按列索引插入列(未修改的列顺序)

Given the following data frame:

给定以下数据框架:

> header = c("A1","A2","A3","B1","B2","B3","AB1", "AB2", "AB3")
> df = matrix(c(0,0,0,0,0,0,0,0,0),nrow = 1)
> colnames(df) = header
> df
     A1 A2 A3 B1 B2 B3 AB1 AB2 AB3
[1,]  0  0  0  0  0  0   0   0   0
>