Matrix rows: how to target every 3 rows

Question

I would like to wrap every 3 rows that the matrix fields output with some required html like this:

{exp:channel:entries channel="pages" disable="categories|member_data|pagination"}
 {my_matrix_field}
 {if rows are: FIRST FOURTH SEVENTH etc}
  <div class="row">
 {/if}
 <div class="span4">                   
 <p>{text} <a href="{link}" class="more">Read more</a></p>
 </div>
 {if rows are THIRD SIXTH NINTH etc}
  </div><!-- / row -->
 {/if}
 {/my_matrix_field}
{/exp:channel:entries}

Answer 1

Ah, you want groups of three wrapped and not every third one, wrapped, right? @mediagirl almost has it, but if your returned entries is not evenly divisible by three, you'll have a problem - the final switch will not take effect. You therefore have to close the wrapper conditionally, like this:

{exp:channel:entries channel="pages" disable="categories|member_data|pagination"}
  {my_matrix_field}

      {switch="<div class='row'>||"}

      Your per matrix row data here

      {if row_count != total_rows}{switch="||</div>"}{/if}{!-- closes row at switch interval IF last row IS NOT the current row --}
      {if row_count == total_rows}</div>{/if}{!-- closes row IF last row IS current row --}  

  {/my_matrix_field}
{/exp:channel:entries}

I've done this many times, and it's a great, if not often thought of uses for the switch variable.

Answer 2

Unfortunately, I don't think this is supported natively (thought I would love to be shown otherwise!). The way I achieved this was to use the MX Calculator extension to calculate whether a row's count is divisible by 3 or not:

    {matrix_test}

        <span style="display: none;">
          {exp:mx_calc expression="mod({row_count}, 3)"}
        </span>

            {if '{calc_result}' == '0'}
                divisible by 3: {number} <br />
            {if:else}
                not divisible by 3: {number} <br />
            {/if}
        {/exp:mx_calc}
    {/matrix_test}

This works and you can then customise every third row's outcome, but has a small inconvenience. When you use the exp:mx_calc opening tag to set the {calc_result} variable, it prints out the result of the expression. This prints out:

1 not divisible by 3: 1
not divisible by 3: 2
divisible by 3: 3
not divisible by 3: 4
not divisible by 3: 5
divisible by 3: 6

Unfortunately, I can't use the mx_calc tag right in the conditional, due to parse order issues. The work around is to just wrap the mx_calc tag in a span and give it display: none;

However, I unfortunately skim read your question (which I know I shouldn't do) and I thought you were talking about Grid. In Grid, you can just have mx_calc right in the conditional, because it's not a third party add-on. Thus:

    {test}
        {if '{exp:mx_calc expression="mod({test:count}, 3)"}' == '0'}
            divisible by 3: {test:number} <br />
        {if:else}
            not divisible by 3: {test:number} <br />
        {/if}
    {/test}

Answer 3

Using the switch="" parameter may do what you need. This is untested but I don't see why it wouldn't work.

{switch="||<div class='row'>"}
{switch="||</div>"}

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EDIT
I adjusted the quotes in my code example based on Jean's comment below...