R中按键的数据表的条件和

我有一个数据表'df'，有3列。身份证、膳食、时间 每个id有许多行

library(data.table)
id = c(1,1,2,2,3,3)
meal = c(1,1,0,0,1,0)
time = c(10,9,12,13,7,15)
df <- data.table(id, meal, time)

> df
   id meal time
1:  1    1   10
2:  1    1    9
3:  2    0   12
4:  2    0   13
5:  3    1    7
6:  3    0   15

---

我能做什么？

您可以尝试以下方法：

df[, is.meal.one := as.integer(meal == 1)]
df[, sum(time * is.meal.one), by = "id"]

这个怎么样

library(data.table)

dt <- data.table(
  id = c(1, 1, 2, 2, 3, 3),
  meal = c(1, 5, 3, 2, 10, 1),
  time = c(2, 10, 15, 5, 5, 2)
)

   id meal time
1:  1    1    2
2:  1    5   10
3:  2    3   15
4:  2    2    5
5:  3   10    5
6:  3    1    2

dt[, list(Meal1Time = sum(time[which(meal == 1)])), by=id]
   id Meal1Time
1:  1         2
2:  2         0
3:  3         2

库（data.table）
在你的问题中加入a会增加你得到答案的机会。你可以在计算完总数后进行合并。您可以使用
ifelse
如果您想要解决方案，请不要忘记将其标记为“正确”。
library(data.table)

dt <- data.table(
  id = c(1, 1, 2, 2, 3, 3),
  meal = c(1, 5, 3, 2, 10, 1),
  time = c(2, 10, 15, 5, 5, 2)
)

   id meal time
1:  1    1    2
2:  1    5   10
3:  2    3   15
4:  2    2    5
5:  3   10    5
6:  3    1    2

dt[, list(Meal1Time = sum(time[which(meal == 1)])), by=id]
   id Meal1Time
1:  1         2
2:  2         0
3:  3         2