JOIN header row values across a row based on non-blank values in cells

Question

So I have two rows:

ID	TagDog	TagCat	TagChair	TagArm	Grouped Tags (need help with this)
1		TRUE		TRUE	TagDog,TagArm

Row 1 consists mainly of Tags, while rows 2+ are entries. This data ties ENTRIES to TAGS.

What I'm needing to do is concatenate/join the tag names per entry. For example, look at the last column above.

I suspect we could write a formula that would:

• Create an array of non-empty cells in the row. (IE: [2,4])
• Return it with the header row A (IE: [A2,A4])
• Then join them together by a comma

But I am unsure how to write the formula, or if this is even the best approach.

Answer 1

Here's the formula:

={
  "Grouped Tags (need help with this)";
  ARRAYFORMULA(
    REGEXREPLACE(TRIM(
      TRANSPOSE(QUERY(TRANSPOSE(
        IF(NOT(B2:E11),, B1:E1)
      ),, COLUMNS(B1:E1)))
    ), "\s+", ",")
  )
}

The trick used is called vertical query smash. That's the part:

TRANSPOSE(QUERY(TRANSPOSE(...),, Nnumber_of_columns))

You can find a brief description of this one and his friends here.