The way of the program¶

The goal of this book is to teach you to think like a computer scientist. I like the way computer scientists think because they combine some of the best features of Mathematics, Engineering, and Natural Science. Like mathematicians, computer scientists use formal languages to denote ideas (specifically computations). Like engineers, they design things, assembling components into systems and evaluating tradeoffs among alternatives. Like scientists, they observe the behavior of complex systems, form hypotheses, and test predictions.

The single most important skill for a computer scientist is problem-solving. By that I mean the ability to formulate problems, think creatively about solutions, and express a solution clearly and accurately. As it turns out, the process of learning to program is an excellent opportunity to practice problem-solving skills. That’s why this chapter is called “The way of the program.”

On one level, you will be learning to program, which is a useful skill by itself. On another level you will use programming as a means to an end. As we go along, that end will become clearer.

What is a programming language?¶

The programming language you will be learning is Java, which is relatively new (Sun released the first version in May, 1995). Java is an example of a high-level language; other high-level languages you might have heard of are Python, C or C++, and Perl.

As you might infer from the name “high-level language,” there are also low-level languages, sometimes called machine language or assembly language. Loosely-speaking, computers can only run programs written in low-level languages. Thus, programs written in a high-level language have to be translated before they can run. This translation takes time, which is a small disadvantage of high-level languages.

The advantages are enormous. First, it is much easier to program in a high-level language: the program takes less time to write, it’s shorter and easier to read, and it’s more likely to be correct. Second, high-level languages are portable, meaning that they can run on different kinds of computers with few or no modifications. Low-level programs can only run on one kind of computer, and have to be rewritten to run on another.

Due to these advantages, almost all programs are written in high-level languages. Low-level languages are only used for a few special applications.

There are two ways to translate a program; interpreting and 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 running any of the commands. Often you compile the program as a separate step, and then run 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.

Java is both compiled and interpreted. Instead of translating programs into machine language, the Java compiler generates 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 program on one machine, transfer the byte code to another machine, and then interpret the byte code on the other machine. This ability is an advantage of Java over many other high-level languages.

Although this process may seem complicated, in most program development environments these steps are automated for you. Usually you will only have to write a program and press a button or type a single command to compile and run it. On the other hand, it is useful to know what steps are happening in the background, so if something goes wrong you can figure out what it is.

What is a program?¶

A program is a sequence of instructions that specifies how to perform a computation [1]. 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, which we will call statements, look different in different programming languages, but there are a few basic operations most languages perform:

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 run the appropriate sequence of statements.
repetition:: Perform some action repeatedly, usually with some variation.

That’s pretty much all there is to it. Every program you’ve ever used, no matter how complicated, is made up of statements that perform these operations. Thus, one way to describe programming is the process of breaking a large, complex task up into smaller and smaller subtasks until the subtasks are simple enough to be performed with one of these basic operations.

What is debugging?¶

For whimsical reasons, programming errors are called bugs and the process of tracking them down and correcting them is called debugging.

There are a three kinds of errors that can occur in a program, and it is useful to distinguish them to track them down more quickly.

Syntax 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 weeks of your programming career, you will probably spend a lot of time tracking down syntax errors. As you gain experience, 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.

Java tends to be a safe language, which means that the compiler catches a lot of errors. So run-time errors are rare, especially for simple programs.

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 logic or semantic error. If there is a logic error in your program, it will compile and run without generating 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 semantics, or meaning of the program, are wrong. Identifying logic errors can be tricky because you have to work backwards, looking at the output of the program and trying to figure out what it is doing.

Experimental debugging¶

One of the most important skills you will acquire in this class is debugging. Although debugging can be frustrating, it is one of the most interesting, challenging, and valuable parts of programming.

Debugging is like detective work. You are confronted with clues and you have to infer the processes and events that lead to the results you see.

Debugging is also like an experimental science. Once you have an idea what is going wrong, you modify your program and try again. If your hypothesis was correct, then you can predict the result of the modification, and you take a step closer to a working program. If your hypothesis was wrong, you have to come up with a new one. As Sherlock Holmes pointed out, “When you have eliminated the impossible, whatever remains, however improbable, must be the truth.” (From A. Conan Doyle’s The Sign of Four.)

For some people, programming and debugging are the same thing. That is, programming is the process of gradually debugging a program until it does what you want. The idea is that you should always start with a working program that does something, and make small modifications, debugging them as you go, so that you always have a working program.

For example, Linux is an operating system that contains thousands of lines of code, but it started out as a simple program Linus Torvalds used to explore the Intel 80386 chip. According to Larry Greenfield, “One of Linus’s earlier projects was a program that would switch between printing AAAA and BBBB. This later evolved to Linux” (from The Linux Users’ Guide Beta Version 1).

In later chapters I make more suggestions about debugging and other programming practices.

Formal and natural languages¶

Natural languages are the languages that people speak, like English, Spanish, and French. They were not designed by people (although people try to impose order on them); they evolved naturally.

Formal languages are languages designed by people for specific applications. For example, the notation that mathematicians use is a formal language that is particularly good at denoting relationships among numbers and symbols. Chemists use a formal language to represent the chemical structure of molecules. And most importantly:

Programming languages are formal languages that have been designed to express computations.

Formal languages have strict rules about syntax. For example, $3+3=6$ is a syntactically correct mathematical statement, but $3 \$ =$ is not. Also, $H_2O$ is a syntactically correct chemical name, but $_2Zz$ is not.

Syntax rules come in two flavors, pertaining to tokens and structure. Tokens are the basic elements of the language, like words and numbers and chemical elements. One of the problems with $3 \$ =$ is that $\$$ is not a legal token in mathematics (at least as far as I know). Similarly, $_2Zz$ is not legal because there is no element with the abbreviation $Zz$.

The second type of syntax rule pertains to the structure of a statement; that is, the way the tokens are arranged. The statement $3 \$ =$ is structurally illegal, because you can’t have an equals sign at the end of an equation. Similarly, molecular formulas have to have subscripts after the element name, not before.

When you read a sentence in English or a statement in a formal language, you have to figure out what the structure of the sentence is (although in a natural language you do this unconsciously). This process is called parsing.

Although formal and natural languages have features in common—tokens, structure, syntax and semantics—there are differences.

ambiguity:: Natural languages are full of ambiguity, which people deal with by using contextual clues and other information. Formal languages are designed to be unambiguous, which means that any statement has exactly one meaning, regardless of context.
redundancy:: To make up for ambiguity and reduce misunderstandings, natural languages are often redundant. Formal languages are more concise.
literalness:: Natural languages are full of idiom and metaphor. Formal languages mean exactly what they say.

People who grow up speaking a natural language (everyone) often have a hard time adjusting to formal languages. In some ways the difference between formal and natural language is like the difference between poetry and prose, but more so:

Poetry:: Words are used for their sounds as well as for their meaning, and the whole poem together creates an effect or emotional response. Ambiguity is common and deliberate.
Prose:: The literal meaning of words is more important and the structure contributes more meaning.
Programs:: The meaning of a computer program is unambiguous and literal, and can be understood entirely by analysis of the tokens and structure.

Here are some suggestions for reading programs (and other formal languages). First, remember that formal languages are much more dense than natural languages, so it takes longer to read them. Also, the structure is important, so it is usually not a good idea to read from top to bottom, left to right. Instead, learn to parse the program in your head, identifying the tokens and interpreting the structure. Finally, remember that the details matter. Little things like spelling errors and bad punctuation, which you can get away with in natural languages, can make a big difference in a formal language.

The first program¶

Traditionally the first program people write in a new language is called “hello world” because all it does is display the words “Hello, World.” In Java, this program looks like:

class Hello {

  // main: generate some simple output

  public static void main(String[] args) {
    System.out.println("Hello, world.");
  }
}

This program includes features that are hard to explain to beginners, but it provides a preview of topics we will see in detail later.

Java programs are made up of class definitions, which have the form:

class CLASSNAME {

  public static void main (String[] args) {
    STATEMENTS
  }
}

Here CLASSNAME indicates a name chosen by the programmer. The class name in the example is Hello.

main is a method, which is a named collection of statements. The name main is special; it marks the place in the program where execution begins. When the program runs, it starts at the first statement in main and ends when it finishes the last statement.

main can have any number of statements, but the example has one. It is a print statement, meaning that it displays a message on the screen. Confusingly, “print” can mean “display something on the screen,” or “send something to the printer.” In this book I won’t say much about sending things to the printer; we’ll do all our printing on the screen. The print statement ends with a semi-colon (;).

System.out.println is a method provided by one of Java’s libraries. A library is a collection of class and method definitions.

Java uses squiggly-braces ({ and }) to group things together. The outermost squiggly-braces (lines 1 and 8) contain the class definition, and the inner braces contain the definition of main.

Line 3 begins with //. That means it’s a comment, which is a bit of English text that you can put a program, usually to explain what it does. When the compiler sees //, it ignores everything from there until the end of the line.

Glossary¶

problem-solving:: The process of formulating a problem, finding a solution, and expressing the solution.
high-level language:: A programming language like Java that is designed to be easy for humans to read and write.
low-level language:: A programming language that is designed to be easy for a computer to run. Also called “machine language” or “assembly language.”
formal language:: Any of the languages people have designed for specific purposes, like representing mathematical ideas or computer programs. All programming languages are formal languages.
natural language:: Any of the languages people speak that have evolved naturally.
portability:: A property of a program that can run on more than one kind of computer.
interpret:: To run a program in a high-level language by translating it one line at a time.
compile:: To translate a program in a high-level language into a low-level language, all at once, in preparation for later execution.
source code:: A program in a high-level language, before being compiled.
object code:: The output of the compiler, after translating the program.
executable:: Another name for object code that is ready to run.
byte code:: A special kind of object code used for Java programs. Byte code is similar to a low-level language, but it is portable, like a high-level language.
statement:: A part of a program that specifies a computation.
print statement:: A statement that causes output to be displayed on the screen.
comment:: A part of a program that contains information about the program, but that has no effect when the program runs.
method:: A named collection of statements.
library:: A collection of class and method definitions.
bug:: An error in a program.
syntax:: The structure of a program.
semantics:: The meaning of a program.
parse:: To examine a program and analyze the syntactic structure.
syntax error:: An error in a program that makes it impossible to parse (and therefore impossible to compile).
exception:: An error in a program that makes it fail at run-time. Also called a run-time error.
logic error:: An error in a program that makes it do something other than what the programmer intended.
debugging:: The process of finding and removing any of the three kinds of errors.

Exercises¶

Exercise¶

Computer scientists have the annoying habit of using common English words to mean something other than their common English meaning. For example, in English, statements and comments are the same thing, but in programs they are different.

The glossary at the end of each chapter is intended to highlight words and phrases that have special meanings in computer science. When you see familiar words, don’t assume that you know what they mean!

In computer jargon, what’s the difference between a statement and a comment?
What does it mean to say that a program is portable?
What is an executable?

Exercise¶

Before you do anything else, find out how to compile and run a Java program in your environment. Some environments provide sample programs similar to the example in Section The first program.

Type in the “Hello, world” program, then compile and run it.
Add a print statement that prints a second message after the “Hello, world!”. Something witty like, “How are you?” Compile and run the program again.
Add a comment to the program (anywhere), recompile, and run it again. The new comment should not affect the result.

This exercise may seem trivial, but it is the starting place for many of the programs we will work with. To debug with confidence, you have to have confidence in your programming environment. In some environments, it is easy to lose track of which program is executing, and you might find yourself trying to debug one program while you are accidentally running another. Adding (and changing) print statements is a simple way to be sure that the program you are looking at is the program you are running.

Exercise¶

It is a good idea to commit as many errors as you can think of, so that you see what error messages the compiler produces. Sometimes the compiler tells you exactly what is wrong, and all you have to do is fix it. But sometimes the error messages are misleading. You will develop a sense for when you can trust the compiler and when you have to figure things out yourself.

Remove one of the open squiggly-braces.
Remove one of the close squiggly-braces.
Instead of main, write mian.
Remove the word static.
Remove the word public.
Remove the word System.
Replace println with Println.
Replace println with print. This one is tricky because it is a logic error, not a syntax error. The statement System.out.print is legal, but it may or may not do what you expect.
Delete one of the parentheses. Add an extra one.

[1]	This definition does not apply to all programming languages; for alternatives, see Wikipedia.

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