CSC 201 Chapter 16- Programming and Languages, Spring 2007

CCHHAAPPTTEERR 1166 PPRRO OG GRRAAM MM MIINNG G AANNDD LLAANNG GUUAAG GEESS Programming and Programmers 3 Programming and Languages Notre Dame University Programming and Programmers Learning Module Objectives When you have completed this learning module you will have: • Understood what programmers do and do not do • Knew how programmers define a problem, plan the solution, and the code, test, and document the program Programming and Programmers 4 Programming and Languages Notre Dame University Understand what is programming, what programmers do and do not do Why Programming You may already have used software, perhaps for word processing or spreadsheets, to solve problems. Perhaps now you are curious to learn how programmers develop software. A program is a set of step-by-step instructions that directs the computer to do the tasks you want it to do and produce the results you want. A set of rules that provides a way of telling a computer what operations to perform is called a programming language. There is not, however, just one programming language; there are many. What programmers do In general, the programmer's job is to convert problem solutions into instructions for the computer. That is, the programmer prepares the instructions of a computer program and runs those instructions on the computer, tests the program to see if it is working properly, and makes corrections to the program. The programmer also writes a report on the program. These activities are all done for the purpose of helping a user fill a need, such as paying employees, billing customers, or admitting students to college. The programming activities just described could be done, perhaps, as solo activities, but a programmer typically interacts with a variety of people. For example, if a program is part of a system of several programs, the programmer coordinates with other programmers to make sure that the programs fit together well. If you were a programmer, you might also have coordination meetings with users, managers, and systems analysts, as well as with peers who evaluate your work--just as you evaluate theirs. Programming and Programmers 5 Programming and Languages Notre Dame University The Programming Process Introduction Developing a program involves steps similar to any problem-solving task. There are five main ingredients in the programming process: (1) defining the problem, (2) planning the solution, (3) coding the program, (4) testing the program, and (5) documenting the program. Defining the Problem Suppose that, as a programmer, you are contacted because your services are needed. You meet with users from the client organization to analyze the problem, or you meet with a systems analyst who outlines the project. Specifically, the task of defining the problem consists of identifying what it is you know (input--the data given) and what it is you want to obtain (output--the result). Eventually, you produce a written agreement that, among other things, specifies the kind of input, processing, and output required. This is not a simple process. Programming and Programmers 6 Programming and Languages Notre Dame University Planning the solution Two common ways of planning the solution to a problem are to draw a flowchart and to write pseudocode, or possibly both. Essentially, a flowchart is a pictorial representation of a step-by-step solution to a problem. It consists of arrows representing the direction the program takes and boxes and other symbols representing actions. It is a map of what your program is going to do and how it is going to do it. The American National Standards Institute (ANSI) has developed a standard set of flowchart symbols. As a practical matter, few programmers use flowcharting in their work, but flowcharting retains its value as a visual representation of the problem-solving process. Pseudocode is an English-like nonstandard language that lets you state your solution with more precision than you can in plain English but with less precision than is required when using a formal programming language. Pseudocode permits you to focus on the program logic without having to be concerned just yet about the precise rules of a particular programming language. However, pseudocode is not executable on the computer. Programming and Programmers 7 Programming and Languages Notre Dame University Coding the program As the programmer, your next step is to code the program--that is, to express your solution in a programming language. You will translate the logic from the flowchart or pseudocode or some other tool to a programming language. There are many programming languages: BASIC, COBOL, Pascal, FORTRAN, and C are some examples. The different types of languages will be discussed in detail later in this chapter. Although programming languages operate grammatically, somewhat like the English language, they are much more precise. To get your program to work, you have to follow exactly the rules--the syntax--of the language you are using. Of course, using the language correctly is no guarantee that your program will work, any more than speaking grammatically correct English means you know what you are talking about. The point is that correct use of the language is the required first step. You will key your program as you compose it, using a terminal or personal computer. One more note here: Programmers usually use a text editor, which is somewhat like a word processing program, to create a file that contains the program. However, as a beginner, you will probably want to write your program code on paper first. Programming and Programmers 8 Programming and Languages Notre Dame University Testing the program In theory, a well-designed program can be written correctly the first time. However, the imperfections of the world are still with us, so most programmers get used to the idea that their newly written programs will probably have a few errors. Therefore, after coding the program, you must prepare to test it on the computer. This step involves these phases • Desk-Checking: is a mental checking or proofreading of the program before it is run. A walkthrough is a process in which a group of programmers--your peers--review your program and offer suggestions in a collegial way. • Translating: a translator program converts the program into a form the computer can understand and in the process detects programming language errors, which are called syntax errors. A common translator is a compiler, which translates the entire program at one time and gives error messages called diagnostic. The original program, called a source module, is translated to an object module, to which prewritten programs may be added during the link/load phase to create an executable load module. • Debugging: involves running the program to detect, locate, and correct mistakes known as logic errors. Programming and Programmers 9 Programming and Languages Notre Dame University Documenting the program An ongoing process, documentation is a detailed written description of the programming cycle and specific facts about the program. Typical program documentation materials include the origin and nature of the problem, a brief narrative description of the program, logic tools such as flowcharts and pseudocode, data-record descriptions, program listings, and testing results. Comments in the program itself are also considered an essential part of documentation. Many programmers document as they code. In a broader sense, program documentation can be part of the documentation for an entire system Programming and Programmers 10 Programming and Languages Notre Dame University Computer Languages Learning Module Objectives When you have completed this learning module you will have: • Understood the level of languages • Understood major programming languages Programming and Programmers 11 Programming and Languages Notre Dame University Know the Levels of language Introduction Programming languages are said to be "lower" or "higher," depending on how close they are to the language the computer itself uses (0s and 1s--low) or to the language people use (more English-like--high). There are five levels of language, numbered 1 through 5 to correspond to levels, or generations. In terms of ease of use and capabilities, each generation is an improvement over its predecessors. The five generations of languages are (1) machine language, (2) assembly languages, (3) high-level languages, (4) very high level languages, and (5) natural languages. Machine Language Humans do not like to deal in numbers alone; they prefer letters and words. But, strictly speaking, numbers are what machine language is. This lowest level of programming language, machine language, represents data and program instructions as 0s and 1s, binary digits corresponding to the on and off electrical states in the computer. This is really the only language the computer truly understands; all other languages must be translated to the machine language before execution. Each type of computer has its own machine language. Primitive by today's standards, machine language programs are not convenient for people to read and use. The computer industry quickly moved to develop assembly languages. Programming and Programmers 12 Programming and Languages Notre Dame University Assembly Languages Today, assembly languages are considered very low level--that is, they are not as convenient for people to use as more recent languages. At the time they were developed, however, they were considered a great leap forward. To replace the 0s and 1s used in machine language, assembly languages use mnemonic codes, abbreviations that are easy to remember: A for add, C for compare, MP for multiply, STO for storing information in memory, and so on. Furthermore, assembly languages permit the use of names--perhaps RATE or TOTAL--for memory locations instead of actual address numbers. As with machine language, each type of computer has its own assembly language. Since machine language is the only language the computer can actually execute, a translator, called an assembly program, is required to convert the assembly language program into machine language. Assembly language may be easier to read than machine language, but it is still tedious. Programming and Programmers 13 Programming and Languages Notre Dame University High-Level Languages The first widespread use of high-level languages in the early 1960s transformed programming into something quite different from what it had been. Programs were written in an English-like manner, thus making them more convenient to use. As a result, a programmer could accomplish more with less effort, and programs could now direct much more complex tasks. Of course, a translator is needed to translate the symbolic statements of a high-level language into computer-executable machine language; this translator is usually a compiler. There are many compilers for each language and at least one for each type of computer. Programming and Programmers 14 Programming and Languages Notre Dame University Very High- Level Languages Languages called very high-level languages are often known by their generation number; that is, they are called fourth-generation languages or, more simply, 4GLs. The 4GLs are essentially shorthand programming languages. An operation that requires hundreds of lines in a third-generation language typically requires only 5 to 10 lines in a 4GL. However, beyond the basic criterion of conciseness, 4GLs are difficult to describe because there are so many different types. Most experts say the average productivity improvement factor is about 10; that is, you can be 10 times more productive in a fourth-generation language than in a third- generation language. Consider this request: Produce a report showing the total units sold for each product, by customer, in each month and year, and with a subtotal for each customer. In addition, each new customer must start on a new page. A 4GL request looks something like this: TABLE FILE SALES SUM UNITS BY MONTH BY CUSTOMER BY PRODUCT ON CUSTOMER SUBTOTAL PAGE BREAK END Even though some training is required to do even this much, you can see that it is pretty simple. The third-generation language COBOL, however, typically requires more than 500 statements to fulfill the same request. It would be naive, however, to assume that all programs should be written using 4GLs; a third-generation language makes more sense for commercial applications that require a high degree of precision. Programming and Programmers 15 Programming and Languages Notre Dame University Natural Languages The word natural has become almost as popular in computing circles as it has in the supermarket. The newest level of languages, called fifth-generation languages, is even more ill-defined than fourth-generation languages. They are most often called natural languages because of their resemblance to the "natural" spoken English language; that is, they resemble the way that you speak. A user of one of these languages can say the same thing in any number of ways. For example, "Get me tennis racket sales for January" works just as well as "I want January tennis racket revenues." The natural language translates human instructions--bad grammar, slang, and all--into code the computer understands. If it is not sure what the user has in mind, it politely asks for further explanation. Programming and Programmers 16 Programming and Languages Notre Dame University Know the major programming languages FORTRAN: The First High-Level Language Developed by IBM and introduced in 1954, Fortran--for FORmula TRANslator--was the first high-level language. FORTRAN is a scientifically oriented language; in the early days, use of the computer was primarily associated with engineering, mathematical, and scientific research tasks. FORTRAN is noted for its brevity, and this characteristic is part of the reason it remains popular. This language is very good at serving its primary purpose, which is the execution of complex formulas such as those used in economic analysis and engineering. COBOL: The Language of Business By the mid-1950s FORTRAN had been developed, but there was still no accepted high-level programming language appropriate for business. The U.S. Department of Defense in particular was interested in creating such a standardized language and called together a committee that, in 1959, introduced COBOL, for COmmon Business-Oriented Language. COBOL is very good for processing large files and performing relatively simple business calculations, such as payroll or interest. COBOL is English-like; even if you know nothing about programming, you may still understand what the program does. However, the feature that makes COBOL so useful--its English-like appearance and easy readability--is also a weakness, because a COBOL program can be incredibly verbose. Today, many consider COBOL old-fashioned and inelegant. In fact, many companies devoted to fast, nimble program development have converted to the language called C. BASIC: For Beginners and Others Beginners' All-purpose Symbolic Instruction Code--is a common language that is easy to learn. Developed at Dartmouth College, BASIC was introduced by John Kemeny and Thomas Kurtz in 1965 and was originally intended for use by students in an academic environment. The use of BASIC has extended to business and personal computer systems. The primary feature of BASIC is one that may be of interest to many readers of this book: BASIC is easy to learn, even for a person who has never programmed before. Thus the language is often used to train Programming and Programmers 17 Programming and Languages Notre Dame University students in the classroom. Programming and Programmers 18 Programming and Languages Notre Dame University Pascal: The Language of Simplicity Named for Blaise Pascal, the seventeenth-century French mathematician, Pascal was developed as a teaching language by a Swiss computer scientist, Niklaus Wirth, and became available in 1971. Its use spread first in Europe and then in the United States, particularly in schools offering computer science programs, although its popularity is now in decline. An attractive feature of Pascal is that it is simpler than other languages--it has fewer features and is less wordy than most. In addition to being popular in college computer science departments, the language has also made large inroads in the personal computer market as a simple yet sophisticated alternative to BASIC. Today, Borland's Turbo Pascal is used by the business community and is often the choice of nonprofessional programmers who need to write their own programs. C: A Portable Language A language invented by Dennis Ritchie at Bell Labs in 1972, C produces code that approaches assembly language in efficiency while still offering the features of a high-level language. C was originally designed to write systems software but is now considered a general-purpose language. C contains some of the best features from other languages, including Pascal. C compilers are simple and compact. A key attraction is that there are C compilers available for different operating systems, a fact that contributes to the portability of C programs. An interesting side note is that the availability of C on personal computers has greatly enhanced the value of personal computers for budding software entrepreneurs. Today C is fast being replaced by its enhanced cousin, C++. Programming and Programmers 19 Programming and Languages Notre Dame University Java Programming languages rarely attain media darling status. But it seems that the language called Java, from developers at Sun Microsystems, has had continuous hard-core coverage in the computer press. Java is a network-friendly programming language, derived from the C++ language, that permits a piece of software to run on many different platforms. A platform is the hardware and software combination that composes the basic functionality of a computer. For example, a popular platform today is based on some version of Microsoft's Windows operating system and Intel's processors, a combination nicknamed Wintel. Traditionally, programmers have been limited to writing a program for a single platform. Coding has had to be redone for other platforms. But a programmer can write a program in Java, which operates across platforms, and have it run anywhere. So how does Java accomplish this cross-platform feat? Programs written in Java can be understood by a universal platform, called the Java platform, that sits atop a computer's regular platform. Essentially, then, this universal platform is an extra layer of software that has been accepted as a standard by most of the computer industry--no small feat. The Java platform translates Java instructions into instructions that the platform underneath can understand. When you consider that Java can run across many platforms, it is easy to see why it is relevant to Internet development; in fact, Java's earliest incarnations were on web applications. Java has a good start on becoming the universal language of Internet computing.