Excerpts from "How to Think Like a Computer Scientist," http://openbookproject.net//thinkCSjav/chap01.htm
There are two ways to translate a program; interpreting or compiling. An interpreter is a program that reads a high-level program and does what it says. In effect, it translates the program line-by-line, alternately reading lines and carrying out commands.
A compiler is a program that reads a high-level program and translates it all at once, before executing any of the commands. Often you compile the program as a separate step, and then execute the compiled code later. In this case, the high-level program is called the source code, and the translated program is called the object code or the executable.
Then, depending on what your programming environment is like, you might leave the text editor and run the compiler. The compiler would read your source code, translate it, and create a new file named program.o to contain the object code, or program.exe to contain the executable.
The Java language is unusual because it is both compiled and interpreted. Instead of translating Java programs into machine language, the Java compiler generates Java byte code. Byte code is easy (and fast) to interpret, like machine language, but it is also portable, like a high-level language. Thus, it is possible to compile a Java program on one machine, transfer the byte code to another machine over a network, and then interpret the byte code on the other machine. This ability is one of the advantages of Java over many other high-level languages.
Although this process may seem complicated, the good news is that in most programming environments (sometimes called development environments), these steps are automated for you. Usually you will only have to write a program and type a single command to compile and run it. On the other hand, it is useful to know what the steps are that are happening in the background, so that if something goes wrong you can figure out what it is.
1.2 What is a program?
A program is a sequence of instructions that specifies how to perform a computation. The computation might be something mathematical, like solving a system of equations or finding the roots of a polynomial, but it can also be a symbolic computation, like searching and replacing text in a document or (strangely enough) compiling a program.
The instructions (or commands, or statements) look different in different programming languages, but there are a few basic functions that appear in just about every language:
- input
- Get data from the keyboard, or a file, or some other device.
- output
- Display data on the screen or send data to a file or other device.
- math
- Perform basic mathematical operations like addition and multiplication.
- testing
- Check for certain conditions and execute the appropriate sequence of statements.
- repetition
- Perform some action repeatedly, usually with some variation.
Believe it or not, that's pretty much all there is to it. Every program you've ever used, no matter how complicated, is made up of functions that look more or less like these. Thus, one way to describe programming is the process of breaking a large, complex task up into smaller and smaller subtasks until eventually the subtasks are simple enough to be performed with one of these simple functions.
1.3 What is debugging?
Programming is a complex process, and since it is done by human beings, it often leads to errors. For whimsical reasons, programming errors are called bugs and the process of tracking them down and correcting them is called debugging.
There are a few different kinds of errors that can occur in a program, and it is useful to distinguish between them in order to track them down more quickly.
Compile-time errors
The compiler can only translate a program if the program is syntactically correct; otherwise, the compilation fails and you will not be able to run your program. Syntax refers to the structure of your program and the rules about that structure.
For example, in English, a sentence must begin with a capital letter and end with a period. this sentence contains a syntax error. So does this one
For most readers, a few syntax errors are not a significant problem, which is why we can read the poetry of e e cummings without spewing error messages.
Compilers are not so forgiving. If there is a single syntax error anywhere in your program, the compiler will print an error message and quit, and you will not be able to run your program.
To make matters worse, there are more syntax rules in Java than there are in English, and the error messages you get from the compiler are often not very helpful. During the first few weeks of your programming career, you will probably spend a lot of time tracking down syntax errors. As you gain experience, though, you will make fewer errors and find them faster.
Run-time errors
The second type of error is a run-time error, so-called because the error does not appear until you run the program. In Java, run-time errors occur when the interpreter is running the byte code and something goes wrong.
The good news for now is that Java tends to be a safe language, which means that run-time errors are rare, especially for the simple sorts of programs we will be writing for the next few weeks.
Later on in the semester, you will probably start to see more run-time errors, especially when we start talking about objects and references (Chapter 8).
In Java, run-time errors are called exceptions, and in most environments they appear as windows or dialog boxes that contain information about what happened and what the program was doing when it happened. This information is useful for debugging.
Logic errors and semantics
The third type of error is the logical or semantic error. If there is a logical error in your program, it will compile and run successfully, in the sense that the computer will not generate any error messages, but it will not do the right thing. It will do something else. Specifically, it will do what you told it to do.
The problem is that the program you wrote is not the program you wanted to write. The meaning of the program (its semantics) is wrong. Identifying logical errors can be tricky, since it requires you to work backwards by looking at the output of the program and trying to figure out what it is doing.
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