Nine Algorithms That Changed the Future: The Ingenious Ideas That Drive Today's Computers (27 page)

Various ways of running a program with a particular file as its input.

Microsoft Excel run with “photo.jpg” as its input. The output is garbage, but the important point is that you can, in principle, run any program on any input you want.

Obviously, you can get rather unexpected results if you open a file using a program it was not intended for. In the figure above, you can see what happens if I open the picture file “photo.jpg” with my spreadsheet program, Microsoft Excel. In this case, the output is garbage and is no use to anyone. But the spreadsheet program did run, and did produce some output.

This may already seem ridiculous, but we can take the craziness one step further. Remember that computer programs are themselves stored on the computer's disk as files. Often, these programs have a name that ends in “.exe,” which is short for “executable”—this just means that you can “execute,” or run, the program. So because computer programs are just files on the disk, we can feed one computer program as input to another computer program. As one specific example, the Microsoft Word program is stored on my computer as the file “WINWORD.EXE.” So by running my spreadsheet program with the file WINWORD.EXE as input, I can produce the wonderful garbage you see in the figure on the facing page.

Microsoft Excel examines Microsoft Word. When Excel opens
the file WINWORD.EXE, the result is—unsurprisingly—garbage.

Again, it would be well worth trying this experiment for yourself. To do that, you will need to locate the file WINWORD.EXE. On my computer, WINWORD.EXE lives in the folder “C:\Program Files\Microsoft Office\Office12,” but the exact location depends on what operating system you are running and what version of Microsoft Office is installed. You may also need to enable the viewing of “hidden files” before you can see this folder. And, by the way, you can do this experiment (and one below) with any spreadsheet and word processing programs, so you don't need Microsoft Office to try it.

One final level of stupidity is possible here. What if we ran a computer program
on itself
? For example, what if I ran Microsoft Word, using the file WINWORD.EXE as input? Well, it's easy enough to try this experiment. The figure on the next page shows the result when I try it on my computer. As with the previous few examples, the program runs just fine, but the output on the screen is mostly garbage. (Once again, try it for yourself.)

So, what is the point of all this? The purpose of this section was to acquaint you with some of the more obscure things you can do when running a program. By now, you should be comfortable with three slightly strange ideas that will be very important later. First, there is the notion that any program can be run with any file as input, but the resulting output will usually be garbage unless the input file was intentionally produced to work with the program you chose to run. Second, we found out that computer programs are stored as files on computer disks, and therefore one program can be run with another program as its input file. Third, we realized that a computer program can be run using its own file as the input. So far, the second and third activities always produced garbage, but in the next section we will see a fascinating instance in which these tricks finally bear some fruit.

Microsoft Word examines itself. The open document is the file WINWORD.EXE, which is the actual computer program run when you click on Microsoft Word.

SOME PROGRAMS CAN'T EXIST

Computers are great at executing simple instructions—in fact, modern computers execute simple instructions billions of times every second. So you might think that any task that could be described in simple, precise English could be written down as a computer program and executed by a computer. My objective in this section is to convince you that the opposite is true: there are some simple, precise English statements that are literally impossible to write down as a computer program.

Some Simple Yes-No Programs

To keep things as simple as possible in this section, we will consider only a very boring set of computer programs. We'll call these “yes-no” programs, because the only thing these programs can do is pop up a single dialog box, and the dialog box can contain either the word “yes” or the word “no.” For example, a few minutes ago I wrote a computer program called ProgramA.exe, which does nothing but produce the following dialog box:

Note that by looking in the title bar of the dialog box, you can see the name of the program that produced this output—in this case, ProgramA.exe.

I also wrote a different computer program called ProgramB.exe, which outputs “no” instead of “yes”:

ProgramA and ProgramB are extremely simple—so simple, in fact, that they do not require any input (if they do receive input, they ignore it). In other words, they are examples of programs that really do behave exactly the same every time they are run, regardless of any input they may be given.

As a more interesting example of one of these yes-no programs, I created a program called SizeChecker.exe. This program takes one file as input and outputs “yes” if that file is bigger than 10 kilobytes and otherwise outputs “no.” If I right-click on a 50-megabyte video file (say, mymovie.mpg), choose “Open with…,” and select SizeChecker.exe, I will see the following output:

On the other hand, if I run the same program on a small 3-kilobyte e-mail message (say, myemail.msg), I will, of course, see a different output:

Therefore, SizeChecker.exe is an example of a yes-no program that sometimes outputs “yes” and sometimes “no.”

Now consider the following slightly different program, which we'll call NameSize.exe. This program examines the
name
of its input file. If the file name is at least one character long, NameSize.exe outputs “yes”; otherwise, it outputs “no.” What are the possible outputs of this program? Well, by definition, the name of any input file is at least one character long (otherwise, the file would have no name at all, and you couldn't select it in the first place). Therefore, NameSize.exe will always output “yes,” regardless of its input.

By the way, the last few programs mentioned above are our first examples of programs that do not produce garbage when they are given other programs as input. For example, it turns out that the size of the file NameSize.exe is only about 8 kilobytes. So if I run SizeChecker.exe with NameSize.exe as the input, the output is “no” (because NameSize.exe is not more than 10 kilobytes). We can even run SizeChecker.exe on itself. The output this time is “yes,” because it turns out that SizeChecker.exe is larger than 10 kilobytes—about 12 kilobytes, in fact. Similarly, we could run NameSize.exe with itself as input; the output would be “yes” since the file name “Name-Size.exe” contains at least one character. All of the yes-no programs we have discussed this far are admittedly rather dull, but it's important to understand their behavior, so work through the table on the facing page line by line, making sure you agree with each output.