Imagine an ordinary pocket calculator which can add, subtract, multiply, and divide, and which has a few memory registers which you can store numbers in. At a grossly oversimplified level (so simple that we'll abandon it in just a minute), we can think of a computer as a calculator which is able to push its own buttons. A computer program is simply the list of instructions that tells the computer which buttons to push. (Actually, there are ``keystroke programmable'' calculators which you program in just about this way.)
Imagine using such a calculator to perform the following task:
Given a list of numbers, compute the average of all the numbers, and also find the largest number in the list.You can imagine giving the calculator a list of instructions for performing this task, or you can imagine giving a list of instructions to a very stupid but very patient person who is able to follow instructions blindly but accurately, as long as the instructions consist of pushing buttons on the calculator and making simple yes/no decisions. (For our purposes just now, either imaginary model will work.)
Your instructions might look something like this:
``We're going to use memory register 1 to store the running total of all the numbers, memory register 2 to store how many numbers we've seen, and register 3 to store the largest number we've seen. For each number in the input list, add it to register 1. Add 1 to register 2. If the number you just read is larger than the number in register 3, store it in register 3. When you've read all the numbers, divide register 1 by register 2 to compute the average, and also retrieve the largest number from register 3.''There are several things to notice about the above list of instructions:
1. The first sentence, which explains what the registers are used for, is more for our benefit than the entity who will be pushing the buttons on the calculator. The entity pushing the buttons doesn't care what the numbers mean, it just manipulates them as directed. Similarly, the words ``to compute the average'' and ``largest'' in the last sentence are extraneous; they don't tell the entity pushing the button anything it needs to know (or that it can even understand).
2. The instructions use the word ``it'' several times. Even in English, where we're used to a certain amount of ambiguity which we can usually work out from the context, pronouns like ``it'' can cause problems in sentences, because sometimes it isn't obvious what they mean. (For example, in the preceding sentence, does ``they'' refer to ``pronouns,'' ``problems,'' or ``sentences?'') In programming, you can never get away with ambiguity; you have to be quite precise about which ``it'' you're referring to.
3. The instructions are pretty vague about the details of reading the next number in the input list and detecting the end of the list.
4. The ``program'' contains several bugs! It uses registers 1, 2, and 3, but we never say what to store in them in the first place. Unless they all happen to start out containing zero, the average or maximum value computed by the ``program'' will be incorrect. (Actually, if all of the numbers in the list are negative, having register 3 start out as 0 won't work, either.)
Here is a somewhat more detailed version of the ``program,'' which removes some of the extraneous information and ambiguity, makes the input list handling a bit more precise, and fixes at least some of the bugs. (To make the concept of ``the number just read from the list'' unambiguous, this ``program'' stores it in register 4, rather than referring to it by ``it.'' Also, for now, we're going to assume that the numbers in the input list are non-negative.)
``Store 0 in registers 1, 2, and 3. Read the next number from the list. If you're at the end of the list, you're done. Otherwise, store the number in register 4. Add register 4 to register 1. Add 1 to register 2. If register 4 is greater than register 3, store register 4 in register 3. When you're done, divide register 1 by register 2 and print the result, and print the contents of register 3.''When we add the initialization step (storing 0 in the registers), we realize that it's not quite obvious which steps happen once only and which steps happen once for each number in the input list (that is, each time through the processing loop). Also, we've assumed that the calculator can do arithmetic operations directly into memory registers. To make the loop boundaries explicit, and the calculations even simpler (assuming that all the calculator can do is store or recall memory registers from or to the display, and do calculations in the display), the instructions would get more elaborate still:
``Store 0 in register 1. Store 0 in register 2. Store 0 in register 3. Here is the start of the loop: read the next number from the list. If you're at the end of the list, you're done. Otherwise, store the number in register 4. Recall from register 1, recall from register 4, add them, store in register 1. Recall from register 2, add 1, store in register 2. Recall from register 3, recall from register 4, if greater store in register 3. Go back to the beginning of the loop. When you're done: recall from register 1, recall from register 2, divide them, print; recall from register 3, print.''We could continue to ``dumb down'' this list of instructions even further, but hopefully you're getting the point: the instructions we use when programming computers have to be very precise, and at a level of pickiness and detail which we don't usually use with each other. (Actually, things aren't quite as bad as these examples might suggest. The ``dumbing down'' we've been doing has been somewhat in the direction of assembly language, which wise programmers don't use much any more. In a higher-level language such as C, you don't have to worry so much about register assignment and individual arithmetic operators.)
Real computers can do quite a bit more than 4-function pocket calculators can; for one thing, they can manipulate strings of text and other kinds of data besides numbers. Let's leave pocket calculators behind, and start looking at what real computers (at least under the control of programming languages like C) can do.
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This page by Steve Summit // Copyright 1995, 1996 // mail feedback