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

# Average and worst case analysis

Algorithmic Complexity and Information Theory

This section provides an overview of the chapter, introducing the importance of fundamental algorithms in computer programming and their relevance to algorithmic complexity and information theory.

The chapter aims to provide a comprehensive understanding of fundamental algorithms, which are essential building blocks of computer programming. The algorithms discussed in this chapter are used in a wide range of applications, from data processing to artificial intelligence.

Algorithmic complexity and information theory are two areas of study that are closely related to fundamental algorithms. Algorithmic complexity is concerned with the resources (such as time and space) required to execute an algorithm, while information theory is concerned with the amount of information that can be transmitted over a communication channel.

In this context, understanding fundamental algorithms is crucial for optimizing the performance of computer programs and ensuring efficient use of resources. Additionally, understanding fundamental algorithms can help in the development of new and innovative applications of computer programming.

## Section 2: Basic Concepts in Algorithm Analysis

This section will introduce basic concepts in algorithm analysis, such as algorithmic efficiency, time complexity, and space complexity. It will provide an overview of the different methods used to analyze algorithms.

### Algorithmic Efficiency

Algorithmic efficiency refers to how well an algorithm performs in terms of the resources it requires to execute. Resources can include time, memory, and processing power. Efficient algorithms are those that can perform a task using the fewest possible resources.

### Time Complexity

Time complexity is a measure of the amount of time required to execute an algorithm as a function of the size of its input. It is typically expressed using big O notation, which provides an upper bound on the time required to execute the algorithm. For example, an algorithm that has a time complexity of O(n) will take no more than n units of time to execute, where n is the size of the input.

### Space Complexity

Space complexity is a measure of the amount of memory required to execute an algorithm as a function of the size of its input. It is also typically expressed using big O notation, which provides an upper bound on the amount of memory required to execute the algorithm.

### Methods of Algorithm Analysis

There are several methods used to analyze algorithms, including:

- Empirical analysis: This involves running the algorithm on various inputs and measuring its performance. This method is useful for gaining insights into how the algorithm behaves in practice.
- Mathematical analysis: This involves using mathematical techniques to derive formulas for the time and/or space complexity of an algorithm. This method is useful for obtaining precise estimates of an algorithm's performance.
- Asymptotic analysis: This involves analyzing the performance of an algorithm as the size of its input goes to infinity. This method is useful for obtaining a high-level understanding of an algorithm's performance and for comparing the performance of different algorithms.

By understanding these basic concepts and methods of algorithm analysis, programmers can gain insights into the performance of their algorithms and optimize them for efficiency.

## Section 3: Average Case Analysis

Average case analysis is a method of analyzing the performance of an algorithm by considering the expected runtime over all possible inputs. This is in contrast to worst-case analysis, which considers the performance over the input that leads to the worst runtime. Average case analysis is useful when the input is not known in advance and the algorithm will be run on a variety of inputs.

To perform average case analysis, we must first define a probability distribution over the inputs. This distribution represents the likelihood of each input occurring. We can then calculate the expected runtime by taking the weighted average of the runtimes over all possible inputs, where the weights are the probabilities of each input occurring.

For example, consider an algorithm that searches for a target element in an array of n elements. If the target element is present in the array, the algorithm will stop as soon as it finds the target. If the target element is not present, the algorithm will search the entire array. The worst-case runtime of this algorithm is O(n), as it may have to search the entire array. However, if we assume that the target element is equally likely to be any element in the array, the average-case runtime is O(n/2), as the algorithm will, on average, search half the array before finding the target.

It is important to note that average case analysis requires a probability distribution over the inputs, which may not always be known or easy to define. In addition, the expected runtime calculated using average case analysis may not be a good approximation of the actual runtime for any specific input. Despite these limitations, average case analysis can be a useful tool for understanding the performance of an algorithm in situations where the input is not known in advance.

## Section 4: Worst Case Analysis

Worst case analysis is a method of analyzing the performance of an algorithm by considering the longest possible runtime over all possible inputs. This is in contrast to average case analysis, which considers the expected runtime over all possible inputs. Worst case analysis is useful when we need to ensure that the algorithm will always finish within a certain time limit, regardless of the input.

To perform worst case analysis, we must first identify the input that leads to the longest possible runtime. We can then calculate the runtime of the algorithm on this input. The worst-case runtime is the maximum runtime over all possible inputs.

For example, consider an algorithm that sorts an array of n elements using the bubble sort algorithm. The worst-case runtime of the bubble sort algorithm is O(n^2), as it may have to perform n^2 comparisons and swaps to sort the array. This worst-case occurs when the input array is already sorted in reverse order.

It is important to note that worst case analysis provides a guarantee on the maximum runtime of an algorithm, regardless of the input. However, the worst-case runtime may not be a good approximation of the actual runtime for most inputs. In practice, we often use worst case analysis as a safety measure to ensure that the algorithm will always finish within a certain time limit, even if the input is unfavorable.

## Section 5: Comparing Average and Worst Case Analysis

Average case and worst case analysis are two methods for analyzing the performance of algorithms. Average case analysis considers the expected runtime over all possible inputs, while worst case analysis considers the longest possible runtime over all possible inputs. In this section, we will compare and contrast these two methods and discuss the pros and cons of each.

### Pros and Cons of Average Case Analysis

One advantage of average case analysis is that it provides a more realistic estimate of an algorithm's performance than worst case analysis. In many cases, the expected runtime is closer to the actual runtime than the worst-case runtime. Average case analysis is also useful when the input is not known in advance and the algorithm will be run on a variety of inputs.

However, there are also some disadvantages to average case analysis. First, it requires a probability distribution over the inputs, which may not always be known or easy to define. Second, the expected runtime calculated using average case analysis may not be a good approximation of the actual runtime for any specific input. Finally, average case analysis may not be appropriate for algorithms that have non-linear runtime behavior, as the expected runtime may not be well-defined.

### Pros and Cons of Worst Case Analysis

One advantage of worst case analysis is that it provides a guarantee on the maximum runtime of an algorithm, regardless of the input. This is useful when we need to ensure that the algorithm will always finish within a certain time limit, even if the input is unfavorable. Worst case analysis is also relatively easy to perform, as it only requires identifying the input that leads to the longest possible runtime.

However, there are also some disadvantages to worst case analysis. First, the worst-case runtime may not be a good approximation of the actual runtime for most inputs. Second, worst case analysis does not provide any information about the expected runtime or the runtime on typical inputs. Finally, worst case analysis may be overly pessimistic for algorithms that have good average-case behavior.

### When to Use Each Method

In general, average case analysis is more appropriate for algorithms that are expected to be run on a variety of inputs and for which a probability distribution over the inputs can be defined. Worst case analysis is more appropriate for algorithms that need to operate under strict time constraints or for which the worst-case behavior is a critical consideration.

It is also worth noting that in some cases, a combination of both methods may be appropriate. For example, we may use average case analysis to estimate the expected runtime of an algorithm and worst case analysis to provide a guarantee on the maximum runtime.