05-2 Intersecting Two Lists Algorithm

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Questions and Answers

What is the primary goal of the Intersecting two Lists (I2L) algorithm?

  • To merge two lists into a single, combined list.
  • To remove duplicate elements from two lists.
  • To sort the elements of two lists into alphabetical order.
  • To identify and extract the common elements present in both lists. (correct)

In the Intersecting two Lists (I2L) algorithm, what is the initial step for comparing elements?

  • Remove any duplicate entries from both lists.
  • Place a marker at the beginning of each list. (correct)
  • Compare the last elements of each list.
  • Sort both lists in ascending order.

During the execution of the Intersecting two Lists (I2L) algorithm, if the elements pointed to by the markers are different, what action is taken?

  • The algorithm terminates and reports an error.
  • The right marker is advanced to the next element in its list. (correct)
  • Both markers are reset to the beginning of their respective lists.
  • The left marker is advanced to the next element in its list.

What happens in the Intersecting two Lists (I2L) algorithm when the right marker reaches the end of its list and the elements are still different?

<p>The right marker is reset to the beginning, and the left marker is advanced. (A)</p> Signup and view all the answers

In the Intersecting two Lists (I2L) algorithm, what occurs when the elements pointed to by the markers are equal?

<p>The element is added to the intersection, the left marker is advanced, and the right marker is reset. (A)</p> Signup and view all the answers

According to the pseudo-code provided for intersecting two lists, what is the purpose of the variable s?

<p>It keeps track of the index for storing elements in the intersection list. (C)</p> Signup and view all the answers

In the pseudo-code for the Intersecting two Lists (I2L) algorithm, what is the role of the nested repeat loops?

<p>To iterate through each element of both lists to compare them. (B)</p> Signup and view all the answers

Considering the Intersecting two Alphabetized Lists (IAL) algorithm, what initial condition must be met before applying the intersection logic?

<p>The lists must be sorted in alphabetical order. (B)</p> Signup and view all the answers

What is the significance of a 'barrier' in the context of the Intersecting two Alphabetized Lists (IAL) algorithm?

<p>It indicates an element that appears in all lists being intersected, ensuring all pointers reach it before proceeding. (D)</p> Signup and view all the answers

Which of the following scenarios represents the 'worst case' for comparing two lists of 100 elements using the Intersecting two Lists (IL) algorithm?

<p>When the lists have no elements in common. (D)</p> Signup and view all the answers

Which of the following is a valid reason that algorithms might sometimes be unclear?

<p>Natural language used to describe the algorithm can be imprecise. (B)</p> Signup and view all the answers

What does it imply when it is said that 'algorithms always finish'?

<p>Algorithms must eventually terminate, either with an answer or an indication that no answer is possible. (D)</p> Signup and view all the answers

Which of the following is a key reason for understanding why an algorithm works, rather than just knowing that it works?

<p>Understanding enables one to verify its correctness and applicability, ensuring reliable outcomes. (D)</p> Signup and view all the answers

Why is it difficult to absolutely verify programs with loops?

<p>Loops can have too many possible cases to test exhaustively. (C)</p> Signup and view all the answers

What is a primary consideration when evaluating algorithms, beyond just whether they provide an answer?

<p>The efficiency and use of resources like space and time. (B)</p> Signup and view all the answers

When comparing the Intersecting Alphabetized Lists (IAL) and Intersecting Lists (IL) algorithms, what key difference affects their efficiency?

<p>IAL leverages the alphabetical order of the lists to reduce comparisons, whereas IL does not. (D)</p> Signup and view all the answers

What distinguishes the Intersecting two Alphabetized Lists (IAL) algorithm from the Intersecting two Lists (I2L) algorithm, in terms of input data requirements?

<p>IAL requires lists to be sorted alphabetically, while I2L does not have this requirement. (D)</p> Signup and view all the answers

In the context of algorithm design, what does it mean for an algorithm to be 'clear and simple'?

<p>The algorithm is easy to understand and implement with minimal steps. (C)</p> Signup and view all the answers

Consider two lists, each containing 100 elements. In the worst-case scenario, how many comparisons would the Intersecting Lists (IL) algorithm perform?

<p>10,000 (A)</p> Signup and view all the answers

If an algorithm is described as 'efficient', what does this imply regarding its performance?

<p>It performs its task using a minimal amount of resources, such as time and memory. (D)</p> Signup and view all the answers

What is the primary advantage of using the Intersecting two Alphabetized Lists (IAL) algorithm over the Intersecting two Lists (I2L) algorithm when dealing with sorted data?

<p>IAL can terminate sooner because it leverages the sorted property to skip unnecessary comparisons. (D)</p> Signup and view all the answers

Given two alphabetized lists, List A and List B, how does the IAL algorithm exploit their sorted nature to find common elements?

<p>It uses a binary search approach to quickly locate matching elements. (A)</p> Signup and view all the answers

Which of the following is a practical implication of the statement 'Algorithms can be given at different levels of detail depending on the abilities of the agent?'

<p>Algorithms can be simplified to be executed by individuals with limited computational skills or less powerful computers. (D)</p> Signup and view all the answers

What strategy can be used to ensure an algorithm works correctly, especially in algorithms with loops where testing every possible case is not feasible?

<p>Find one or more properties that ensure the algorithm works and explain why these properties ensure correct operation. (D)</p> Signup and view all the answers

Consider the Intersecting two Lists (I2L) algorithm. If the left list is significantly shorter than the right list, how would this generally affect the algorithm's performance?

<p>It would significantly improve the algorithm's performance because there are fewer elements to check in the left list. (B)</p> Signup and view all the answers

In the context of algorithmic efficiency, what does 'space' typically refer to?

<p>The amount of memory required by the algorithm during its execution. (A)</p> Signup and view all the answers

An algorithm is designed to process user data, but due to an oversight, it is vulnerable to accepting and processing malicious code embedded within the user input. Which fundamental property of algorithms is most directly compromised in this scenario?

<p>Correctness. (A)</p> Signup and view all the answers

You are tasked with designing an algorithm to find the median of a very large dataset that is constantly being updated. What primary consideration should guide your choice of algorithm?

<p>The algorithm's efficiency in terms of both time and space complexity, particularly its ability to handle continuous updates. (A)</p> Signup and view all the answers

In the Intersecting Alphabetized Lists (IAL) algorithm, under what specific condition is the 'barrier' concept most crucial for ensuring the algorithm's correctness?

<p>When elements are duplicated across lists, and it's critical to ensure the right number of common elements are accounted for. (D)</p> Signup and view all the answers

What distinguishes an ordinary algorithm from an algorithm suitable for 'algorithmic thinking'?

<p>An algorithm for algorithmic thinking must involve critical thinking and problem-solving skills to optimize and adapt solutions. (C)</p> Signup and view all the answers

In software engineering, the cost of fixing a bug generally increases over time. Given this, what strategy might a software engineer incorporate into their work on an algorithm?

<p>Practice test-driven development to frontload bug detection. (B)</p> Signup and view all the answers

You're optimizing an algorithm that processes large amounts of data. You've identified a section that performs redundant calculations. Which algorithmic thinking principle would be most effective in addressing this?

<p>Employing dynamic programming to cache and reuse previously computed results. (A)</p> Signup and view all the answers

What level of code indentation is required for the pseudo code shown?

<p>The level of indentation depends on where a statement is located in the nested <code>repeat</code> loops or <code>if</code> conditional (B)</p> Signup and view all the answers

Why is algorithm study often associated with mathematics? Can you pinpoint a key reason that inherently links the two fields?

<p>Mathematics allows for the precise expression and validation of algorithmic correctness and efficiency. (B)</p> Signup and view all the answers

An algorithm's time complexity is expressed as $O(n^2)$. If the input size doubles, what happens to the execution time?

<p>It Quadruples. (D)</p> Signup and view all the answers

What is the ultimate purpose of algorithmic thinking in computer science and related fields?

<p>To develop efficient, scalable, and reliable solutions to complex problems. (D)</p> Signup and view all the answers

Considering the trade-offs in algorithm design, which of the following scenarios represents a situation where sacrificing some time efficiency might be justified?

<p>When designing an encryption algorithm where security is paramount. (A)</p> Signup and view all the answers

When is it beneficial to consider alternative algorithms to solve a specific problem?

<p>When the existing algorithm is slow, has high complexity, or doesn't scale well. (B)</p> Signup and view all the answers

Which of the following is a good use case for the Intersecting two Lists algorithm?

<p>Generating a list of mutual friends from two friends' friend lists. (D)</p> Signup and view all the answers

Flashcards

Intersecting two Lists (I2L)

Finding common elements between two lists.

Markers in List Intersection

Place a marker at the start of each list to track the current element.

Advancing the Right Marker

Move the marker of the list with the smaller element.

Resetting Markers

Move the left marker to the next element. Reset the right marker to the beginning.

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Equal Elements

Copy the element to the intersection, advance left marker, and reset right marker.

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Intersecting two Lists Pseudo-code

An empty variable is initialized, list1 is traversed to compare the values in list2, if matched, the value is stored in the intersection.

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Alphabetized Lists

Lists are already sorted alphabetically before intersection begins.

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Alphabetized List Barriers

If name appears in all lists, it forms a barrier than ones that have.

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IAL vs IL Comparison

IAL: 100+100 = 200 comparisons. IL: 100x100 = 10,000.

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Algorithm works

The way to know that an algorithm works is to know why it works.

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Algorithms Usage

Daily, we continually create them as we instruct other people in how to do something.

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Algorithm Evaluations

Algorithms always get to the final result and are evaluated depending on their use of resources.

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Study Notes

Intersecting two Lists (I2L)

  • The friends of Mary and John are written on two separate pieces of paper.
  • The intersection, or common friends, are copied onto a new piece of paper.

Intersecting 2 Lists Algorithm

  • A marker is placed at the beginning of each list.
  • The elements pointed to by each marker are compared to see if they are equal or different.
  • If the elements are different, the marker on the right list is advanced.
  • If the right marker reaches the end of its list, it is reset to the beginning, and the left marker is advanced.
  • If the elements are equal, the element is copied to the end of the Intersection list, the left marker is advanced and the right marker is reset.
  • This process continues until the left list reaches its end.
  • The algorithm then stops because all elements have been compared.

Intersecting two lists in pseudo-code

  • First, prepare an empty variable for the intersection and set 's' to 0
  • Then repeat for 'i' from 1 to the length of list1
  • Then again, repeat for 'j' from 1 to the length of list2
  • If the element in position 'i' of list1 is equal to the element in position 'j' of list2
  • Increment 's' by 1
  • Store the element in position i of list1 into position s of the intersection

Intersecting two Alphabetized Lists (IAL)

  • Friends of both lists are written on paper in alphabetical order and then copied to show intersections

Comparison of IAL & IL

  • IAL and IL are different algorithms
  • They accomplish the same task from the same data in a different order.
  • IL is much slower than IAL.
  • For two lists of 100 elements, the worst case:
    • IAL requires 200 comparisons,
    • IL requires 10,000.
  • How many steps are needed to put a list in alphabetical order is a course topic.

How Do We Know It Works?

  • If the algorithm's solution is simple, clear and efficient
  • If there are no loops, the program runs to the end, allowing the result to be checked.
  • Programs with loops cannot be verified completely due to too many possibilities.
  • Algorithm verification stems from knowing why it works.

Strategy for knowing why an algorithm works

  • Find properties that ensure the algorithm works.
  • Explain why those properties allow it to work by using the program.

Why IAL Works

  • If a name appears in all lists, this forms a barrier.
  • Pointers not at the barrier will move faster than those that already reached it.
  • When they all reach the barrier, the name is recorded.
  • None can advance past the next barrier until it is reached.

Summary

  • Algorithms are created and used daily, especially while teaching.
  • Algorithms in natural language can be unclear.
  • Algorithms have five fundamental properties.
  • Algorithms vary in detail depending on the agent's abilities.
  • Problems are solved by different algorithms in their own ways.
  • Algorithms always finish and give an answer or find that none is possible; they are also evaluated by space and time used.

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