VARIABLE ROLE: Document the format and add a test.

[pspp] / doc / dev / system-file-format.texi

diff --git a/doc/dev/system-file-format.texi b/doc/dev/system-file-format.texi
index 8315762cef52cb26ea20f1aa2757ca3d6f622554..9bb636144030ea83c595a2ebd48a099192a61003 100644 (file)
--- a/doc/dev/system-file-format.texi
+++ b/doc/dev/system-file-format.texi
@@ -33,12 +33,28 @@ floating-point numbers, and translates as needed.  However, only IEEE
 has actually been observed in system files, and it is likely that
 other formats are obsolete or were never used.
 
-The PSPP system-missing value is represented by the largest possible
-negative number in the floating point format (@code{-DBL_MAX}).  Two
-other values are important for use as missing values: @code{HIGHEST},
-represented by the largest possible positive number (@code{DBL_MAX}),
-and @code{LOWEST}, represented by the second-largest negative number
-(in IEEE 754 format, @code{0xffeffffffffffffe}).
+System files use a few floating point values for special purposes:
+
+@table @asis
+@item SYSMIS
+The system-missing value is represented by the largest possible
+negative number in the floating point format (@code{-DBL_MAX}).
+
+@item HIGHEST
+HIGHEST is used as the high end of a missing value range with an
+unbounded maximum.  It is represented by the largest possible positive
+number (@code{DBL_MAX}).
+
+@item LOWEST
+LOWEST is used as the low end of a missing value range with an
+unbounded minimum.  It was originally represented by the
+second-largest negative number (in IEEE 754 format,
+@code{0xffeffffffffffffe}).  System files written by SPSS 21 and later
+instead use the largest negative number (@code{-DBL_MAX}), the same
+value as SYSMIS.  This does not lead to ambiguity because LOWEST
+appears in system files only in missing value ranges, which never
+contain SYSMIS.
+@end table
 
 System files are divided into records, each of which begins with a
 4-byte record type, usually regarded as an @code{int32}.
@@ -815,8 +831,8 @@ Ordinal Scale
 Continuous Scale
 @end table
 
-SPSS 14 sometimes writes a @code{measure} of 0 for string variables.
-PSPP interprets this as nominal scale.
+SPSS sometimes writes a @code{measure} of 0.  PSPP interprets this as
+nominal scale.
 
 @item int32 width;
 The width of the display column for the variable in characters.
@@ -1173,6 +1189,32 @@ will contain a variable attribute record with the following contents:
 00000030  0a 29                                             |.)              |
 @end example
 
+@menu
+* Variable Roles::
+@end menu
+
+@node Variable Roles
+@subsection Variable Roles
+
+A variable's role is represented as an attribute named @code{$@@Role}.
+This attribute has a single element whose values and their meanings
+are:
+
+@table @code
+@item 0
+Input.  This, the default, is the most common role.
+@item 1
+Output.
+@item 2
+Both.
+@item 3
+None.
+@item 4
+Partition.
+@item 5
+Split.
+@end table
+
 @node Extended Number of Cases Record
 @section Extended Number of Cases Record
 
@@ -1237,7 +1279,9 @@ Record type.  Always set to 7.
 @item int32 subtype;
 Record subtype.  May take any value.  According to Aapi
 H@"am@"al@"ainen, value 5 indicates a set of grouped variables and 6
-indicates date info (probably related to USE).
+indicates date info (probably related to USE).  Subtype 24 appears to
+contain XML that describes how data in the file should be displayed
+on-screen.
 
 @item int32 size;
 Size of each piece of data in the data part.  Should have the value 1,