R - Exctracting a vector of doubles from data.frame

问题

I got this question using read.table() with or without header=T, trying to extract a vector of doubles from the resulting data.frame with as.double(as.character()) (see ?factor).

But that's just how I realized that I don't understand R's logic. So you won't see e.g. read.table in the code below, only the necessary parts. Could you tell me what's the difference between the following options?

With header=T equivalent:

(a <- data.frame(array(c(0.5,0.5,0.5,0.5), c(1,4))))
as.character(a)
# [1] "0.5" "0.5" "0.5" "0.5"

Without header=T equivalent:

b <- data.frame(array(c("a",0.5,"b",0.5,"c",0.5,"d",0.5), c(2,4)))
(a <- b[2,])
as.character(a)
# [1] "1" "1" "1" "1"

(a <- data.frame(a, row.names=NULL)) # now there's not even a visual difference
as.character(a)
# [1] "1" "1" "1" "1"

回答1:

The problem lies in the default setting of data.frame, where one of the options, stringsAsFactors is set to TRUE. This is a problem in your scenario because when you use header = FALSE, the presence of character values in that row coerces the entire column to characters, which is then converted to factors (unless you set stringsAsFactors = FALSE).

Here are some examples to play with:

## Two similar `data.frame`s -- just one argument different

b <- data.frame(array(c("a",0.5,"b",0.5,"c",0.5,"d",0.5), c(2,4)))
b2 <- data.frame(array(c("a",0.5,"b",0.5,"c",0.5,"d",0.5), c(2,4)),
                stringsAsFactors = FALSE)

## First with "b"

as.character(b[2, ])
# [1] "1" "1" "1" "1"

sapply(b[2, ], as.character)
#    X1    X2    X3    X4 
# "0.5" "0.5" "0.5" "0.5"
as.matrix(b)[2, ]
#    X1    X2    X3    X4 
# "0.5" "0.5" "0.5" "0.5"
as.double(as.matrix(b)[2, ])
# [1] 0.5 0.5 0.5 0.5

## Now with "b2"

as.character(b2[2, ])
# [1] "0.5" "0.5" "0.5" "0.5"
as.double(as.character(b2[2, ]))
# [1] 0.5 0.5 0.5 0.5

来源：https://stackoverflow.com/questions/26194973/r-exctracting-a-vector-of-doubles-from-data-frame

标签

class

dataframe

double

character