确定一个值更改的日期，并用R中的sum()和diff()来总结数据

Sample Data:

样本数据:

 product_id <- c("1000","1000","1000","1000","1000","1000", "1002","1002","1002","1002","1002","1002")
    qty_ordered <- c(1,2,1,1,1,1,1,2,1,2,1,1)
    price <- c(2.49,2.49,2.49,1.743,2.49,2.49,  2.093,2.093,2.11,2.11,2.11, 2.97)
    date <- c("2/23/15","2/23/15",  '3/16/15','3/16/15','5/16/15',  "6/18/15",  "2/19/15","3/19/15","3/19/15","3/19/15","3/19/15","4/19/15")
    sampleData <- data.frame(product_id,    qty_ordered,    price,  date)
 product_id <- c("1000"