Programs
General Description
BASIC is a line-oriented language. A BASIC program is a sequence of lines, the last of which shall be an end-line and each of which contains a keyword. Each line shall contain a unique line-number which serves as a label for the statement contained in that line.
Syntax
- program = block* end-line
- block = (line/for-block)*
- line = line-number statement end-of-line
- line-number = digit digit? digit? digit?
- end-of-line = [implementation-defined]
- end-line = line-number end-statement end-of-line
- end-statement = END
- statement = data-statement / def-statement / dimension -statement / gosub-statement / goto-statement / if-then-statement / input-statement / let-statement / on-goto-statement / option-statement / print-statement / randomize-statement / read-statement / remark-statement / restore-statement / return-statement / stop-statement
Examples
999 END
Semantics
A BASIC program shall be composed of a sequence of lines ordered by line-numbers, the last of which contains an end-statement. Program lines shall be executed in sequential order, starting with the first line, until
- some other action is dictated by a control statement, or
- an exception condition occurs, which results in a termination of the program, or
- a stop-statement or end-statement is executed.
Special conventions shall be observed regarding spaces. With the following exceptions, spaces may occur anywhere in a BASIC program without affecting the execution of that program and may be used to improve the appearance and readability of the program.
Spaces shall not appear:
- at the beginning of a line
- within keywords
- within numeric constants
- within line numbers
- within function or variable names
- within two-character relation symbols
All keywords in a program shall be preceded by at least one space and, if not at the end of a line, shall be followed by at least one space.
Each line shall begin with a line-number. The values of the integers represented by the line-numbers shall be positive nonzero; leading zeroes shall have no effect. Statements shall occur in ascending line-number order.
The manner in which the end of a statement line is detected is determined by the implementation; e.g. the end-of-line may be a carriage-return character, a carriage-return character followed by a line-feed character, or the end of a physical record. Lines in a standard-conforming program may contain up to 72 characters; the end-of-line indicator is not included within this 72 character limit.
The end-statement serves both to mark the physical end of the main body of a program and to terminate the execution of the program when encountered.
Exceptions
None.
Remarks
Local editing facilities may allow for the entry of statement lines in any order and also allow for duplicate line-numbers and lines containing only a line-number. Such editing facilities usually sort the program into the proper order and in the case of duplicate line-numbers, the last line entered with that line-number is retained. In many implementations, a line containing only a line-number (without trailing spaces) is usually deleted from the program.