NAME¶
FunCombine - Combining Region and Table Filters
SYNOPSIS¶
This document discusses the conventions for combining region and table filters,
especially with regards to the comma operator.
DESCRIPTION¶
Comma Conventions
Filter specifications consist of a series of boolean expressions, separated by
commas. These expressions can be table filters, spatial region filters, or
combinations thereof. Unfortunately, common usage requires that the comma
operator must act differently in different situations. Therefore, while its
use is intuitive in most cases, commas can be a source of confusion.
According to long-standing usage in IRAF, when a comma separates two table
filters, it takes on the meaning of a boolean
and. Thus:
foo.fits[pha==1,pi==2]
is equivalent to:
foo.fits[pha==1 && pi==2]
When a comma separates two spatial region filters, however, it has traditionally
taken on the meaning of a boolean
or. Thus:
foo.fits[circle(10,10,3),ellipse(20,20,8,5)]
is equivalent to:
foo.fits[circle(10,10,3) ⎪⎪ ellipse(20,20,8,5)]
(except that in the former case, each region is given a unique id in programs
such as funcnts).
Region and table filters can be combined:
foo.fits[circle(10,10,3),pi=1:5]
or even:
foo.fits[pha==1&&circle(10,10,3),pi==2&&ellipse(20,20,8,5)]
In these cases, it is not obvious whether the command should utilize an
or or
and operator. We therefore arbitrarily chose to implement
the following rule:
- •
- if both expressions contain a region, the operator used is or.
- •
- if one (or both) expression(s) does not contain a region, the operator
used is and.
This rule handles the cases of pure regions and pure column filters properly. It
unambiguously assigns the boolean
and to all mixed cases. Thus:
foo.fits[circle(10,10,3),pi=1:5]
and
foo.fits[pi=1:5,circle(10,10,3)]
both are equivalent to:
foo.fits[circle(10,10,3) && pi=1:5]
[NB: This arbitrary rule
replaces the previous arbitrary rule
(pre\-funtools 1.2.3) which stated:
- •
- if the 2nd expression contains a region, the operator used is
or.
- •
- if the 2nd expression does not contain a region, the operator used is
and.
In that scenario, the
or operator was implied by:
pha==4,circle 5 5 1
while the
and operator was implied by
circle 5 5 1,pha==4
Experience showed that this non-commutative treatment of the comma operator was
confusing and led to unexpected results.]
The comma rule must be considered provisional: comments and complaints are
welcome to help clarify the matter. Better still, we recommend that the comma
operator be avoided in such cases in favor of an explicit boolean operator.
SEE ALSO¶
See
funtools(7) for a list of Funtools help pages