THE PINPOINT TRICK
Now that we know about checksums, we can go back to the original problem of both detecting and correcting communication errors. We already know how to do this, either inefficiently using the repetition trick or efficiently using the redundancy trick. But let's return to this now, because we never really found out how to create the code words that form the key ingredient in this trick. We did have the example of using English words to describe numerals, but this particular set of code words is less efficient than the ones computers actually use. And we also saw the real example of a Hamming code, but without any explanation of how the code words were produced in the first place.
So now we will learn about another possible set of code words that can be used to perform the redundancy trick. Because this is a very special case of the redundancy trick that allows you to quickly pinpoint an error, we'll call this the “pinpoint trick.”
Just as we did with the checksum trick, we will work entirely with numerical messages consisting of the digits 0-9, but you should keep in mind that this is just for convenience. It is very simple to take an alphabetical message and translate it into numbers, so the technique described here can be applied to any message whatsoever.
To keep things simple, we'll assume that the message is exactly 16 digits long, but, again, this doesn't limit the technique in practice. If you have a long message, just break it into 16-digit chunks and work with each chunk separately. If the message is shorter than 16 digits, fill it up with zeroes, until it is 16 digits long.
The first step in the pinpoint trick is to rearrange the 16 digits of the message into a square that reads left to right, top to bottom. So if the actual message is
4837543622563997
it gets rearranged into
Next, we compute a simple checksum of each row and add it to the right-hand side of the row:
These simple checksums are computed just like before. For example, to get the second row checksum you compute 5 + 4 + 3 + 6 = 18 and then take the last digit, which is 8.
The next step in the pinpoint trick is to compute simple checksums for each column and add these in a new row at the bottom:
Again, there's nothing mysterious about the simple checksums. For example, the third column is computed from 3 + 3 + 5 + 9 = 20, which becomes 0 when we take the last digit.
The next step in the pinpoint trick is to reorder everything so it can be stored or transmitted one digit at a time. You do this in the obvious way, reading digits from left to right, top to bottom. So we end up with the following 24-digit message:
483725436822565399784306
Now imagine you have received a message that has been transmitted using the pinpoint trick. What steps do you follow to work out the original message and correct any communication errors? Let's work through an example. The original 16-digit message will be the same as the one above, but to make things interesting, suppose there was a communication error and one of the digits was altered. Don't worry about which is the altered digit yet—we will be using the pinpoint trick to determine that very shortly.
So let's suppose the 24-digit message you received is
483725436827565399784306
Your first step will be to lay the digits out in a 5-by-5 square, recognizing that the last column and last row correspond to checksum digits that were sent with the original message:
Next, compute simple checksums of the first four digits in each row and column, recording the results in a newly created row and column next to the checksum values that you received:
It is crucial to bear in mind that there are two sets of checksum values here: the ones you were sent, and the ones you calculated. Mostly, the two sets of values will be the same. In fact, if they are all identical, you can conclude that the message is very likely correct. But if there was a communication error, some of the calculated checksum values will differ from the sent ones. Notice that in the current example, there are two such differences: the 5 and 0 in the third row differ, and so do the 3 and 8 in the second column. The offending checksums are highlighted in boxes:
Here is the key insight: the location of these differences tells you exactly where the communication error occurred! It must be in the third row (because every other row had the correct checksum), and it must be in the second column (because every other column had the correct checksum). And as you can see from the following diagram, this narrows it down to exactly one possibility—the 7 highlighted in a solid box:
But that's not all—we have located the error, but not yet corrected it. Fortunately, this is easy: we just have to replace the erroneous 7 with a number that will make both of the checksums correct. We can see that the second column was meant to have a checksum of 3, but it came out to 8 instead—in other words, the checksum needs to be reduced by 5. So let's reduce the erroneous 7 by 5, which leaves 2:
You can even double-check this change, by examining the third row—it now has a checksum of 5, which agrees with the received checksum. The error has been both located and corrected! The final obvious step is to extract the corrected original 16-digit message from the 5-by-5 square, by reading top to bottom, left to right (and ignoring the final row and column, of course). This gives
4837543622563997
which really is the same message that we started with.
In computer science, the pinpoint trick goes by the name of “two-dimensional parity.” The word parity means the same thing as a simple checksum, when working with the binary numbers computers normally use. And the parity is described as two-dimensional because the message gets laid out in a grid with two dimensions (rows and columns). Two-dimensional parity has been used in some real computer systems, but it is not as effective as certain other redundancy tricks. I chose to explain it here because it is very easy to visualize and conveys the flavor of how one can both find and correct errors without requiring the sophisticated math behind the codes popular in today's computer systems.
ERROR CORRECTION AND DETECTION IN THE REAL WORLD
Error-correcting codes sprang into existence in the 1940s, rather soon after the birth of the electronic computer itself. In retrospect, it's not hard to see why: early computers were rather unreliable, and their components frequently produced errors. But the true roots of error-correcting codes lie even earlier, in communication systems such as telegraphs and telephones. So it is not altogether surprising that the two major events triggering the creation of error-correcting codes both occurred in the research laboratories of the Bell Telephone Company. The two heroes of our story, Claude Shannon and Richard Hamming, were both researchers at Bell Labs. Hamming we have met already: it was his annoyance at the weekend crashes of a company computer that led directly to his invention of the first error-correcting codes, now known as Hamming codes.
However, error-correcting codes are just one part of a larger discipline called information theory, and most computer scientists trace the birth of the field of information theory to a 1948 paper by Claude Shannon. This extraordinary paper, entitled “The Mathematical Theory of Communication,” is described in one biography of Shannon as “the Magna Carta of the information age.” Irving Reed (co-inventor of the Reed-Solomon codes mentioned below) said of the same paper: “Few other works of this century have had greater impact on science and engineering. By this landmark paper…he has altered most profoundly all aspects of communication theory and practice.” Why the high praise? Shannon demonstrated through mathematics that it was possible, in principle, to achieve surprisingly high rates of error-free communication over a noisy, error-prone link. It was not until many decades later that scientists came close to achieving Shannon's theoretical maximum communication rate in practice.
Incidentally, Shannon was apparently a man of extremely diverse interests. As one of the four main organizers of the 1956 Dartmouth AI conference (discussed at the end of chapter 6), he was intimately involved in the founding of another field: artificial intelligence. But it doesn't stop there. He also rode unicy
cles and built an improbable-sounding unicycle with an elliptical (i.e., noncircular) wheel, meaning that the rider moved up and down as the unicycle moved forward!
Shannon's work placed Hamming codes in a larger theoretical context and set the stage for many further advances. Hamming codes were thus used in some of the earliest computers and are still widely used in certain types of memory systems. Another important family of codes is known as the Reed-Solomon codes. These codes can be adapted to correct for a large number of errors per codeword. (Contrast this with the (7,4) Hamming code in the figure on page 67, which can correct only one error in each 7-digit code word.) ReedSolomon codes are based on a branch of mathematics called finite field algebra, but you can think of them, very roughly, as combining the features of the staircase checksum and the two-dimensional pinpoint trick. They are used in CDs, DVDs, and computer hard drives.
Checksums are also widely used in practice, typically for detecting rather than correcting errors. Perhaps the most pervasive example is Ethernet, the networking protocol used by almost every computer on the planet these days. Ethernet employs a checksum called CRC-32 to detect errors. The most common internet protocol, called TCP (for Transmission Control Protocol), also uses checksums for each chunk, or packet, of data that it sends. Packets whose checksums are incorrect are simply discarded, because TCP is designed to automatically retransmit them later if necessary. Software packages published on the internet are often verified using checksums; popular ones include a checksum called MD5, and another called SHA-1. Both are intended to be cryptographic hash functions, providing protection against malicious alteration of the software as well as random communication errors. MD5 checksums have about 40 digits, SHA-1 produces about 50 digits, and there are some even more error-resistant checksums in the same family, such as SHA-256 (about 75 digits) and SHA-512 (about 150 digits).
The science of error-correcting and error-detecting codes continues to expand. Since the 1990s, an approach known as low-density parity-check codes has received considerable attention. These codes are now used in applications ranging from satellite TV to communication with deep space probes. So the next time you enjoy some high-definition satellite TV on the weekend, spare a thought for this delicious irony: it was the frustration of Richard Hamming's weekend battle with an early computer that led to our own weekend entertainment today.
6
Pattern Recognition: Learning from Experience
The Analytical Engine has no pretensions whatever to originate anything. It can do whatever we know how to order it to perform.
—ADA LOVELACE, from her 1843 notes on the Analytical Engine
In each previous chapter, we've looked at an area in which the ability of computers far outstrips the ability of humans. For example, a computer can typically encrypt or decrypt a large file within a second or two, whereas it would take a human many years to perform the same computations by hand. For an even more extreme example, imagine how long it would take a human to manually compute the PageRank of billions of web pages according to the algorithm described in chapter 3. This task is so vast that, in practice, it is impossible for a human. Yet the computers at web search companies are constantly performing these computations.
In this chapter, on the other hand, we examine an area in which humans have a natural advantage: the field of pattern recognition. Pattern recognition is a subset of artificial intelligence and includes tasks such as face recognition, object recognition, speech recognition, and handwriting recognition. More specific examples would include the task of determining whether a given photograph is a picture of your sister, or determining the city and state written on a hand-addressed envelope. Thus, pattern recognition can be defined more generally as the task of getting computers to act “intelligently” based on input data that contains a lot of variability.
The word “intelligently” is in quotation marks here for good reason: the question of whether computers can ever exhibit true intelligence is highly controversial. The opening quotation of this chapter represents one of the earliest salvos in this debate: Ada Lovelace commenting, in 1843, on the design of an early mechanical computer called the Analytical Engine. Lovelace is sometimes described as the world's first computer programmer because of her profound insights about the Analytical Engine. But in this pronouncement, she emphasizes that computers lack originality: they must slavishly follow the instructions of their human programmers. These days, computer scientists disagree on whether computers can, in principle, exhibit intelligence. And the debate becomes even more complex if philosophers, neuroscientists, and theologians are thrown into the mix.
Fortunately, we don't have to resolve the paradoxes of machine intelligence here. For our purposes, we might as well replace the word “intelligent” with “useful.” So the basic task of pattern recognition is to take some data with extremely high variability—such as photographs of different faces in different lighting conditions, or samples of many different words handwritten by many different people—and do something useful with it. Humans can unquestionably process such data intelligently: we can recognize faces with uncanny accuracy, and read the handwriting of virtually anyone without having to see samples of their writing in advance. It turns out that computers are vastly inferior to humans at such tasks. But some ingenious algorithms have emerged that enable computers to achieve good performance on certain pattern recognition tasks. In this chapter, we will learn about three of these algorithms: nearest-neighbor classifiers, decision trees, and artificial neural networks. But first, we need a more scientific description of the problem we are trying to solve.
WHAT'S THE PROBLEM?
The tasks of pattern recognition might seem, at first, to be almost absurdly diverse. Can computers use a single toolbox of pattern recognition techniques to recognize handwriting, faces, speech, and more? One possible answer to this question is staring us (literally) in the face: our own human brains achieve superb speed and accuracy in a wide array of recognition tasks. Could we write a computer program to achieve the same thing?
Before we can discuss the techniques that such a program might use, we need to somehow unify the bewildering array of tasks and define a single problem that we are trying to solve. The standard approach here is to view pattern recognition as a classification problem. We assume that the data to be processed is divided up into sensible chunks called samples, and that each sample belongs to one of a fixed set of possible classes. For example, in a face recognition problem, each sample would be a picture of a face, and the classes would be the identities of the people the system can recognize. In some problems, there are only two classes. A common example of this is in medical diagnosis for a particular disease, where the two classes might be “healthy” and “sick,” while each data sample could consist of all the test results for a single patient (e.g., blood pressure, weight, x-ray images, and possibly many other things). So the computer's task is to process new data samples that it has never seen before and classify each sample into one of the possible classes.
To make things concrete, let's focus on a single pattern recognition task for now. This is the task of recognizing handwritten digits. Some typical data samples are shown in the figure on the facing page. There are exactly ten classes in this problem: the digits 0, 1, 2, 3, 4, 5, 6, 7, 8, and 9. So the task is to classify samples of handwritten digits as belonging to one of these ten classes. This is, of course, a problem of great practical significance, since mail in the United States and many other countries is addressed using numeric postcodes. If a computer can rapidly and accurately recognize these postcodes, mail can be sorted by machines much more efficiently than by humans.
Obviously, computers have no built-in knowledge of what handwritten digits look like. And, in fact, humans don't have this built-in knowledge either: we learn how to recognize digits and other handwriting, through some combination of explicit teaching by other humans and by seeing examples that we use to teach ourselves. These two strategies (explicit teaching and learn
ing from examples) are also used in computer pattern recognition. However, it turns out that for all but the simplest of tasks, explicit teaching of computers is ineffective. For example, we can think of the climate controls in my house as a simple classification system. A data sample consists of the current temperature and time of day, and the three possible classes are “heat on,” “air-conditioning on,” and “both off.” Because I work in an office during the day, I program the system to be “both off” during daytime hours, and outside those hours it is “heat on” if the temperature is too low and “air-conditioning on” if the temperature is too high. Thus, in the process of programming my thermostat, I have in some sense “taught” the system to perform classification into these three classes.
Unfortunately, no one has ever been able to explicitly “teach” a computer to solve more interesting classification tasks, such as the handwritten digits on the next page. So computer scientists turn to the other strategy available: getting a computer to automatically “learn” how to classify samples. The basic strategy is to give the computer a large amount of labeled data: samples that have already been classified. The figure on page 84 shows an example of some labeled data for the handwritten digit task. Because each sample comes with a label (i.e., its class), the computer can use various analytical tricks to extract characteristics of each class. When it is later presented with an unlabeled sample, the computer can guess its class by choosing the one whose characteristics are most similar to the unlabeled sample.
Nine Algorithms That Changed the Future: The Ingenious Ideas That Drive Today's Computers Page 9
