I have a data.frame (or a matrix or any other tabular data structure object for that matter):
我有一个data.frame（或矩阵或任何其他表格数据结构对象）：
df = data.frame(field1 = c(1,1,1),field2 = c(2,2,2),field3 = c(3,3,3))
And I want to copy part of its columns - given in the vector below:
我想复制其部分列 - 在下面的向量中给出：
fields = c("field1","field2")
to a new data.table that already has 1 or more columns:
到已有1列或更多列的新data.table：
dt = data.table(fieldX = c("x","x","x"))
I'm looking for something more efficient (and elegant) than:
我正在寻找比以下更有效（和优雅）的东西：
for(f in 1:length(fields))
{
dt[,fields[f]] = df[,fields[f]]
}
2 个解决方案
#1
17
You can use cbind:
你可以使用cbind：
cbind(dt, df[fields])
However, the most efficient way is still probably going to be to use data.table's assign by reference:
但是，最有效的方法仍然可能是使用data.table通过引用分配：
dt[, (fields) := df[fields]]
#2
2
I think you want cbind
我想你想要cbind
cbind(dt, df[, 1:2])
# fieldX field1 field2
# 1 x 1 2
# 2 x 1 2
# 3 x 1 2
#1
17
You can use cbind:
你可以使用cbind：
cbind(dt, df[fields])
However, the most efficient way is still probably going to be to use data.table's assign by reference:
但是，最有效的方法仍然可能是使用data.table通过引用分配：
dt[, (fields) := df[fields]]
#2
2
I think you want cbind
我想你想要cbind
cbind(dt, df[, 1:2])
# fieldX field1 field2
# 1 x 1 2
# 2 x 1 2
# 3 x 1 2