////
Included in:

- user-manual: Counters
////

// document attributes and counters are NOT the same thing, but modifying a document attribute with the same name as the counter modifies the counter at the same time.

Counters are used to store and display ad-hoc sequences of numbers or latin characters.
They are designed for simple use cases.
Possible uses include inline itemized lists, paragraph numbers and serial item numbers.

A counter is implemented as a specialized document attribute.
You declare and display a counter using an augmented attribute reference, in which the attribute name is prefixed with `counter:` (e.g., `+{counter:name}+`).
Since counters are attributes, counter names follow the same rules as <<attribute-restrictions,attribute names>>.
The most important rule to note is that letters in counter names _must be lowercase_.

The counter value is incremented and displayed every time the `counter:` attribute reference is used.
The term [.term]_increment_ means advance to the next value in the sequence.
If the counter value is an integer, add 1.
If the counter value is a character, move to the next letter in the Latin alphabet.
The default start value of a counter is 1.

To create a sequence starting at 1, use the simple form `+{counter:name}+` as shown here:

[source]
The salad calls for {counter:seq1}) apples, {counter:seq1}) oranges and {counter:seq1}) pears.

Here's the resulting output:

====
:!seq1:
The salad calls for {counter:seq1}) apples, {counter:seq1}) oranges and {counter:seq1}) pears.
====

To increment the counter without displaying it (i.e., to skip an item in the sequence), use the `counter2` prefix instead:

[source]
{counter2:seq1}

WARNING: A `counter2` attribute reference on a line by itself will produce an empty paragraph.
You'll need to adjoin it to the nearest content to avoid this side effect.

To display the current value of the counter without incrementing it, reference the counter name as you would any other attribute:

[source]
{counter2:pnum}This is paragraph {pnum}.

To create a character sequence, or start a number sequence with a value other than 1, specify a start value by appending it to the first use of the counter:

[source]
Dessert calls for {counter:seq1:A}) mangoes, {counter:seq1}) grapes and {counter:seq1}) cherries.

CAUTION: Character sequences either run from a,b,c,...x,y,z,{,|... or A,B,C,...,X,Y,Z,[,... depending on the start value.
Therefore, they aren't really useful for more than 26 items.

The start value of a counter is only recognized if the counter is _unset_ at that point in the document.
Otherwise, the start value is ignored.

To reset a counter attribute, unset the corresponding attribute using an attribute entry.
The attribute entry must be adjacent to a block or else it is ignored.

[source]
----
The salad calls for {counter:seq1:1}) apples, {counter:seq1}) oranges and {counter:seq1}) pears.

:!seq1:
Dessert calls for {counter:seq1:A}) mangoes, {counter:seq1}) grapes and {counter:seq1}) cherries.
----

This gives:

====
:!seq1:
The salad calls for {counter:seq1:1}) apples, {counter:seq1}) oranges and {counter:seq1}) pears.

:!seq1:
Dessert calls for {counter:seq1:A}) mangoes, {counter:seq1}) grapes and {counter:seq1}) cherries.
====

Here's a full example that shows how to use a counter for part numbers in a table.

[source]
----
include::ex-counter.adoc[tag=base]
----

Here's the output of that table:

====
include::ex-counter.adoc[tag=base]
====