Drop columns by name data frame vs matrix

Question

I want to drop columns by name in a matrix, I noticed that it does not work the same as for data frame:

df <- as.matrix(data.frame(x=1:5, y=2:6, z=3:7, u=4:8))

df[ , -which(names(df) %in% c("z","u"))]


df <- data.frame(x=1:5, y=2:6, z=3:7, u=4:8)

df[ , -which(names(df) %in% c("z","u"))]

Why and how can I fix this?

Because matrices and data frames are different. Columns aren't as special in matrices as they are in data frames. colnames will work for both. — Gregor Thomas
– Gregor Thomas, Commented Oct 13, 2017 at 15:46
data.frame is essentially list with properties of matrix in R. You can't expect matrix to be having every method same as data.frame. — platypus
– platypus, Commented Oct 13, 2017 at 15:50

Gregor Thomas · Accepted Answer · 2017-10-13 16:03:58Z

1

With a matrix you can use colnames or rownames (or, if you need to generalize up to an array with more dimensions, the dimnames list).

names isn't defined for a matrix. It is defined for a list, and thus for a data.frame, where columns are generally more important (in some sense, at least), so it is a safe convention that names refers to column names. But in an array there is no reason to prefer one dimension to another.

A similar question is Extract matrix column values by matrix column name.

answered Oct 13, 2017 at 16:03

Gregor Thomas

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