[Solved] Find a function to return value based on condition using R

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If I understand well, your requirement is to compare sold quantity in month t with the sum of quantity sold in months t-1 and t-2. If so, I can suggest using dplyr package that offer the nice feature of grouping rows and mutating columns in your data frame.

resultData <- group_by(data, KId) %>% 
    arrange(sales_month) %>% 
    mutate(monthMinus1Qty = lag(quantity_sold,1), monthMinus2Qty = lag(quantity_sold, 2)) %>% 
    group_by(KId, sales_month) %>%
    mutate(previous2MonthsQty = sum(monthMinus1Qty, monthMinus2Qty, na.rm = TRUE)) %>%  
    mutate(result = ifelse(quantity_sold/previous2MonthsQty >= 0.6,0,1)) %>%
    select(KId,sales_month, quantity_sold, result)

The result is as below:
example of data frame processed by the above code

Adding 

select(KId,sales_month, quantity_sold, result)

at the end let us display only columns we care about (and not all these intermediate steps).

I believe this should satisfy your requirement. NA is the result column are due to 0/0 division or no data at all for the previous months.
Should you need to expand your calculation beyond one calendar year, you can add year column and adjust group_by() arguments appropriately.
For more information on dplyr package, follow this link

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solved Find a function to return value based on condition using R