R 語言- 使用循環創建滯後並根據列名計算累計和

問題：

id = c("a", "a", "a", "b", "b")
date = seq(as.Date("2015-12-01"), as.Date("2015-12-05"), by="days")
v1 = sample(seq(1, 20), 5)
v2 = sample(seq(1, 20), 5)
df = data.frame(id, date, v1, v2)

希望將這一份數據轉化便成爲以下的結構：

id   date         v1   v2   v1Cum   v2Cum
a    2015-12-01   1    13     0       0
a    2015-12-02   7    11     1       13
a    2015-12-03   12   2      8       24
b    2015-12-04   18   6      0       0
b    2015-12-05   4    9      18      6

解決方案：
原始數據：

set.seed(123)
df = data.frame(
  id = c("a", "a", "a", "b", "b"),
  date = seq(as.Date("2015-12-01"), as.Date("2015-12-05"), by="days"),
  v1 = sample(seq(1, 20), 5),
  v2 = sample(seq(1, 20), 5),
  v3 = sample(seq(1, 20), 5)
)

> df
  id       date v1 v2 v3
1  a 2015-12-01  6  1 20
2  a 2015-12-02 15 11  9
3  a 2015-12-03  8 17 13
4  b 2015-12-04 16 10 10
5  b 2015-12-05 17  8  2

實現代碼：

df %>%
  group_by(id) %>%
  arrange(date) %>%
  mutate_at(vars(v1:v3), funs(Cum = cumsum(lag(., default = 0)))) %>%
  ungroup()


# A tibble: 5 x 8
# Groups: id [2]
  id     date          v1    v2    v3 v1_Cum v2_Cum v3_Cum
  <fctr> <date>     <int> <int> <int>  <int>  <int>  <int>
1 a      2015-12-01     6     1    20      0      0      0
2 a      2015-12-02    15    11     9      6      1     20
3 a      2015-12-03     8    17    13     21     12     29
4 b      2015-12-04    16    10    10      0      0      0
5 b      2015-12-05    17     8     2     16     10     10