Can I create and manipulate a Map in jekyll liquid? - jekyll

Question

I want to calculate total sums while processing a table, so as rendering goes row by row but I need totals over the columns, I need a counter per column.
{% assign cols = "col1,col2" | split: "," %}
{% assign rows = "row1,row2" | split: "," %}
{% assign total = ????? %}
...
{% for row in rows %}
<tr>
{% for col in cols %}
<td>
{% for post in site.posts %}
{% if post.colThing == col and post.rowThing == row %}
{% assign total[row] = total[row] | plus: post.thatNumber %}
.... {{ post.thatNumber }} ...
{% endif %}
{% endfor %}
</td>
{% endfor %}
</tr>
{% endfor %}
<tr>
{% for col in cols %}
<td>
.... {{ total[row] }} ...
</td>
{% endfor %}
</tr>
The best I could come up with is to parse the site.posts again for the total:
...
<tr>
{% for col in cols %}
{% assign total = 0 %}
{% for post in site.posts %}
{% if post.colThing == col %}
{% assign total = total | plus: post.thatNumber %}
{% endif %}
{% endfor %}
<td>
.... {{ total }} ...
</td>
{% endfor %}
</tr>
Is that the most efficient way of getting that total?

Answer 1

Your idea of using a hash to store totals is evident except that Jekyll cannot manipulate ruby hashes. total[row] is a hash entry that cannot be populated from liquid.
The way you're printing your totals is one of the solutions.
On the Efficiency side,it might be interesting to explore an other way if you want to avoid to loop over all posts each time you want to populate a cell, by using where and where_exp filters. Depending of the number of posts you're manipulating, this can be more efficient :
{% assign cols = "col1,col2" | split: "," %}
{% assign rows = "row1,row2" | split: "," %}
<thead><tr>{% for col in cols %}<th>{{ col }}</th>{% endfor %}</tr></thead>
{% for row in rows %}
<tr>
{% for col in cols %}
<td>
{% comment %}
where_exp: "post", "post.colThing == col and post.rowThing == row"
returns null if no post fulfill conditions
or
returns an array of posts fulfilling conditions
first
returns the first post of the returned array
{% endcomment %}
{% assign post = site.posts | where_exp: "post", "post.colThing == col and post.rowThing == row" | first %}
{% if post != null %}
{{ post.title }} : {{ post.thatNumber }}
{% else %}
NO DATAS AT THIS POSITION
{% endif %}
</td>
{% endfor %}
</tr>
{% endfor %}
<tfoot><tr>
{% for col in cols %}
{% assign total = 0 %}
{% comment %}
Select posts that are in each column
{% endcomment %}
{% assign posts = site.posts | where: "colThing", col %}
{% for post in posts %}
{% assign total = total | plus: post.thatNumber %}
{% endfor %}
<td>{{ total }}</td>
{% endfor %}
</tr></tfoot>