How to check if `folderitem` is the first array element in Jekyll (Liquid)?

Question

The following code is supposed to add an <h1> for every page in PDF output. Basically, it steps through the YAML file that defines my guide and, if the URL of the current page matches the URL in the YAML block, it places the title key of the parent block within an <h1> tag.

{% assign sidebar = site.data.sidebars[page.sidebar].entries %}
    
{% for entry in sidebar %}
  {% for folder in entry.folders %}
    {% if folder.title and folder.type != "navi" and folder.type != "frontmatter" %}
      {% for folderitem in folder.folderitems %}
        {% if folderitem.url == page.url %}
<h1 class="post-title-main" id="{{page.permalink | replace: '/', '' }}">{{ folder.title }}</h1>
        {% endif %}
      {% endfor %}
    {% endif %}
  {% endfor %}
{% endfor %}

The problem is that the current code does this for every page, like this:

<h1>The Title I Want to Put in the H1</h1>
<h2>The Title of the Page Which I *Do* Want the <H1> to Appear Above</h2>
. . .
<h1>The Title I Want to Put in the H1</h1>
<h2>The Title of the Page Which I *Don't* Want the <H1> to Appear Above</h2>

I need another conditional check, something like "if folderitem is the first one" that would let the rest of the conditional logic proceed only if the item is the first in the list, resulting in:

<h1>The Title I Want to Put in the H1</h1>
<h2>The Title of the Page Which I *Do* Want the <H1> to Appear Above</h2>

What is the syntax for selecting the first item? first appears to be for returning the first item of an array, which doesn't seem to apply to checking if the current item is the first item in the YAML data structure that I'm working with.

Here's an example YAML structure:

entries:
- title:
  pdftitle: foobar.pdf
  product:
  version:
  folders:

  - title:
    output: pdf
    type: frontmatter
    folderitems:

    - title:
      url: /titlepage.html
      output: pdf
      type: frontmatter

  - title: My Amazing Guide
    output: web
    type: navi
    folderitems:

    - title: Home
      url: /index.html
      output: web

- title: The Title I Want to Put in the H1
    url: /section/page/index.html
    output: web, pdf
    folderitems:

    - title: The Title of the Page Which I *Do* Want the <H1> to Appear Above
      url: /section/page/some-page.html
      output: web, pdf

    - title: The Title of the Page, Which I *Don't* Want the <H1> to Appear Above
      url: /section/page/another-page.html
      output: web, pdf

Answer 1

Idea from: jekyll/liquid: given key access value from hash in template

Could not test it but could something like this help you in your loop?

{% assign first_url= site.data.sidebars[page.sidebar].entries | where: "url", page.url | first %}`

Answer 2

I figured it out. Instead of

{% if folderitem.url == page.url %}

I was able to use

{% if folderitem.url == page.url and forloop.first == true %}

This additional condition does, in fact, check precisely what I wanted to check—whether the array item is first. Ironically (because I'm writing documentation?), the problem is that the Liquid documentation on https://shopify.github.io/liquid/ is incomplete, whereas the Liquid documentation on https://shopify.dev/api/liquid/ is far more comprehensive.

Here is the documentation on forloop.first . It is here that I also learned that I could use properties like forloop.index (1-based), forloop.index0 , and so forth.