[R] row.names are dropped when extracting one column ?

[ryszard.czerminski@pharma.novartis.com]

Apparently row names are dropped when I extract
single column from a data frame. Why this behaviour ?

> y <- as.matrix(df[,1:2]); length(row.names(y))
[1] 324
> y <- as.matrix(df[,1:1]); length(row.names(y))
[1] 0

Best regards,

Ryszard