Date Difference calculation referencing grouping columns

[#1]

I am trying to calculate a Date Difference in Days between the current row and the last row where a comment is non-missing. Any help greatly appreciated!

If( (:ID == Lag( :ID ) | Row() == 1) & !Is Missing( :Comments ), 0,
Date Difference( :Sampled Date, Lag(:Sampled Date, n=last row where !Is Missing(:Comments)), "Day" )
)

[#2]

If the table is always sorted by date you could do this with two columns, as shown below.

New Column( "Last Comment Date",
	Numeric,
	"Continuous",
	Format( "m/d/y h:m", 19 ),
	Input Format( "Monddyyyy h:m" ),
	Formula(
		If(
			Row() == 1, :Sampled Date,
			:Comments == Lag( :Comments, 1 ), Lag( :Sampled Date, 1 ),
			:Sampled Date
		)
	),
	Set Display Width( 136 )
);
New Column( "Days Since Last Comment New",
	Numeric,
	"Continuous",
	Format( "Best", 12 ),
	Formula( (:Sampled Date - :Last Comment Date) / (3600 * 24) ),
	Set Display Width( 109 )
);

[#3]

Thanks ih, but this doesn't work. The new column you created is not populated with the date of the last comment, just the lag sampled date. Thanks for trying!

[#4 · Accepted solution]

Here is the formula that I would use for the calculation

If( Row() == 1 | :ID != Lag( :ID ),
	lastCmtDate = .
);
If( :Comments != "",
	lastCmtDate = :Sampled Date
);
:Days Since Last Comment = (:Sampled Date - lastCmtDate) / In Days( 1 );

[#5]

Thank you! This worked!

[#6]

I think I have a very simular problem. 

I tried the formula you give us, but in my table it don't work. (column 5 (Differenz in Tagen Bauteil 1)).

At first it is write the day difference not in the rows wear the comment (1) is, rather in all other rows.

And more important for me, it's write just the difference to the last date, and not to the date where the last comment (or no comment) was.

Could you please help, again?

[#7]

Try this

If( Row() == 1 | :Source Table != Lag( :Source Table ),
	lastCmtDate = :"Datum:"n,
	lastCmtDate = Lag( :"Datum:"n )
);
If( :Bauteil 1 == "",
	:Differenz in Tagen Bauteil 1 = (:"Datum:"n - lastCmtDate) / In Days( 1 ),
	:Differenz in Tagen Bauteil 1 = .
);

[#8]

Thank you Jim. 

It is still not work, bu with your formulars I have found now a Work around and use 2 new colums to calculated and now it works

New Column( "Letzter Komentar Tag",
Numerisch,
"Stetig",
Format( "t.m.j", 12 ),
Eingabeformat( "t.m.j" ),
Formel(
If( Row() == 1 | :Quelltabelle != Lag( :Quelltabelle ),
lastCmtDate = .
);
If( :Bauteil 1 == "1",
lastCmtDate = :"Datum:"n,
lastCmtDate
)));

New Column( "Differenz Bauteil 1",
Numerisch,
"Stetig",
Format( "Bestes", 12 ),
Formel(
If( :Bauteil 1 == "1",
(:"Datum:"n - Lag( :Letzter Komentar Tag )) / In Days( 1 )
)));