.help expressions Feb07 "IRAF Expressions"

.sh
Introduction

Many
.hr #s_see_also IRAF tasks
use expressions.  These all go through the same basic expression evaluator.
This provides a standard set of operators and functions with a 
common syntax similar to the CL.  However, each application can customize

.ls *
how expressions are input; e.g. from files or with special prefixes
.le
.ls *
how variables (called operands) are specified; e.g. with leading
special characters such as '$' or with single letter capitals
.le
.ls *
where the values are obtained; e.g. from image header keywords or
from data files
.le
.ls *
special, application specific functions; e.g. airmass or arcsep
.le

This help topic covers the common features of expressions.  The
individual task help descriptions may also include this material or
just describe the customization.

.sh
Expressions

Expressions consist of operands, numeric and string constants, operators,
and functions.  Parentheses are also used to control the evaluation order
as in standard algebraic expressions.  String constants are quoted strings
and numeric constants are pure unquoted numbers.  Numbers may be given in
sexagesimal notation and are automatically converted to decimal numbers.
The operators are arithmetic, logical, and string.

.sh
Operands

The way applications refer to operands is generally unique to that task.
Sometimes they are simply alphanumeric identifiers, sometimes they
have prefixes such '$', sometimes they are limited to specific
identifiers, and sometimes they have additional structure such as
<image>.<keyword> where <image> is an identifier for an image
and <keyword> refers to a keyword in the image.  Common types of
operands refer to images, table columns, and keywords.

There is one special common syntax for operands that include characters that
correspond to the operators in expressions.  In this case the operand
identifier must be quoted as a string and then referenced using a leading @
character; e.g.  @'mjd-obs'.

.sh
Operators

The following operators are recognized in expressions.  With the exception
of the operators "?", "?=", and "@", the operator set is equivalent to that
available in the CL.

.nf
        +  -  *  /              arithmetic operators
        **                      exponentiation
        //                      string concatenation
        !  -                    boolean not, unary negation
        <  <= >  >=             order comparison (works for strings)
        == != && ||             equals, not equals, and, or
        ?=                      string equals pattern
        ? :                     conditional expression (ternary operator)
        @                       reference a operand
.fi

The operators "==", "&&", and "||" may be abbreviated as "=", "&", and "|"
if desired.  The ?= operator performs pattern matching upon strings.
The pattern syntax is that described for the task
.hr match \fBmatch\fR.
The @ operator is required to reference keywords with
one of the operator characters.  This is most likely to be used as:

.nf
        @"date-obs"
.fi

A point to be aware of is that in the ?: conditional expression both
possible result values are evaluated though the result of the expression
is only one of them.

.sh
Functions

A number of standard intrinsic functions are recognized within
expressions.  Many of these may not be useful in the context of the
application but are part of the language.  The set of
functions currently supported is shown below.

.nf
	abs     atan2   deg     log     min     real    sqrt
	acos    bool    double  log10   mod     short   str
	asin    cos     exp     long    nint    sin     tan
	atan    cosh    int     max     rad     sinh    tanh
.fi

The trigonometric functions operate in units of radians.
The \fImin\fR and \fImax\fR functions may have any number of arguments up
to a maximum of sixteen or so (configurable).  The arguments need not all
be of the same datatype.

A function call may take either of the following forms:

.nf
        <identifier> '(' arglist ')'
        <string_expr> '(' arglist ')'
.fi

The first form is the conventional form found in all programming languages.
The second permits the generation of function names by string valued
expressions and might be useful on rare occasions.

.sh
SEE ALSO

.hr imexpr \fBimexpr\fR
.hr hedit \fBhedit\fR
.hr hselect \fBhselect\fR
.hr asthedit \fBasthedit\fR
.hr astcalc \fBastcalc\fR
.hr irproc \fBirproc\fR
.hr ircatalog \fBircatalog\fR
.hr ccget \fBccget\fR
.hr mskexpr \fBmskexpr\fR
.hr mskreg \fBmskreg\fR
.hr import \fBimport\fR
.hr export \fBexport\fR
.hr agetcat \fBagetcat\fR
.hr pcalc \fBpcalc\fR
.hr pconvert \fBpconvert\fR
.hr pdump \fBpdump\fR
.hr pselect \fBpselect\fR
.hr tbcalc \fBtbcalc\fR
.hr tbdump \fBtbdump\fR
.hr tbselect \fBtbselect\fR
.hr txcalc \fBtxcalc\fR
.hr txdump \fBtxdump\fR
.hr txselect \fBtxselect\fR

.endhelp