Calculate previous n date's sale for perticular date in table

[shwethaa]

i have data like 

date, sales

1/1/2017, 100

1/7/2017,20

1/21/2017,200

1/28/2017,80

........

i have to calculate rollimg month sum , no of month depends on user input. If user select 2 then output will be

date, sales, outputvalue1

1/1/2017, 100, 100

1/7/2017,20   ,120

1/21/2017,200,  220

1/28/2017,80,  280

How to do it in set expression?

Thanks in advance

[saminea_ANZ]

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@shwethaa wrote:

i have to calculate rollimg month sum , no of month depends on user input. If user select 2 then output will be

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In that, Where is the month data? Can you please supply more realistic values for us.

[shwethaa]

Not exactly rolling month. We want to show week over week percentage change. If input is 2 .then sum of max date and previous to max date. 

One more sum will be sum of sales for3rd maxdate and 4th max date.

(3rd max date sale+4th max date sale)-(1st max date sale+2nd max date sale)

Since there is a date column in table, this expression is taking sales of only corresponding date row. It's not summing up other date

[saminea_ANZ]

If input is 4 then? And are you providing input in box?

[Channa]

before date add month column as Month to table and use expression

RangeSum(Above(Sum(Sales), 0, RowNo()))

[Vegar]

I think the @Channa approach above idea good:
RangeSum(Above(Sum(Sales), 0, RowNo()))
It is flexible because by making selection in the graph you will be able to reduce to the correct number of periods.

However if you want to specify the number of months in your accumulating expression you would, as @saminea_ANZ is saying, need some kind of user input, a variable (or a island field value).
Assuming you have the variable vNumOfMonths you can do this: RangeSum(Above(Sum(Sales), 0, $(vNumOfMonths)))

BR
Vegar