PDS4 field format Conventions

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The <field_format> attribute found in table field definitions is equivalent to the PDS3 FORMAT keyword in COLUMN definitions. But in PDS4 we will be using a subset of the POSIX I/O conversion specifiers (as used in most modern programming languages with a printf statement or the equivalent), rather then the FORTRAN specifiers used in PDS3.



NOTE: As of this update, this is still a proposal, not a standard. The details have been discussed by the DDWG, and the following text edited accordingly for the next round of discussion. There is one outstanding question:

  • If your binary field contains packed data, what should be used for a field format?

Do check for a final standard before submitting data for review or archiving.



Why bother?

The field_format attribute provides two potential benefits to users and the archive:

  1. In character tables, the field_format specification provides width and precision information that can be used in validating individual values in the table data.
  2. In binary tables, the field_format specification can be used as an output format for converting binary numeric values to a character form without losing or overstating significant digits.


The POSIX Standard

The latest edition of the relevant standard is IEEE Std 1003.1-2004. The subset we're selecting is compatible with the 2001 version of the same standard, which in turn defers to the ISO C standard for printf conventions. Specifically, I'm referencing section 5: "File Notation Conventions".

Formation Rule

The formation rule for a field_format value is:

%[+|-]width[.precision]specifier

where square brackets indicate an optional component, and:

width
is the total potential width of the field (i.e., the width of the widest value occuring in the field)
precision
is the number of digits following the decimal point for real numbers (but is otherwise ignored)
specifier
is exactly one of the characters in the set [doxfes]


Breaking this rule down into separate parts...

%
The format must begin with a percent sign ("%").
[+|-]
Either a "+" or "-", but never both. The "-" may be used for string fields, to indicate that the string is (or should be) left-justified in the field. This is actually the preferred way to present most string values in character tables, so the field_format value for fields with a data type of ASCII_String will nearly always begin with a "-". Similarly for any of the date/time type fields. In PDS4 labels, the "-" prefix is forbidden for all numeric fields (integers, floating point numbers, and numbers using scientific notation).
The "+" may be used with numeric fields to indicate that an explicit sign is included in the field for input, and should be displayed on output. In PDS4 labels, the "+" is forbidden for string fields.
width
The width is an integer value indicating the maximum number of characters needed for the complete representation of the largest (in terms of display bytes, not necessarily magnitude) value occuring, or potentially occuring, in the field. This should include bytes for signs, decimal points, and exponents. In the case of string values, it should be the maximum width from the first non-blank character to the last non-blank character. It should not include bytes for field delimiters, which are not considered part of the field. In character tables, it must be the same as the <field_length> for scalar fields.
The width is separated from the precision by a decimal point ("."). If there is no precision specified, the decimal point must be omitted.
precision
The precision value is used in three different ways:
  1. In real numbers, it indicates the number of digits to the right of the decimal point.
  2. In integers, it indicates that the integer will be zero-padded on the left out to the full field width. For example, the value "2" in "%3.3d" format is "002".
  3. In strings, it signifies the maximum number of characters from the actual string value that should be printed. (It is possible in programming, for example, to print no more than the first 10 characters from a string, but require that the output field be left-justified and padded with at least 5 blanks by using a specifier of "%15.10s".) In PDS4 archive labels, if a precision value is included for a string format, it must be equal to the field width.
specifier
The specifier indicates the broad data type for display. It will be one of a subset of the conversion specifiers included in the IEEE standard:
d
A decimal integer
o
An unsigned octal integer
x
An unsigned hexidecimal number
f
A floating point number in the format [-]ddd.ddd, where the actual number of digits before and after the decimal point are determined by the preceding width and precision values (note that the width includes the decimal point and any sign).
e
A floating point number in the format [-]d.ddde+/-dd where "+/-" stands for exactly one character (either "+" or "-"), there is always exactly one digit to the left of the decimal point, and the number of digits to the right of the decimal point is determined by the preceding precision value (note that the width includes all digits, signs, and the decimal point).
s
A string value. Note that strings should generally be left-justified in fixed width character tables and on output from a binary table, so most field_format values ending in "s" should begin with "-"

Variations on the Theme

The proposal is intended to be a very limited subset of the total universe of possible format conversion specifiers. Here are some things that were discussed by the DDWG but not included in the proposed standard, above. If you have an opinion about them, NOW would be an excellent time to tell me.

"i" conversion specifier
The "i" specifier is identical to the "d" specifier in every way. I chose "d" over "i" because it was mnemonic ("d" for decimal, "o" for octal, "x" for hexadecimal - OK, so it's not a perfect system). The DDWG also preferred it in that it makes it clear that we are using a different format specification system from the PDS3 standards.
"X" and "E" specifiers
These uppercase specifiers only matter on output - they cause the letters in their formats ("a"-"f" for hexadecimal, "e" for reals) to be uppercase rather than lowercase. Programmers outputting values from PDS4 label info can certainly convert the lowercase forms to uppercase easily and unambiguously, so these were omitted to keep the archiving requirements simple and to the point.
"-" for numbers
POSIX allows numbers to be left-justified, but this is problematic for documenting numeric fields in an archive. If you have a field that looks like a number that you want to left-justify, you should treat it as a simple string.
"g", "G" and "c" specifiers
The "g|G" specifier switches back and forth between floating point and exponential notation, depending on the magnitude of the output value. I'm not a big fan of that for archive tables, but perhaps it is useful. The "c" specifier is for I/O on a single character at a time - it cannot take a field width or precision, and the output is a single printable character, not the decimal equivalent. I think "%1s" is good enough for us, and the subtle distinction between "%c" and "%1s" is not something I want to spend the next 10 years explaining.