores the maximum value that can be obtained using the first i items and a knapsack of size j. We can fill in the table row by row, starting with the base case (i=0) and filling in the remaining cells using the recurrence relation:

If the weight of the i-th item is greater than the remaining capacity j, then the maximum value that can be obtained with the first i items and a knapsack of size j is the same as the maximum value that can be obtained with the first (i-1) items and a knapsack of size j.

If the weight of the i-th item is less than or equal to the remaining capacity j, then the maximum value that can be obtained with the first i items and a knapsack of size j is the maximum of:

- The maximum value that can be obtained with the first (i-1) items and a knapsack of size j (i.e., not taking the i-th item)
- The maximum value that can be obtained with the first (i-1) items and a knapsack of size (j-w[i]) plus the value of the i-th item (i.e., taking the i-th item)

Once we have filled in the entire table, the solution to the original problem (i.e., the maximum value that can be obtained using all n items and a knapsack of size W) can be found in the cell (n,W).

## Implementation Strategies

Implementing dynamic programming algorithms to solve the Knapsack problem can be challenging. In this section, we will explore different implementation strategies, including top-down and bottom-up approaches.

### Top-Down Approach

The top-down approach, also known as memoization, involves breaking down the problem into smaller subproblems and then recursively solving each subproblem. As each subproblem is solved, its solution is stored in a table for future reference, which can significantly reduce the number of computations required to solve the problem.

The top-down approach can be implemented using a recursive function that takes the current item and the remaining capacity of the knapsack as arguments. The function checks if the solution to the current subproblem has already been computed and stored in the table. If so, it returns the stored solution. Otherwise, it computes the solution recursively by considering two cases: taking the current item or not taking the current item. The function then stores the solution in the table and returns it.

### Bottom-Up Approach

The bottom-up approach involves solving the subproblems in a specific order and building up the solution to the larger problem. This approach can be more efficient than the top-down approach in some cases because it avoids the overhead of recursive function calls.

The bottom-up approach can be implemented using a table to store the solutions to the subproblems. We start by solving the base case (i=0) and filling in the first row of the table. We then move on to the next row (i=1) and use the solutions from the previous row to solve the current row. We continue this process until we have solved all subproblems and computed the solution to the original problem.

### Which Approach to Use?

The choice between the top-down and bottom-up approaches depends on the specific problem and the available resources. In general, the top-down approach is more intuitive and easier to understand, but it can be less efficient than the bottom-up approach due to the overhead of recursive function calls. The bottom-up approach, on the other hand, can be more efficient, but it requires more memory to store the solutions in a table.

In practice, it is often useful to implement both approaches and compare their performance for a given problem. This can help identify the most efficient approach for the specific problem and the available resources.