Stash id for use in category filtering - help with parse order

Question

I'd like to use a stash'd variable to filter a list of categories, but the following is not working:

{if segment_2=="thing1"}
    {exp:stash:set name="cat_id"}22{/exp:stash:set}
{if:elseif segment_2=="thing2"}
    {exp:stash:set name="cat_id"}23{/exp:stash:set}
{/if}

{exp:stash:set_list name="filters" parse_tags="yes"}
    {exp:channel:categories channel="channel1" category_group="{exp:stash:get name='cat_id'}" backspace="6"}
        {stash:category_id}{category_id}{/stash:category_id}
        {stash:category_name}{category_name}{/stash:category_name}
        {stash:category_url_title}{category_url_title}{/stash:category_url_title}
    {/exp:channel:categories}
{/exp:stash:set_list}

I think this is a parsing issue, but how do I fix it?

Update: I tried this after the answer offered below:

{exp:switchee variable="{segment_2}" parse="inward"}    
    {case value="thing1"}
        {exp:stash:set name="cat_id"}22{/exp:stash:set}
    {/case}
    {case value="thing2"}
        {exp:stash:set name="cat_id"}23{/exp:stash:set}
    {/case}
{/exp:switchee}

{exp:stash:set_list name="filters" parse_tags="yes"}
    {exp:channel:categories channel="channel1" category_group="{exp:stash:get name='cat_id'}" backspace="6" parse="inward"}
        {stash:category_id}{category_id}{/stash:category_id}
        {stash:category_name}{category_name}{/stash:category_name}
        {stash:category_url_title}{category_url_title}{/stash:category_url_title}
    {/exp:channel:categories}
{/exp:stash:set_list}

I also tried: {exp:stash:get name='cat_id' process='end' }

But still no luck.

Answer 1

First problem: the variable "cat_id" will get set twice and always have the later value. You could use Switchee instead of if/else. The reason why and some alternative solutions are described here: https://gist.github.com/croxton/9d012297096892ca5c10

Second problem: when you pass a variable in a parameter to a tag, you need to use parse="inward" to tell EE to parse the tag.

{exp:channel:categories channel="channel1" category_group="{exp:stash:get name='cat_id'}" backspace="6" parse="inward"} 
...
{/exp:channel:categories}