How to sum a previous period value and current period value (multiple times)
Hi all,
Have been trying to sum the values from the last week and the current week to obtain a new cumulative value; this value will be the new value to be added to the current week value.
Unfortunately, this does not seem to be so easy in Anaplan.
WEEK1
I start the first week with one value (layersactual) and, of course, the value from the previous week is 0 but it actually comes from a LOOKUP[previous week] formula. I sum these two and I get the layers  cumulative line item.
WEEK2
I start the week with a new value which is to be added to the lookupped value I get from the first week. I sum these two and I get the new cumulative value.
So far so good.
WEEK3
Now, as usual I have the new value for the week which needs to be added to the cumulative value from the last week. However, every attempt I made fails to work due to circular reference.
My formula would be for the cumulative line item:
IF WEEK=1 OR WEEK=2
THEN layers  actual + layers  previous week
ELSE layers  actual + layers  cumulative[LOOKUP: previous period]
Where am I going wrong?
Many thanks in advance,
Luca
Answers

@LucaRicci try this formula for the cumulative line item
'layers  actual' + previous('layers  cumulative')
This will keep accumulating though from beginning to the end of the time period. So if you need to reset at some point you will need to add a condition. E.g.
If <condition to reset> then
'layers  actual'
else
'layers  actual' + previous('layers  cumulative')
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@LucaRicci so week 1 to week 3 in your screenshot is a list rather than time period. One thing you can do is map that week list in to a time period and execute the cumulate against the time period then remap it back to your week list.
E.g in time period module
Add Line item weeklist with formula
Time period week 1 maps to week 1 list item
Time period week 2 maps to week 2 list item
Add lineitem layersactual with formula
Sourcemodule. Layers actual[lookup: weeklist]
then you can apply the previous function as per earlier recoomendation
you can then push the result of the cumulate back to the original module using a lookup to the time period equivalent0 
@LucaRicci
Try this workaround Cumulate adds values across a list.
Create a Mapping module for Weeks and then by using cumulate function you can desired output.
W1 is Week 1 True booleanLet me know if it helps.
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