如何在r中的data.table中评估（或创建）即时列 [英] How can I evaluate (or create) an on the fly column in data.table in r

问题描述

我想创建一个新的data.table或者只是添加一些列到data.table。很容易指定多个新列，但如果我想要第三列根据我创建的列之一计算值，会发生什么。我认为plyr包可以做这样的事情。可以数据表吗？我确定我已经阅读过这个地方。

I want to create a new data.table or maybe just add some columns to a data.table. It is easy to specify multiple new columns but what happens if I want a third column to calculate a value based on one of the columns I am creating. I think plyr package can do something such as that. Can data.table? I am sure I have read about this somewhere

我想做如下：

dt <- data.table(shop=1:10,income=10:19*70)
dt[,list(hope=income*1.05,hopemore=income*1.20,hopemorerealistic=hopemore-100)]  

或可能

dt[,`:=`(hope=income*1.05,hopemore=income*1.20,hopemorerealistic=hopemore-100)]

推荐答案

也可以使用< - code> list 例如

You can also use <- within the call to list eg

DT <- data.table(a=1:5)


DT[, c('b','d') := list(b1 <- a*2, b1*3)]
DT
   a  b  d
1: 1  2  6
2: 2  4 12
3: 3  6 18
4: 4  8 24
5: 5 10 30

或

DT[, `:=`(hope = hope <- a+1, z = hope-1)]
DT
   a  b  d hope z
1: 1  2  6    2 1
2: 2  4 12    3 2
3: 3  6 18    4 3
4: 4  8 24    5 4
5: 5 10 30    6 5

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