Here is one way of writing the formula
colList = {"Column 1", "Column 2", "Column 3"};
x = "";
For( i = 1, i <= N Items( colList ), i++,
If( As Column( colList[i] ) != "",
If( x != "", x = x || "_" );
x = x || Char( As Column( colList[i] ) );
)
);
Trim( x );
Jim