Compute a mathematical operation depending on the row value

[RoyBatty296]

I have a dataframe with many variables, but I want to focus on two of them:

Time   | Value
Start     200
1         50
2         50
4         50
8         50
12        80
24        80
Start     300
1         50
2         50
4         50
8         50
12        60
24        70
Start     250

These columns represent the value for a given time.

I want to obtain another column that represents the difference in value between a given time and the start.

To make it clear, I would like something like:

Time   | Value   |  Change
Start     200         200
1         50          150
2         50          150
4         50          150
8         50          150
12        80          120
24        80          120
Start     300         300
1         50          250
2         50          250
4         50          250
8         50          250
12        60          240
24        70          230
Start     250         250

So it’s a simple mathematical operation.

However, the tricky part is to compute always against the last start value, and I haven’t find a way of doing this so far.

Any help is highly appreciated.

[elsamuel]

You haven’t really given us much to work with.

Assuming the rows under investigation are always chunks of 7, the following approach can work:

[gonhaddock]

Without loop or for uneven chunks:

Rule Engine [Base]:
$Start$ LIKE "Start" => $Value$

Missing Value [Base]:
Previous Value

Math Formula [Change]:
if($Base$ == $Value$, $Value$, $Base$-$Value$)

BR