Core: Analyzing Search Algorithms

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Del curso dictado por University of California San Diego

Data Structures and Performance

1356 calificaciones

University of California San Diego

Data Structures and Performance

1356 calificaciones

Curso 4 de 4 en SpecializationObject Oriented Programming in Java

How do Java programs deal with vast quantities of data? Many of the data structures and algorithms that work with introductory toy examples break when applications process real, large data sets. Efficiency is critical, but how do we achieve it, and how do we even measure it? This is an intermediate Java course. We recommend this course to learners who have previous experience in software development or a background in computer science, and in particular, we recommend that you have taken the first course in this specialization (which also requires some previous experience with Java). In this course, you will use and analyze data structures that are used in industry-level applications, such as linked lists, trees, and hashtables. You will explain how these data structures make programs more efficient and flexible. You will apply asymptotic Big-O analysis to describe the performance of algorithms and evaluate which strategy to use for efficient data retrieval, addition of new data, deletion of elements, and/or memory usage. The program you will build throughout this course allows its user to manage, manipulate and reason about large sets of textual data. This is an intermediate Java course, and we will build on your prior knowledge. This course is designed around the same video series as in our first course in this specialization, including explanations of core content, learner videos, student and engineer testimonials, and support videos -- to better allow you to choose your own path through the course!

De la lección

Efficiency Analysis and Benchmarking

Welcome to week 3! The text-editor application you worked with last week does something, but it doesn't do it particularly fast. This week we'll start talking about efficiency. We'll introduce the concept of "Big-O" notation, which sounds a little silly, but is really a powerful (and extremely common) way of analyzing a program's efficiency, independent of the system that it's running on and the exact details of how it's implemented. Then we'll go the other direction and dive into the details, talking about how to measure the actual running time of a piece of code to get an idea of how it really performs in practice.

Core: Our Motivation for Asymptotic Analysis8:28

Core: Counting Operations9:57

Core: Introduction to Asymptotic Analysis, Part 19:11

Core: Introduction to Asymptotic Analysis, Part 23:48

Core: Computing Big O with Consecutive Operations5:19

Core: Computing Big O with Nested Operations5:22

Concept Challenge: Classifying Functions using Big O7:52

Support: Analyzing Selection Sort8:41

Concept Challenge: Estimating Big O from Code6:08

Core: Worst, Best, and Average Cases8:16

In the Real World: Worst Case Analysis1:30

Core: Analyzing Search Algorithms6:25

Core: Analyzing Sorting Algorithms9:25

When I struggled: Algorithm performance1:31

Core: Merge Sort11:07

Core: A Summary of Sorting4:00

Core: Common Pitfalls in Asymptotic Analysis5:51

Conoce a los instructores

Christine Alvarado
Associate Teaching Professor
Computer Science and Engineering
Mia Minnes
Assistant Teaching Professor
Computer Science and Engineering
Leo Porter
Assistant Teaching Professor
Computer Science and Engineering

[MUSIC]

So now we're ready to put together everything we've been talking

about with performance analysis.

And in particular think about the asymptotic balance,

the big-O classes of searching algorithms.

So at this point we've got two searching algorithms that we've talked about, so

by the end of this video you'll be able to both state and

justify the big-O runtime for both of these search algorithms.

And both in the best case and in the worst case.

So what we're going to be doing is filling in this little table, and

so let's start with a linear search.

So our linear search basic algorithm is that when we're searching

through a list of data, we start at the beginning and we go to the end, and

we look for our desired piece of data.

So, in the best case, let's think back to

the example in your search that we've done, that was that hasLetter method,

in the best case we find the piece of data right away.

And so, our best case performance is a constant time big O(1).

Now in the worst case, we talked about this also with the hasLetter method.

What happens if we're unlucky is that we go through the entire string or

entire list of data and we don't find what we're looking for, or

we only find it at the very end.

But in either one of those situations in this very unlucky situation, we have to go

through all data and so our performance is growing just like the input is.

So, it's big O(n).

And so if the worst case linear search performs like big O(n).

Okay, so we've got linear search and now let's move on to binary search.

We haven't talked about binary search in this course so far, but

if you think back to the first course in the specialization we did do several

examples with this algorithm.

And so you can go back there to refresh your memory.

The binary search algorithm is similarly as before looking for

data in a list of data, but something that we have

to assume in order to use binary search is that this data is already sorted.

It's organized from smallest to biggest in some measure of order.

Okay, so if we make that assumption that we have sorted data,

what's the best case situation?

What's the worst case situation?

Well, we have to go back to what the algorithm is in the first place before we

can analyze it.

So the nuts and bolts of the binary search algorithm is that we take our data and

we start by looking in the very middle.

And maybe we're lucky and we found it.

Sweet.

If not, if we didn't find the item that we're looking for

in the very middle then what we have to do is either look at the first half

of the list or the second half of the list.

And we choose which half to look at based on comparing the value that we're looking

for with the value in the middle and seeing, should I be before it or

should I be after it?

And so what we're doing in binary search is over and

over cutting the amount of information that we need to look at in half again and

again and again until we focus in onto where our element

ought to be in the list and then checking if it really is there.

Okay so that's big picture what we're doing, now how does it perform?

And we ask ourselves, best case, worst case?

And best case is super lucky, we zoom into the middle of the list and

we've found our element that we're looking for.

And we're done and we can return, and that took lots of time.

So the best case is really a similar situation to linear search,

where we're really lucky and we find our item at the first chance we could.

So the very first thing that we test is actually

the element that we're looking for.

Okay, so that's the best case, constant time, okay.

But what about the worst case?

And let's think through what happens in this worst case and

what it would mean to have a really unlucky search.

And just like before, what it would mean to have a really unlucky search is if we

don't find the element at all.

If we keep looking, keep looking, keep looking, and

we can't find this element in the list.

So if the element is missing from the list, and

let's think about what this algorithm would do in that that case.

So we looked in the middle, and we didn't find our element.

And now, we go and look at just half the list, and

then we basically repeat the algorithm.

And so what we do is we look in the middle of that half and look for our algorithm,

and we're not gonna find it there.

And so we look at half of the remaining and look in the middle of that.

We're not gonna find it there.

Okay, so we only need to consider half of the remaining.

Look in the middle of that, keep on going, keep on going, and

the question is when do we get to stop?

So, every time we repeat this process, every time we

have a new high element and a low element, and we compute a new midpoint,

what we're doing is we're decreasing the list in consideration by half.

And so we get to stop when the number of elements that we need to consider,

well, is just one.

And so we need to ask ourselves, how many times does it take for

this process to reduce the list that we need to consider to just one element?

Or another way of saying that is,

how many times can we divide the size of the element by 2 and get to 1?

So, we're repeatedly dividing by 2, dividing by 2, dividing by 2,

and we want to get to 1.

But the number of times we can do that is

exactly the logarithm base 2 of the size of our list.

And there's a support video to go through that calculation a little bit

more in detail if you're interested.

So, for binary search we filled in that last cell in the table, and

in the worst case this search runs in big O of logarithmic time.

So, we've got this table, it's great.

But, we've got that little asterisk that says, all of these calculations for

binary search are assuming that our data is sorted.

Our algorithm for binary search requires the data to be sorted.

And so it's not really fair to have this table and

compare linear search head to head with binary search because linear search

doesn't make any assumptions on the organization of the data.

And so if we really wanted to compare them head to head,

these two algorithms, what we would need to do is say, well, how long would it take

to take an unorganized list and sort it and then run binary search on it?

So in the next video we'll be summarizing some approaches for sorting and

doing their performance analysis as well.

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