cumulative sum of array of rows

I have this data frame:

I want to take the cumulative sum of the each set of predicted values based on the sequential day numbers (i.e. cumsum of the first 2 rows, cumsum of the next 2, and the cumsum of the last 4)

so the results would be .003, .003, .008

>Solution :

Using R base

sapply(split(df$Predicted_value,cumsum(c(1,diff(df$day_number)!=1))), sum)
   1     2     3 
0.003 0.003 0.008