## Summary

This document discusses the conventions for combining region and table
filters, especially with regards to the comma operator.

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.

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##### Last updated: November 16, 2005