|
History Prior to the mid-1960s, computers were extremely expensive tools used only for special-purpose tasks. A simple batch processing arrangement ran only a single "job" at a time, one after another. During the 1960s, however, computer prices began to decrease and even small companies could afford them, and speed increased to the point where they often sat idle without jobs to run. Programming languages in the batch programming era tended to be designed, like the machines on which they ran, for specific purposes (such as scientific formula calculations or business data processing or eventually for text editing). Since even the newer less expensive machines were still major investments, there was strong tendency to consider efficiency (ie, execution speed, and such) to be the most important feature of a language. In general, these specialized languages were difficult to use and had widely disparate syntax. As prices decreased, the possibility of sharing computer access began to move from research labs to commercial use. The time-sharing idea had arrived. In such a system the processing capability of the computer CPU was "sliced up" in time and each user given a small amount in alternation with the other users. The machines had become fast enough that most users could feel they had the machine all to themselves. In theory, timesharing reduced the cost of computing tremendously, as a single machine could be shared among (up to) hundreds of users. Early years — the mini computer era The original BASIC language was designed in 1963 by John Kemeny and Thomas Kurtz and implemented by a team of Dartmouth students under their direction. BASIC was designed to allow students to write programs for the Dartmouth Time-Sharing System. It intended to address the complexity issues of older languages with a new language design specifically for the new class of users time-sharing systems allowed — that is, a less technical user who did not require the computation cycles more traditional users needed, and so was not as interested in speed. Being able to use a computer to support teaching and research was quite attractive enough. In the following years, as other dialects of BASIC appeared, Kemeny and Kurtz' original BASIC dialect became known as Dartmouth BASIC. The eight design principles of BASIC were: The language was based partly on FORTRAN II and partly on ALGOL 60, with additions to make it suitable for timesharing. (The features of other time-sharing systems such as JOSS and CORC, and to a lesser extent LISP, were also considered). It had been preceded by other teaching-language experiments at Dartmouth such as the DARSIMCO (1956) and DOPE (1962 implementations of SAP and DART (1963) which was a simplified FORTRAN II). Initially, BASIC concentrated on supporting straightforward mathematical work, with matrix arithmetic support and full string functionality being added by 1965. BASIC was first implemented on the GE-265 mainframe which supported multiple terminals. Contrary to popular belief, it was a compiled language at the time of its introduction. It was also quite efficient, beating FORTRAN II and ALGOL 60 implementations on the 265 at several fairly computationally intensive programming problems such as maximization Simpson's Rule. The designers of the language decided to make the compiler available without charge so that the language would become widespread. They also made it available to high schools in the Dartmouth area and put a considerable amount of effort into promoting the language. As a result, knowledge of BASIC became relatively widespread (for a computer language) and BASIC was implemented by a number of manufacturers, becoming fairly popular on newer minicomputers like the DEC PDP series and the Data General Nova. In these instances the language tended to be implemented as an interpreter, instead of (or in addition to) a compiler. Several years after its release, highly-respected computer professionals, notably Edsger W. Dijkstra, expressed their opinions that the use of GOTO statements, which existed in many languages including BASIC, promoted poor programming practices. Some have also derided BASIC as too slow (most interpreted versions are slower than equivalent compiled versions) or too simple (many versions, especially for small computers left out important features and capabilities). Explosive growth — the home computer era
Maturity — the personal computer era Many newer BASIC versions were created during this period. Microsoft sold several versions of BASIC for MS-DOS/PC-DOS including BASICA, GW-BASIC (a BASICA-compatible version that did not need IBM's ROM) and QuickBASIC. Turbo Pascal-publisher Borland published Turbo BASIC 1.0 in 1985 (successor versions are still being marketed by the original author under the name PowerBASIC). These languages introduced many extensions to the original home computer BASIC, such as improved string manipulation and graphics support, access to the file system and additional data types. More important were the facilities for structured programming, including additional control structures and proper subroutines supporting local variables. However, by the latter half of the 1980s newer computers were far more capable with more resources. At the same time, computers had progressed from a hobbyist interest to tools used primarily for applications written by others, and programming became less important for most users. BASIC started to recede in importance, though numerous versions remained available. Compiled BASIC or CBASIC is still used in many IBM 4690 OS point of sale systems. BASIC's fortunes reversed once again with the introduction of Visual Basic by Microsoft. It is somewhat difficult to consider this language to be BASIC, because of the major shift in its orientation towards an object-oriented and event-driven perspective. While this could be considered an evolution of the language, few of the distinctive features of early Dartmouth BASIC, such as line numbers and the INPUT keyword, remain.Many BASIC dialects have also sprung up in the last few years, including Bywater BASIC and True BASIC (the direct successor to Dartmouth BASIC from a company controlled by Kurtz). Recently, the remaining community using Microsoft's pre-Visual Basic products have begun to switch wholesale to FreeBASIC, a GPLed compiler which has moved BASIC onto a GCC backend. Many other BASIC variants and adaptations have been written by hobbyists, equipment developers, and others, as it is a relatively simple language to develop translators for. An example of an open source interpreter, written in C, is MiniBasic. The ubiquity of BASIC interpreters on personal computers was such that textbooks once included simple TRY IT IN BASIC exercises that encouraged students to experiment with mathematical and computational concepts on classroom or home computers. Futurist and sci-fi writer David Brin mourns the loss of ubiquitous BASIC in a recent Salon article Why Johnny Can't Code. Syntax
Procedures and flow control Most BASICs do not use an external library for common operations like other languages such as C. Instead, the interpreter (or compiler) contains an extensive built-in library of intrinsic procedures, rather like most Pascals. These procedures include most of the tools a programmer needs to learn programming and write simple applications, including functions for math, strings, console input/output, graphics and file manipulation. Some BASIC dialects do not provide builtin structuring facilities, eg, a Procedure construct. In these programmers must write their programs using GOTO statements to emulate these structures. This can result in considerably confusing programs, commonly referred to as spaghetti code. GOSUB statements branch to simple kinds of subroutines without (though sometimes with) parameters or local variables. Most modern versions of BASIC such as Microsoft QuickBASIC have added support for full subroutines and functions. This is another area where BASIC differs from many other programming languages. Most BASICs, like Pascal, makes a distinction between a procedure which does not return a value (called a subroutine) and a procedure which does (called a function). Many other languages (notably C) make no distinction and consider everything a function, all returning values, though some return a "void" value. While functions, in the larger sense of subroutines returning values, were a latecomer to BASIC dialects, many early systems supported the definition of one-line mathematical functions by DEF FN ("DEFine FunctioN"). The original Dartmouth BASIC also supported Algol-like functions and subroutines from an early date, as did the BASIC09 variant from about 1979. Data types BASIC is known for good string manipulation functions. Early dialects already had a set of fundamental functions (LEFT$, MID$, RIGHT$) for string operations. Because strings are often used in everyday applications, this was a considerable convenience advantage over many other languages at the time of its introduction. The original Dartmouth BASIC supported only numeric and string data types. There was no integer type, leaving all numeric variables as floating point. Strings were dynamic, and could change their length. Arrays of both numbers and strings were supported, as well as simple matrices (ie, two dimensional arrays). Every modern BASIC dialect at least has a numeric and a string data type. Data types are usually distinguished by a sigil, or suffixed character; typically, string identifiers end with '$', whereas integer numerics usually end with a '%'. In some dialects, variables must be declared (using the DIM statement) on their first usage; other dialects do not require it, but can optionally enforce it—typically using a directive such as Option Explicit, In Visual Basic it is off by default but can be turned on using Option Explicit On. Many dialects also support such additional types as 16- and 32-bit integers and floating-point numbers. Some have "polynomial", "complex", "list" and specialized types designed to support particular end users' needs. Additionally, some allow user-defined types in a manner similar to Pascal "records" or C "structs". Most BASIC dialects beyond the most primitive also support arrays of numerics or of other types. In some, arrays must be pre-declared (ie, allocated with the DIM statement) before they can be used. Support for two- and higher-dimensional arrays, as well as arrays of non-numeric types, is common in advanced BASICs. DIM myIntArray(100) AS INTEGER — some versions will only allow: DIM myIntArray%(100) DIM myStringArray(50) AS STRING — some versions will only allow: DIM myStringArray$(50) Depending on the dialect of BASIC, arrays are 0-based (ie, the first element has subscript '0'), while others are 1-based (the first element is subscripted '1'). So, in some, commands like the first in the example will define a 101-element array of integers with elements from 0 to 100, others may define a 100-element array of integers with elements from 1 to 100 or, rarely, from 0 to 99. It may also be possible to set a default beginning element number other than 0 or 1 with an additional command such as Option Base. The Microsoft BASIC example above will create an array with elements numbered from 0 to 100. When working with strings, commands like the second may allocate a 50-element array of variable-length strings, or may allocate a single string of 50 characters. Note that they are also subject to the basing differences mentioned above. These non-standard and, to some, less than intuitive dimensioning examples often confuse the novice (and even the advanced) programmer. Newer dialects offer specific dimensioning, such as DIM myIntArray (10 TO 20) AS INTEGER which would be an 11-element array with elements numbered from 10 to 20. Relational, logical operators = equal <= less than or equal NOT logical negation <> not equal >= greater than or equal AND logical conjunction < less than OR logical disjunction > greater than Note that there is no lexical distinction between the assignment operator and the equality operator in BASIC; a single equal sign is used for both. There is, however, a method available to the programmer if a visible difference between the two is wanted: the optional LET keyword allows for assignments to be clearly and unambiguously distinguished from the use of the equality operator. Example: IF X=7 THEN LET Y=3.Also note that the AND, OR, NOT operators are actually bitwise arithmetic operations. They can also be used as logical operations, because most BASIC dialects represent the boolean values by -1 (true) and 0 (false). However, they resemble the C bitwise arithmetic operators &, | and ~ respectively, more than the C logical operators &&, || and !: for example, AND and OR always evaluate both their arguments (they are not short-circuiting operators).A first program New BASIC programmers on a home computer might start with a simple program similar to the Hello world program made famous by Kernighan and Ritchie. This generally involves a simple use of the language's PRINT statement to display the a message (such as the programmer's name) to the screen. Often an infinite loop was used to fill the display with the message. Classic BASIC Note that this example is actually well structured, demonstrating that use of the GOTO statement need not necessarily lead to an unstructured program. 10 INPUT "What is your name: "; U$ 20 PRINT "Hello "; U$ 30 REM 40 INPUT "How many stars do you want: "; N 50 S$ = "" 60 FOR I = 1 TO N 70 S$ = S$ + " 90 PRINT S$ 100 REM 110 INPUT "Do you want more stars? "; A$ 120 IF LEN(A$) = 0 THEN GOTO 110 130 A$ = LEFT$(A$, 1) 140 IF (A$ = "Y") OR (A$ = "y") THEN GOTO 40 150 PRINT "Goodbye "; 160 FOR I = 1 TO 200 170 PRINT U$; " "; 180 NEXT I 190 PRINT Modern BASIC "Modern" structured BASICs (for example, QuickBASIC, FreeBasic, PureBasic, BlitzMax, PowerBASIC, and TrueBASIC) support classic commands such as GOTO statements to varying degrees, while adding many more modern keywords. The previous example in QuickBASIC: INPUT "What is your name"; UserName$ PRINT "Hello "; UserName$ DO INPUT "How many stars do you want"; NumStars Stars$ = "" Stars$ = REPEAT$(" Stars$ = STRING$(NumStars, " DO INPUT "Do you want more stars"; Answer$ LOOP UNTIL Answer$ <> "" Answer$ = LEFT$(Answer$, 1) LOOP WHILE UCASE$(Answer$) = "Y" PRINT "Goodbye "; FOR I = 1 TO 200 PRINT UserName$; " "; NEXT I For comparison, the same program in the more modern PureBasic: OpenConsole() Print("What is your name ") UserName$ = Input() PrintN("Hello " + UserName$) Repeat Print("How many stars do you want ") NumStars = Val(Input()) Stars$ = RSet("", NumStars, " Repeat Print("Do you want more stars ") Answer$ = Input() Until Answer$ <> "" Answer$ = Left(Answer$, 1) Until UCase(Answer$) <> "Y" Print("Goodbye ") For I = 1 To 200 Print(UserName$ + " ") Next I PrintN("") CloseConsole() See also Notes Standards | |||||||||||||
|
| ||||||||||||||
 |
|
| |