一种方式使用data.table
df <- structure(list(V1 = structure(c(1L, 1L, 2L, 2L, 2L, 3L),
.Label = c("A ", "B ", "C "), class = "factor"),
V2 = c(45, 66, 99, 34, 71, 22)),
.Names = c("V1", "V2"),
class = "data.frame", row.names = c(NA, -6L))
require(data.table)
dt <- data.table(df, key="V1")
# get maximum index
idx <- max(dt[, .N, by=V1]$N)
# get final result
dt[, list(V2 = c(V2, rep(0, idx-length(V2)))), by=V1]
# V1 V2
# 1: A 45
# 2: A 66
# 3: A 0
# 4: B 99
# 5: B 34
# 6: B 71
# 7: C 22
# 8: C 0
# 9: C 0