Provided by: libfuntools-dev_1.4.8-1_amd64 
NAME
Funfilters - Filtering Rows in a Table
SYNOPSIS
This document contains a summary of the user interface for filtering rows in binary
tables.
DESCRIPTION
Table filtering allows a program to select rows from an table (e.g., X-ray event list) by
checking each row against one or more expressions involving the columns in the table. When
a table is filtered, only valid rows satisfying these expressions are passed through for
processing.
A filter expression is specified using bracket notation appended to the filename of the
data being processed:
foo.fits[pha==1&&pi==2]
It is also possible to put region specification inside a file and then pass the filename
in bracket notation:
foo.fits[@my.reg]
Filters must be placed after the extension and image section information, when such
information is present. The correct order is:
• file[fileinfo,sectioninfo][filters]
• file[fileinfo,sectioninfo,filters]
where:
• file is the Funtools file name
• fileinfo is an ARRAY, EVENT, FITS extension, or FITS index
• sectioninfo is the image section to extract
• filters are spatial region and table (row) filters to apply
See Funtools Files for more information on file and image section specifications.
Filter Expressions
Table filtering can be performed on columns of data in a FITS binary table or a raw event
file. Table filtering is accomplished by means of table filter specifications. An table
filter specification consists of one or more filter expressions Filter specifications also
can contain comments and local/global processing directives.
More specifically, a filter specification consist of one or more lines containing:
# comment until end of line
# include the following file in the table descriptor
@file
# each row expression can contain filters separated by operators
[filter_expression] BOOLOP [filter_expression2], ...
# each row expression can contain filters separated by the comma operator
[filter_expression1], [filter_expression2], ...
# the special row# keyword allows a range of rows to be processed
row#=m:n
# or a single row
row#=m
# regions are supported -- but are described elsewhere
[spatial_region_expression]
A single filter expression consists of an arithmetic, logical, or other operations
involving one or more column values from a table. Columns can be compared to other
columns, to header values, or to numeric constants. Standard math functions can be applied
to columns. Separate filter expressions can be combined using boolean operators. Standard
C semantics can be used when constructing expressions, with the usual precedence and
associativity rules holding sway:
Operator Associativity
-------- -------------
() left to right
!! (logical not) right to left
! (bitwise not) - (unary minus) right to left
* / left to right
+ - left to right
< <= > >= left to right
== != left to right
& (bitwise and) left to right
^ (bitwise exclusive or) left to right
⎪ (bitwise inclusive or) left to right
&& (logical and) left to right
⎪⎪ (logical or) left to right
= right to left
For example, if energy and pha are columns in a table, then the following are valid
expressions:
pha>1
energy == pha
(pha>1) && (energy<=2)
max(pha,energy)>=2.5
Comparison values can be integers or floats. Integer comparison values can be specified in
decimal, octal (using '0' as prefix), hex (using '0x' as prefix) or binary (using '0b' as
prefix). Thus, the following all specify the same comparison test of a status mask:
(status & 15) == 8 # decimal
(status & 017) == 010 # octal
(status & 0xf) == 0x8 # hex
(status & 0b1111) == 0b1000 # binary
The special keyword row# allows you to process a range of rows. When row# is specified,
the filter code skips to the designated row and only processes the specified number of
rows. The "*" character can be utilized as the high limit value to denote processing of
the remaining rows. Thus:
row#=100:109
processes 10 rows, starting with row 100 (counting from 1), while:
row#=100:*
specifies that all but the first 99 rows are to be processed.
Spatial region filtering allows a program to select regions of an image or rows of a table
(e.g., X-ray events) using simple geometric shapes and boolean combinations of shapes.
For a complete description of regions, see Spatial Region Filtering.
Separators Also Are Operators
As mentioned previously, multiple filter expressions can be specified in a filter
descriptor, separated by commas or new-lines. When such a comma or new-line separator is
used, the boolean AND operator is automatically generated in its place. Thus and
expression such as:
pha==1,pi=2:4
is equivalent to:
(pha==1) && (pi>=2&&pi<=4)
[Note that the behavior of separators is different for filter expressions and spatial
region expressions. The former uses AND as the operator, while the latter user OR. See
Combining Region and Table Filters for more information about these conventions and how
they are treated when combined.]
Range Lists
Aside from the standard C syntax, filter expressions can make use of IRAF-style range
lists which specify a range of values. The syntax requires that the column name be
followed by an '=' sign, which is followed by one or more comma-delimited range
expressions of the form:
col = vv # col == vv in range
col = :vv # col <= vv in range
col = vv: # col >= vv in range
col = vv1:vv2 # vv1 <= col <= vv2 in range
The vv's above must be numeric constants; the right hand side of a range list cannot
contain a column name or header value.
Note that, unlike an ordinary comma separator, the comma separator used between two or
more range expressions denotes OR. Thus, when two or more range expressions are combined
with a comma separator, the resulting expression is a shortcut for more complicated
boolean logic. For example:
col = :3,6:8,10:
is equivalent to:
(col=6 && col =10)
Note also that the single-valued rangelist:
col = val
is equivalent to the C-based filter expression:
col == val
assuming, of course, that val is a numeric constant.
Math Operations and Functions
It is permissible to specify C math functions as part of the filter syntax. When the
filter parser recognizes a function call, it automatically includes the math.h and links
in the C math library. Thus, it is possible to filter rows by expressions such as these:
• (pi+pha)>(2+log(pi)-pha)
• min(pi,pha)*14>x
• max(pi,pha)==(pi+1)
• feq(pi,pha)
• div(pi,pha)>0
The function feq(a,b) returns true (1) if the difference between a and b (taken as double
precision values) is less than approximately 10E-15. The function div(a,b) divides a by
b, but returns NaN (not a number) if b is 0. It is a safe way to avoid floating point
errors when dividing one column by another.
Include Files
The special @filename directive specifies an include file containing filter expressions.
This file is processed as part of the overall filter descriptor:
foo.fits[pha==1,@foo]
Header Parameters
The filter syntax supports comparison between a column value and a header parameter value
of a FITS binary tables (raw event files have no such header). The header parameters can
be taken from the binary table header or the primary header. For example, assuming there
is a header value MEAN_PHA in one of these headers, you can select photons having exactly
this value using:
• pha==MEAN_PHA
Table filtering is more easily described by means of examples. Consider data containing
the following table structure:
• double TIME
• int X
• int Y
• short PI
• short PHA
• int DX
• int DY
Tables can be filtered on these columns using IRAF/QPOE range syntax or any valid C
syntax. The following examples illustrate the possibilities:
• pha=10
• pha==10
select rows whose pha value is exactly 10
• pha=10:50
select rows whose pha value is in the range of 10 to 50
• pha=10:50,100
select rows whose pha value is in the range of 10 to 50 or is equal to 100
• pha>=10 && pha<=50
select rows whose pha value is in the range of 10 to 50
• pi=1,2&&pha>3
select rows whose pha value is 1 or 2 and whose pi value is 3
• pi=1,2 ⎪⎪ pha>3
select rows whose pha value is 1 or 2 or whose pi value is 3
• pha==pi+1
select rows whose pha value is 1 less than the pi value
• (pha==pi+1) && (time>50000.0)
select rows whose pha value is 1 less than the pi value and whose time value is
greater than 50000
• (pi+pha)>20
select rows in which the sum of the pi and pha values is greater than 20
• pi%2==1
select rows in which the pi value is odd
Currently, integer range list limits cannot be specified in binary notation (use decimal,
hex, or octal instead). Please contact us if this is a problem.
SEE ALSO
See funtools(7) for a list of Funtools help pages