pebble loop through array

(# -- index is an array-- #) 
{%- for col in index.columns %}
     {%- if loop.first and not loop.last %}
          {%- set ColumnString = ColumnString + col.name  + "," -%}
      {%- else -%}
          {%- set ColumnString = ColumnString + col.name -%}
      {% endif -%}
{%- endfor %}