How can I spilt a row data in to below other rows

Question

In below variable I want to divide 900000 in below its respective three rows where zero is present, similarly for other values. The number of zeroes to be counted dynamically be

Answer 1

We can use data.table. We convert the 'data.frame' to data.table' (setDT(df1)), create a grouping variable ('grp') by checking where 'A_B' is not equal to 0 to create a logical vector and getting the cumulative sum. Grouped by 'grp', we set the 'i' where 'grp' is not equal to 0, create logical vector 'i1' where the 'A_B' is not 0. Get the sum of 'i1', and use pmin to get the minimum value when compared with 4. Then we subset the 'A_B' based on 'i1' and do an integer division with 'i2'. We group by 'grp', get the row index ('.I`) based on the logical conditions and then assign the 'A_BnewNew' to 0. If not needed, the 'grp' can be assigned to NULL.

library(data.table)
iN <- setDT(df1)[, grp := cumsum(A_B!=0)][grp!=0, A_BnewNew := {
           i1 <- A_B!=0; i2 <- pmin(sum(!i1),4)
           A_B[i1]%/%i2}  , by = grp
       ][, .I[!(A_B ==0 & grp !=0 & (1:.N) <= pmin(5, .N))]  , by = grp]$V1
 df1[iN, A_BnewNew:=0][, grp := NULL][]
#       A_B A_Bnew A_BnewNew
# 1:      0      0         0
# 2:      0      0         0
# 3: 900000      0         0
# 4:      0 300000    300000
# 5:      0 300000    300000
# 6:      0 300000    300000
# 7:  10000      0         0
# 8:      0   5000      5000
# 9:      0   5000      5000
#10:  20000      0         0
#11:      0   5000      5000
#12:      0   5000      5000
#13:      0   5000      5000
#14:      0   5000      5000
#15:      0      0         0
#16:      0      0         0
#17:      0      0         0

data

df1 <- structure(list(A_B = c(0L, 0L, 900000L, 0L, 0L, 0L, 10000L, 0L, 
 0L, 20000L, 0L, 0L, 0L, 0L, 0L, 0L, 0L), A_Bnew = c(0L, 0L, 0L, 
 300000L, 300000L, 300000L, 0L, 5000L, 5000L, 0L, 5000L, 5000L, 
 5000L, 5000L, 0L, 0L, 0L)), .Names = c("A_B", "A_Bnew"), 
 class = "data.frame", row.names = c(NA, -17L))