= &Attribute References

.Syntax
[source,unlang]
----
&Attribute-Name
----

The `&Attribute-Name` operator returns a reference to the named
attribute.

When used as an existence check in a condition, the condition
evaluates to `true` if the attribute exists.  Otherwise, the condition
evaluates to `false`.

When used elsewhere, such as in xref:switch.adoc[switch], it returns
the value of the named attribute.

.Examples
[source,unlang]
----
&User-Name
&NAS-IP-Address
----

== Lists

The attribute reference can also be qualified with a
xref:list.adoc[list] reference.  When no list is given, the server
looks in the input packet list for the named attribute.

.Examples

[source,unlang]
----
&request.User-Name
&reply.NAS-IP-Address
----

== Array References

.Syntax
[source,unlang]
----
&Attribute-Name[<integer]
----

When an attribute appears multiple times in a list, this syntax allows
you to address the attributes as if they were array entries.  The
`<integer>` value defines which attribute to address.  The `[0]` value
refers to the first attributes, `[1]` refers to the second attribute,
etc.

.Examples
[source,unlang]
----
&EAP-Message[1]
&reply.NAS-IP-Address[2]
----

== Parent / child references

.Syntax
[source,unlang]
----
&Attribute-Name.Attribute-Name
----

In some cases, attributes are nested or grouped.  The child reference
syntax is used to address a particular child attribute.  Multiple
levels of parent / child nesting can be performed by simply appending
the name of another child attribute, separated by the `.` character.

.Examples

[source,unlang]
----
&TLV-One.Child-one
&TLV-One.Child-one.Child-Three
&reply.TLV[3].Child-Two
----

== Putting it All Together

All of the above syntaxes can be used interchangeably, at any nesting level.  The examples are presented

.Examples
[source,unlang]
----
&TLV-One.Child-one[3].Child-Three
&reply.TLV[3].Child-Two
----

== Removing ambuguity from the configuration files

The server does not use the `&` character to distinguish attribute names
from other strings.  

Without the `&`, it is more difficult to parse the configuration file
clearly. You could interpret a string as `hello-there`
either as a literal string "hello-there", or as a reference to an
attribute named `hello-there`.  

Adding the leading `&` character means that attribute references are
now easily distinguishable from literal strings.  The use of the leading 
`&` character is highly recommended.


// Copyright (C) 2020 Network RADIUS SAS.  Licenced under CC-by-NC 4.0.
// Development of this documentation was sponsored by Network RADIUS SAS.
