使用列名变量按行选择数据框值 [英] Select data frame values row-wise using a variable of column names

问题描述

假设我有一个如下所示的数据框:

Suppose I have a data frame that looks like this:

dframe = data.frame(x = c(1, 2, 3), y = c(4, 5, 6))
#   x y
# 1 1 4
# 2 2 5
# 3 3 6

还有一个列名向量，数据框的每一行一个:

And a vector of column names, one per row of the data frame:

colname = c('x', 'y', 'x')

对于数据框的每一行，我想从向量中的相应列中选择值.类似于 dframe[, colname] 但对于每一行.

For each row of the data frame, I would like to select the value from the corresponding column in the vector. Something similar to dframe[, colname] but for each row.

因此，我想获得 c(1, 5, 3)(即第 1 行:colx"；第 2 行:coly"；第 3 行:colx"；x")

Thus, I want to obtain c(1, 5, 3) (i.e. row 1: col "x"; row 2: col "y"; row 3: col "x")

推荐答案

我最喜欢的旧矩阵索引会解决这个问题.只需传递带有相应行/列索引的 2 列矩阵:

My favourite old matrix-indexing will take care of this. Just pass a 2-column matrix with the respective row/column index:

rownames(dframe) <- seq_len(nrow(dframe))
dframe[cbind(rownames(dframe),colname)]
#[1] 1 5 3

或者，如果您不想添加行名:

Or, if you don't want to add rownames:

dframe[cbind(seq_len(nrow(dframe)), match(colname,names(dframe)))]
#[1] 1 5 3

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