Get next and previous elements out of an array in liquid

Question

Short version:

I want to add 1 to a number in a liquid template and use the result as an array index.

{% capture plus_one %}{{ 0 | plus: 1 }}{% endcapture %}
<div>-Value of plus_one: {{plus_one}}</div>
<div>-This works: {{site.posts[1].title}}</div>
<div>-This doesn't: {{site.posts[plus_one].title}}</div>

Result:

-Value of plus_one: 1
-This works: The Zone
-This doesn't:

---

Long version:

I'm using Jekyll, with no plugins. I want to give the current post a link to the next post that is in the same category. (The category is hardcoded to 'journal' in this code.)

My code loops over all posts in the category array, looking for the current post. When it is found, I try to grab the next post in the category array.

{% for num in (0..site.categories.journal.size) %}
    {% assign page2 = site.categories.journal[num] %}
        {% if page2.title == page.title and page2.date == page.date %}
            {% capture plus_one %}{{ num | plus: 1 }}{% endcapture %}
        {% endif %}
{% endfor %}

<div>value of plus_one: {{plus_one}}</div>
<div>This doesn't work: {{site.categories.journal[plus_one].title}}</div>
<div>This does: {{site.categories.journal[1].title}}</div>

Result:

<div>value of plus_one: 1</div>
<div>This doesn't work: </div>
<div>This does: A Blog Post Title</div>

I guess the value of my variable 'plus_one' is being treated as a string instead of a number.

Is there any way to convert it to a number?

Or is there another way to achieve what I'm trying to do?

Answer 1

I was having a similar problem, using the answers above I got the following working on my site. Might be of help to someone else.

{% for num in (0..site.categories.work.size) %}
    {% assign page2 = site.categories.work[num] %}
    {% if page2.title == page.title and page2.date == page.date %}
        {% assign next_i = forloop.index0 | plus: 1 %}
        {% assign prev_i = forloop.index0 | minus: 2 %}

        {% if next_i == site.categories.work.size %}
            {% assign next_i = 1 %}
        {% endif %}
    {% endif %}
{% endfor %}

<nav class="pagination">
    <div class="next">
        <a href="work/{{site.categories.work[next_i].slug}}/">Next</a>
    </div>
    <div class="previous">
        <a href="work/{{site.categories.work[prev_i].slug}}/">Previous</a>
    </div>
</nav>

Answer 2

assign preserves the number:

{% for item in items %}
    {% assign next_i = forloop.index0 | plus: 1 %}
    {% assign prev_i = forloop.index0 | minus: 1 %}
    {{ items[next_i] }}
    {{ items[prev_i] }}
{% endfor %}