在 data.table 中插入一行 [英] Insert a row in a data.table
问题描述
如果我有一个数据框
set.seed(12345)
df=data.frame(a=rnorm(5),b=rnorm(5))
我可以通过例如添加一行
I can add a row by e.g.
df[6,] =c(5,6)
如果我现在在 data.table 中做同样的事情
If I now do the equivalent in data.table
library(data.table)
dt=data.table(df)
dt[6,]=c(5,6)
它失败并出现错误.将行插入 data.table 的正确方法是什么?
It fails with an error. What is the right way to insert a row into a data.table?
推荐答案
为了扩展@Franks 的答案,如果在您的特定情况下您要附加一行,它是:
To expand on @Franks answer, if in your particular case you are appending a row, it's :
set.seed(12345)
dt1 <- data.table(a=rnorm(5), b=rnorm(5))
以下是等价的;我发现第一个更容易阅读,但第二个更快:
The following are equivalent; I find the first easier to read but the second faster:
microbenchmark(
rbind(dt1, list(5, 6)),
rbindlist(list(dt1, list(5, 6)))
)
如我们所见:
expr min lq median uq max
rbind(dt1, list(5, 6)) 160.516 166.058 175.089 185.1470 457.735
rbindlist(list(dt1, list(5, 6))) 130.137 134.037 140.605 149.6365 184.326
如果你想在别处插入该行,下面的方法会起作用,但它并不漂亮:
If you want to insert the row elsewhere, the following will work, but it's not pretty:
rbindlist(list(dt1[1:3, ], list(5, 6), dt1[4:5, ]))
甚至
rbindlist(list(dt1[1:3, ], as.list(c(5, 6)), dt1[4:5, ]))
给予:
a b
1: 0.5855288 -1.8179560
2: 0.7094660 0.6300986
3: -0.1093033 -0.2761841
4: 5.0000000 6.0000000
5: -0.4534972 -0.2841597
6: 0.6058875 -0.9193220
如果您正在就地修改行(这是首选方法),则需要提前定义 data.table 的大小,即
If you are modifying a row in place (which is the preferred approach), you will need to define the size of the data.table in advance i.e.
dt1 <- data.table(a=rnorm(6), b=rnorm(6))
set(dt1, i=6L, j="a", value=5) # refer to column by name
set(dt1, i=6L, j=2L, value=6) # refer to column by number
感谢@Boxuan,我已经修改了这个答案以考虑你的建议,它更快更容易阅读.
Thanks @Boxuan, I have modified this answer to take account of your suggestion, which is a little faster and easier to read.
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