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2.11 Chapter Summary

This chapter covered some important topics related to the efficiency of algorithms. Efficiency is an important topic because even the fastest computers will not be able to solve problems in a reasonable amount of time if the programs that are written for them are inefficient. In fact, some problems can't be solved in a reasonable amount of time no matter how the program is written. Nevertheless, it is important that we understand these issues of efficiency. Finding the complexity of a piece of code is an important skill that you will get better at the more you practice. Here are some of the things you should have learned in this chapter. You should:

• know the complexity of storing or retrieving a value from a list or the memory of the computer.

• know how memory is like a post office.

• know how memory is NOT like a post office.

• know how to use the datetime module to get information about the time it takes to complete an operation in a program.

• know how to write an XML file that can be used by the plotting program to plot information about the performance of an algorithm or piece of code.

• understand the definition of big-Oh notation and how it establishes an upper bound on the performance of a piece of code.

• understand why the list + operation is not as efficient as the append operation.

• understand the difference between O(n), O(n2), and other computational complexities and why those differences are important to us as computer programmers.

• Understand Theta notation and what an asymptotically tight bound says about an algorithm.

• Understand Amortized complexity and how to apply it in some simple situations.

2.12 Review Questions

Answer these short answer, multiple choice, and true/false questions to test your mastery of the chapter.

1. How is a list like a bunch of post office boxes?

2. How is accessing an element of a list NOT like retrieving the contents of a post office box?

3. How can you compute the amount of time it takes to complete an operation in a computer using Python?

4. In terms of computational complexity, which is better, an algorithm that is O(n2) or an algorithm that is O(2n)?

5. Describe, in English, what it means for an algorithm to be O(n2).

6. When doing a proof by induction, what two parts are there to the proof?

7. If you had an algorithm with a loop that executed n steps the first time through, then n − 2 the second time, n − 4 the next time, and kept repeating until the last time through the loop it executed 2 steps, what would be the complexity measure of this loop? Justify your answer with what you learned in this chapter.

8. Assume you had a data set of size n and two algorithms that processed that data set in the same way. Algorithm A took 10 steps to process each item in the data set. Algorithm B processed each item in 100 steps. What would the complexity be of these two algorithms?

9. Explain why the append operation on a list is more efficient than the + operator.

10. Describe an algorithm for finding a particular value in a list. Then give the

computational complexity of this algorithm. You may make any assumptions you want, but you should state your assumptions along with your algorithm.

2.13 Programming Problems

1. Devise an experiment to discover the complexity of comparing strings in Python. Does the size of the string affect the efficiency of the string comparison and if so, what is the complexity of the comparison? In this experiment you might want to consider a best case, worst case, and average case complexity. Write a program that produces an XML file with your results in the format specified in this chapter. Then use the PlotData.py program to visualize those results.

2. Conduct an experiment to prove that the product of two numbers does not depend on the size of the two numbers being multiplied. Write a program that plots the results of multiplying numbers of various sizes together. HINT: To get a good reading you may want to do more than one of these multiplications and time them as a group since a multiplication happens pretty quickly in a computer. Verify that it truly is a O(1) operation. Do you see any anomalies? It might be explained by Python's support of large integers. What is the cutoff point for handling multiplications in constant time? Why? Write a program that produces an XML file with your results in the format given in this chapter. Then visualize your results with the PlotData.py program provided in this chapter.

3. Write a program to gather experimental data about comparing integers. Compare integers of different sizes and plot the amount of time it takes to do those comparisons. Plot your results by writing an XML file in the Ploy.py format. Is the comparison operation always O(1)? If not, can you theorize why? HINT: You may want to read about Python's support for large integers.

4. Write a short function that searches for a particular value in a list and returns the position of that value in the list (i.e. its index). Then write a program that times how long it takes to search for an item in lists of different sizes. The size of the list is your n. Gather results from this experiment and write them to an XML file in the PlotData.py format. What is the complexity of this algorithm? Answer this question in a comment in your program and verify that the experimental results match your prediction. Then, compare this with the index method on a list. Which is more efficient in terms of computational complexity? HINT: You need to be careful to consider the average case for this problem, not just a trivial case.

5. Write a short function that given a list, adds together all the values in the list and returns the sum. Write your program so it does this operation with varying sizes of lists. Record the time it takes to find the sum for various list sizes. Record this information in an XML file in the PlotData.py format. What complexity is this algorithm? Answer this in a comment at the top of your program and verify it with your experimental data. Compare this data with the built-in sum function in Python that does the same thing. Which is more efficient in terms of computational complexity? HINT: You need to be careful to consider the average case for this problem, not just a trivial case.

6. Assume that you have a datatype called the Clearable type. This data type has a fixed size list inside it when it is created. So Clearable(10) would create a clearable list of size 10. Objects of the Clearable type should support an append operation and a lookup operation. The lookup operation is called getitem (item). If cl is a Clearable list, then writing cl[item] will return the item if it is in the list and return None otherwise. Writing cl[item] results in a method call of cl. getitem (item). Unlike the append operation described in Sect. 2.10.1, when the Clearable object fills up the list is automatically cleared or emptied on the next call to append by setting all elements of the list back to None. The Clearable object should always keep track of the number of values currently stored in the object. Form a theory about the complexity of the append operation on this datatype. Then write a test program to test the Clearable object on different initial sizes and numbers of append operations. Create one sequence for each different initial size of the Clearable datatype and write your results in the plot format described in this chapter. Then comment on how your theory holds up or does not hold up given your experimentation results.

 
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