Rules for expressions

The expression string can contain constants, column and parameter names, operators, functions, and parentheses. In general the usual rules of algebra and Fortran should be followed, with some minor exceptions as noted below.

1. Spaces are permitted between items, except that a function-name must be followed immediately by a left parenthesis. Spaces are not permitted within items such as names and numerical constants, but can be used within character strings and date/time values in curly braces.

2. Lower-case letters are treated everywhere as identical to the corresponding upper-case letter.

3. Column and parameter names can be up to fifteen characters long, and may consist of letters, digits, and underscores, except that the first character must not be a digit.

4. Vector elements are supported but with a restricted syntax: they may consist of a name followed by an unsigned integer constant subscript enclosed in square brackets, for example FLUX[4] or MAGNITUDE[13]. The first element of the vector is numbered one.

5. CHARACTER constants may be enclosed in a pair of single or double quotes; embedded quotes of the same type may be denoted by doubling up on the quote character within the string, for example 'DON''T' or "DON""T".

6. LOGICAL constants may be .TRUE. or .FALSE. but abbreviations of these words are allowed down to .T. and .F.

7. Numerical constants may appear in any valid form for Fortran 77 (except that embedded spaces are not allowed). Some additional forms are also permitted, as shown below.

8. %Xstring %Ostring %Bstring for hexadecimal, octal and binary INTEGER constants respectively.

9. Angles in sexagesimal notation: colons must be used to separate items, for example hours:minutes:seconds (or degrees:minutes:seconds). If there is a leading sign then the value will be taken as degrees:minutes:seconds, otherwise hours:minutes:seconds. In either case the value is converted to RADIANS.

10. A date/time value may be given as a string enclosed in curly braces; a range of common formats are permitted, with order year-month-day or day-month-year, and the month as a number or three-character abbreviation. The time may follow with colons separating hours:minutes:seconds. Examples of some valid dates:

    1992-JUL-26 12:34:56
    92.7.26
    26/7/92T3:45

11. Relational operators are supported in both Fortran 77 form (for example .GE. .NE.) as well as in the Fortran 90 forms (for example, ${\tt >=}$ ${\tt /=}$ ).

12. Single-symbol forms for .AND. .OR. and .NOT. are provided as an alternative: & ${\tt \vert}$ # respectively.

13. The dots may be left off the Fortran 77 forms of the relational operators and the logical operators .AND. and .OR. where spaces or parentheses separate them from names or constants, but the logical constants and the .NOT. operator need the enclosing dots to distinguish them from other lexical items in all cases.

14. INTEGER division does not result in truncation (as in Fortran) but produces a floating-point result. The NINT or INT function should be used (as appropriate) if an INTEGER result is required.

15. The functions MAX and MIN must have exactly two arguments.

16. All arithmetic is carried out internally in DOUBLE PRECISION (but the compiler works out the effective data type of the result using the normal expression rules).

17. Exponentiation is performed by log/exp functions, with use of ABS to avoid taking logs of negative arguments, thus -2**3 will come out as `+8' not `-8'.