选择适合某些逻辑测试的第一列的列名称 [英] Choose column name of the first column which fits certain logical test

问题描述

我有以下输入：

id <- c("a", "b", "c", "d")
target <- seq(from = 100, to = 400, length.out = 4)
a <- c(300, 304, 100, 405)
b <- c(300, 104, 100, 405)
c <- c(85, 304, 500, 405)
df <- as.data.frame(cbind(id, target, a, b, c))

我想添加一个新列列，表示每行，列a，b，c中的哪一列将是具有小于目标解的值的第一列。
请求的输出如下所示：

I would like to add a new column "column" which indicates per row, which of the columns "a", "b", "c" would be the first column with a value smaller than the target solution. The requested output looks like this:

必需输出：

df$column <- c("c", "b", "a", "NA")
df

我想到了一个合并的每行检查，并将其应用于具有应用功能的所有行。然而，abc列相当长（第20轮，因此将需要循环），行数约为4.000。
有没有人想知道如何解决它？

I thought about a concenated if check per row and apply this to all rows with the apply function. However the abc columns are quite long (round 20, therefore a loop would be required) and the number of rows are about 4.000. Does anybody have an idea on how to solve it?

推荐答案

这是另一个使用哪个。这基本上是所有出现的地方，其中目标更大，并且使用重复的函数获取第一个实例。 p>

Here's another vectorized solution using which. This is basically takes all the occurrences where target is larger and the takes the first instances using the duplicated function.

indx <- which(df[, 3:5] < df[, 2], arr.ind = TRUE)
indx2 <- indx[!duplicated(indx[, "row"]),]
df[indx2[, "row"], "column"] <- names(df)[3:5][indx2[, "col"]]
df
#   id target   a   b   c column
# 1  a    100 300 300  85      c
# 2  b    200 304 104 304      b
# 3  c    300 100 100 500      a
# 4  d    400 405 405 405   <NA>

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