数据表 - 根据另一列的名称选择列的值 [英] Data Table - Select Value of Column by Name From Another Column

问题描述

我有一个包含多个列的数据表。我有另一个列，它定义了哪些列的值需要选择。我无法找到办法做到这一点。

I have a data table with a number of columns containing values. I have another column which defines which one of those columns whose value I need to select. I am having trouble finding a way to do this.

这是一个简单的例子。

Here is a simple example.

> d <- data.table(
     value.1 = c("one", "uno", "1"),
     value.2 = c("two", "dos", "2"),
     name.of.col = c("value.1","value.2","value.1"))

> d
   value.1 value.2 name.of.col
1:     one     two     value.1
2:     uno     dos     value.2
3:       1       2     value.1

我想添加一个列'value.of.col'，其中包含指定列的值by'name.of.col'。

I would like to add a column 'value.of.col' which contains the value of the column specified by 'name.of.col'.

> d
   value.1 value.2 name.of.col  value.of.col
1:     one     two     value.1  one
2:     uno     dos     value.2  dos
3:       1       2     value.1  1

推荐答案

/ p>

Another option:

d[,value.of.col:=diag(as.matrix(d[,d[,name.of.col],with=F]))]
> d
   value.1 value.2 name.of.col value.of.col
1:     one     two     value.1          one
2:     uno     dos     value.2          dos
3:       1       2     value.1            1

EDIT 添加更快的解决方案： / p>

EDIT add a faster solution:

d[,value.of.col:=
      melt(d,id.vars='name.of.col')[name.of.col==variable,value]]

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