CS 99

Summer 2001 07.02

Lecture Outline for W 6.27

A Review of the first Lab assignment

· What you were really doing: In CodeWarrior there are two kinds of files, project files and text files.

· Programming languages are precise: notice what happens when you change random things

· That's the real secret for learning how to program: playing around. You'll never ever learn to program just by listening to someone talk about it or by having someone just show you. You only learn how to program by programming.

Programming Languages

· A high level language like Java is almost like English. It was designed that way on purpose. It's really asking too much to have people talk the language that computers actually understand. The days of punch cards are over!

· Learning a programming language is exactly like learning a foreign language. Unfortunately, unlike say in French, where we start with simple words and then build up to sentences (and then later to essays), in learning a programming language, we have to start almost immediately by writing 'essays'.

· So it's a little more difficult to learn Java in the beginning because you have to absorb a lot of information rather quickly

· On the other hand, Java is a much simpler language than most human languages. There isn't a huge dictionary of definitions for every conceivable circumstance. Once you learn the basics, you'll be able to write your own essays and do them rather well.

· All programs can be written in terms of three control structures:

· Direct sequencing

· Conditional branching

· Conditional looping

· Direct sequencing
The sequencing structure is built into Java. Unless directed otherwise, the computer executes Java statements in the order in which they are written.

· Conditional branching
Java provides three types of conditional branching (or selection structures). They are the if, if/else, and switch statements. We probably will not cover the switch statement in CS 99, but we shall learn the other two statements rather thoroughly (starting next week).

· Conditional looping
Java provides for three types of conditional branching (or repetition structures): the while, do/while and for. We really don't need all three, but it's useful to have the variations for different situations.

· The words if, else, switch, while, do, and for are Java keywords. These words are reserved by Java to implement various features in the language. You can't use them separately for your own purposes.

· Control structures vary only a little from programming language to language. The implementation details may differ, but not the ideas. In learning thoroughly how these structures work in Java, you're learning something quite universal. You'll be able to apply the same ideas to other programming languages rather easily.

Are control structures all we need?

· No. Having these control structures is enough to specify any possible algorithm, but the computer also has to be able to do the things you want it to do

· For instance, say you're writing a program for a robotic chef. If there comes a point in the algorithm where the robot has to 'Beat Eggs', then the computer's language must have the ability to tell the robot 'Beat Eggs' - and the robot will know what to do when it's given that command

· how to 'Beat Eggs' isn't essential to the language, but it's necessary in this circumstance for the programming application to do its job

· Another example: System.out.println … this is also not an essential language construct, but you need it or something like it for the computer to give you output.

· To summarize, in addition to the control structures, the language must have other functions that enable it to both talk to us and do other things we may want it to do

Variables

· But even that's not the end of the story

· A computer has to be able to remember things as well, to store data.

· If I want to write a program that tells me the largest value in a sequence of values I give it, how is the computer going to remember what that largest value is?

· Indeed, for a program to be interesting, we usually have to supply it data (otherwise it'll always do the same thing; unless of course it's got some kind of randomization aspect to it, but even that is input of a kind)

· A computer stores data in variables.

· Because there are all kinds of things we might want a computer to remember - numbers, names, other kinds of data - you can imagine that we have to be able to tell the computer the kinds of data we want it to remember

· Recall, the computer only knows 1s and 0s and switches and registers, so to remember various kinds of data, we have to tell it how much memory to allocate

· So there are different types of data

· In Java, there are ints, floats, doubles, chars, Strings, booleans, and other kinds that may be defined by the user or are supplied by Java natively, such as Color, Graphics, etc.

· More on that next week.