CS101 explores the essential qualities of computers, how they work, what they can and cannot do, and requires not computer background at all. In this first section we'll look at the basic features of computers and get started playing with computer code.
Acknowledgements: thanks to Google for supporting my early research that has helped create this class. Thanks to Mark Guzdial who popularized the idea of using digital media to introduce computers.
The fundamental equation of computers is:
Computer = Powerful + Stupid
Computers are very powerful, looking through large amounts of data quickly. Computers can literally perform billions of operations per second. However, the individual "operations" that computers can perform are extremely simple and mechanical, nothing like a human thought or insight. A typical operation in the language of computers is adding two numbers together.
So although the computers are fast at what they do, the operations that they can do are extremely rigid, simple, and mechanical. Or put another way, computers are not like the HAL 9000 from the movie 2001: A Space Odyssey: HAL 9000 video.
If nothing else, you should not be intimidated by the computer as if it's some sort of brain. The computer is a mechanical tool which can do amazing things, but it requires a human to tell it what to do.
Since the computer is totally mechanical and stupid -- how do they manage to do so many useful things? The gap between the computer and doing something useful is where the human programmer creates solutions. Programming is about a person using their insight about what would be useful and how it could be done, and breaking the steps down into code the computer can follow.
Code refers to the language the computer can understand. For these lectures, we'll write and run short snippets of code to understand what the essential qualities of computers, and especially their strengths and limitations.
Experimenting with code, the nature of computers will come through very clearly ... powerful in their own way, but with a limited, mechanical quality. IMHO, this mixed nature of computers is something everyone should understand, to use them well, to not be intimidated by them.
Within a few hours of lecture, we'll be doing special effects with images such as the following:
But for now we just have print()!
Patience We're going to start by learning a few programming elements, and later we'll recombine these few elements to solve many problems. These first elements are simple, so they are not much to look at on their own. Be patient, soon we'll put these elements together -- like lego bricks -- to make pretty neat projects.
Here is code which calls the "print" function. Click the Run button below, and your computer will run this code, and the output of the code will appear to the right.
Note that print is recognized as a function in the code vs. the "hello" string which is just passive data (like verbs and nouns). The computer ignores the comments, so they are just a way for you to write notes to yourself about what the code is doing. Comments can be use it to temporarily remove a line of code -- "commenting out" the code by placing a "//" to its left.
Syntax The syntax shown above must be rigidly followed or the code will not work: function name, parenthesis, each string has opening and closing quotes, commas separating values for a function call.
The rigidity of the syntax is a reflection of the limitations of computers .. their natural language is fixed and mechanical. This is important to absorb when working with computers, and I think this is where many people get derailed getting started with computers. You write something that any human could understand, but the computer can only understand the code where it fits the computer's mechanical syntax.
When writing for the computer, it is very common to make a trivial, superficial syntax mistakes in the code. The most expert programmers on earth make that sort of error all the time, and think nothing of it. The syntax errors do not reflect some flawed strategy by the author. It's just a natural step in translating our thoughts into the more mechanical language of the computer. As we'll see below, fixing these errors is very fast.
It's important to not be derailed by these little superficial-error cases. To help teach you the patterns, below we have many examples showing typical errors, so you can see what the error messages look like and see how to fix them. For each snippet of code below, what is the error? Sometimes the error message generated by the computer points to the problem accurately, but sometimes the error message just reveals that the error has so deeply confused the computer that it cannot create an accurate error message. Firefox currently produces the most helpful error messages often pointing to the specific line with problems.
These syntax problems are quick to fix.
Change the code below so, when run, it produces the following output:
1 2 buckle 3 4 knock
For the example problems shown in lecture, the solutions are available as shown below. So you can revisit the problem, practice with it, and still see the solution if you like.