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.help selectors Nov96 tables
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NAME
selectors -- Table row/column selector syntax.
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BASIC SYNTAX
Selectors are appended to table names using a bracket notation. The
construct "[c:......]" appended to a table name tells that a column
selector exists. In a similar way, the construct "[r:......]" indicates
a row selector.
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COLUMN SELECTOR
The basic structure of a column selector is a list of column patterns
separated by commas. The column pattern is either a column name, a file
name containing a list of column names, or a pattern using the usual IRAF
pattern matching syntax. For example, the string
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[c:a[1-9], b, time*, @column.lis]
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would be expanded as the column names a1 through a9, b, any column
name beginning with "time", and all the column names in the file
column.lis. If the column list is entirely whitespace, all columns are
selected. If the first non-white character is the negation character (~),
the selected columns will include all columns not matched by the list.
The negation character only has this meaning at the beginning of the list.
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ROW SELECTOR
Rows are selected according to a qpoe filter. The filter is evaluated
at each table row, and the row is selected if the filter is true.
For sake of an example, suppose we have a star catalog with the
columns Name, Ra, Dec, V, B-V, and U-B. The simplest sort of filter is
the equality test. The name of the column appears on the left of an
equals sign and the column value appears on the right. For example,
[name=eta_uma]. (The brackets in this and the following example are
not actually part of the filter.) Column numbers can be used in place
of the column name. This is especially useful for ascii
tables. Values can be either numbers or strings. It is usually not
necessary to place strings in quotes. However, any string (including
a column name) contains embedded blanks or characters significant to
the qpoe filter, such a equal signs, commas, or colons, it should be
placed in quotes.
Ranges of values can be specified by giving the endpoints of the
ranges separated by a colon. For example, [r:v=10:15] selects all rows
with visual magnitude between 10 and 15. Ranges include their
endpoints. Ranges can also be used with strings as well as
numbers. Ranges can also be one sided. The filter [r:dec=80:] selects
all rows with declination greater than or equal to eighty degress and
the filter [r:dec=:-40] selects all declinations less than or equal to
forty degrees south. A filter can contain a list of single values and
ranges. The values in the list should be enclosed in parentheses. For
example, [r:name=(eta_uma,alpha_lyr)] or [r:b-v=(-1:0,0.5:1)].
Individual values or ranges can be negated by placing a ! in front of
them. For example, [r:name=!eta_uma] selects every row except the star
named eta_uma and [r:ra=!0:6] selects all rows except those with right
ascension between zero and six hours. An entire list can be negated by
placing a ! in front of the column name or the parentheses enclosing
the list. The filters [r:!name=(eta_uma,alpha_lyr)] and
[r:name=!(eta_uma,alpha_lyr)] and [r:name=(!eta_uma,!alpha_lyr)] are all
equivalent.
Filters can test more than one column in a table. The individual tests
are separated by commas or semicolons. All tests in the filter must
succeed for the filter to be accepted. For example,
[r:ra=1.3:1.4,dec=40:42] selects a rectangular region in the catalog. A
range of row numbers can also be selected by placing the word row on
the left side of the equals sign. For example, [r:row=10:20] selects
rows from ten to twenty inclusive and [r:row=50:] selects all rows from
fifty on. Row selection can be combined with any other test in a
filter. A filter, can also be placed in an include file, for example
[r:@filter.lis]. Include files can be a part of a larger expression
and include files can contain other files, up to seven levels deep.
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EXAMPLES
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1. "[c:WAVELENGTH,FLUX]" selects columns named "WAVELENGTH" and
"FLUX"
2. "[r:WAVELENGTH=(4000:5000)]" selects all rows in which the WAVELENGTH
column assumes values in between 4000 and
5000.
3. "[c:FLUX][r:row=(25:30)]" selects column FLUX and all rows from 25
to 30.
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SEE ALSO
.endhelp
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