counter

Counts through pages, elements, and more.

With the counter function, you can access and modify counters for pages, headings, figures, and more. Moreover, you can define custom counters for other things you want to count.

Displaying a counter

To display the current value of the heading counter, you call the counter function with the key set to heading and then call the display method on the counter. To see any output, you also have to enable heading numbering.

The display method optionally takes an argument telling it how to format the counter. This can be a numbering pattern or a function.

#set heading(numbering: "1.")

= Introduction
Some text here.

= Background
The current value is:
#counter(heading).display()

Or in roman numerals:
#counter(heading).display("I")

Modifying a counter

To modify a counter, you can use the step and update methods:

• The step method increases the value of the counter by one. Because counters can have multiple levels (in the case of headings for sections, subsections, and so on), the step method optionally takes a level argument. If given, the counter steps at the given depth.

• The update method allows you to arbitrarily modify the counter. In its basic form, you give it an integer (or multiple for multiple levels). For more flexibility, you can instead also give it a function that gets the current value and returns a new value.

The heading counter is stepped before the heading is displayed, so Analysis gets the number seven even though the counter is at six after the second update.

#set heading(numbering: "1.")

= Introduction
#counter(heading).step()

= Background
#counter(heading).update(3)
#counter(heading).update(n => n * 2)

= Analysis
Let's skip 7.1.
#counter(heading).step(level: 2)

== Analysis
Still at #counter(heading).display().

To define your own counter, call the counter function with a string as a key. This key identifies the counter globally.

#let mine = counter("mycounter")
#mine.display() \
#mine.step()
#mine.display() \
#mine.update(c => c * 3)
#mine.display() \

How to step

When you define and use a custom counter, in general, you should first step the counter and then display it. This way, the stepping behaviour of a counter can depend on the element it is stepped for. If you were writing a counter for, let's say, theorems, your theorem's definition would thus first include the counter step and only then display the counter and the theorem's contents.

#let c = counter("theorem")
#let theorem(it) = block[
  #c.step()
  *Theorem #c.display():* #it
]

#theorem[$1 = 1$]
#theorem[$2 < 3$]

The rationale behind this is best explained on the example of the heading counter: An update to the heading counter depends on the heading's level. By stepping directly before the heading, we can correctly step from 1 to 1.1 when encountering a level 2 heading. If we were to step after the heading, we wouldn't know what to step to.

Because counters should always be stepped before the elements they count, they always start at zero. This way, they are at one for the first display (which happens after the first step).

Page counter

The page counter is special. It is automatically stepped at each pagebreak. But like other counters, you can also step it manually. For example, you could have Roman page numbers for your preface, then switch to Arabic page numbers for your main content and reset the page counter to one.

#set page(numbering: "(i)")

= Preface
The preface is numbered with
roman numerals.

#set page(numbering: "1 / 1")
#counter(page).update(1)

= Main text
Here, the counter is reset to one.
We also display both the current
page and total number of pages in
Arabic numbers.

Time travel

Counters can travel through time! You can find out the final value of the counter before it is reached and even determine what the value was at any particular location in the document.

#let mine = counter("mycounter")

= Values
#locate(loc => {
  let start-val = mine.at(loc)
  let elements = query(<intro>, loc)
  let intro-val = mine.at(
    elements.first().location()
  )
  let final-val = mine.final(loc)
  [Starts as: #start-val \
   Value at intro is: #intro-val \
   Final value is: #final-val \ ]
})

#mine.update(n => n + 3)

= Introduction <intro>
#lorem(10)

#mine.step()
#mine.step()

Let's dissect what happens in the example above:

• We call locate to get access to the current location in the document. We then pass this location to our counter's at method to get its value at the current location. The at method always returns an array because counters can have multiple levels. As the counter starts at zero, the first value is thus (0,).

• We now query the document for all elements with the <intro> label. The result is an array from which we extract the first (and only) element's location. We then look up the value of the counter at that location. The first update to the counter sets it to 0 + 3 = 3. At the introduction heading, the value is thus (3,).

• Last but not least, we call the final method on the counter. It tells us what the counter's value will be at the end of the document. We also need to give it a location to prove that we are inside of a locate call, but which one doesn't matter. After the heading follow two calls to step(), so the final value is (5,).

Other kinds of state

The counter type is closely related to state type. Read its documentation for more details on state management in Typst and why it doesn't just use normal variables for counters.

Create a new counter identified by a key.