根据行值提取列

Question

Following Data Set:

df <- read.table(text = "  a b c 
                 X Y Z", header = T)

The command

 df[, sapply (df[1, ], as.character) %in% c("Y", "X")]

returns

  a b
1 X Y

but the command

df[, sapply (df[1, ], as.character) %in% c("Y")]

returns

[1] Y
Levels: Y

and not

   b
1  Y

Any idea why? And how I can retrieve the correct column name

Answer 1

Likely a duplicate...but to provide an answer...

You've fallen into one of the major traps of the R Inferno; 8.1.44 where "By default dimensions of arrays are dropped when subscripting makes the dimension length 1. Subscripting with drop=FALSE overrides the default."

So, this will return what you expect:

df[, sapply (df[1, ], as.character) %in% c("Y"), drop = FALSE]