Coerce variables in data frame to appropriate format

问题

I'm working a data frame which consists of multiple different data types (numerics, characters, timestamps), but unfortunately all of them are received as characters. Hence I need to coerce them into their "appropriate" format dynamically and as efficiently as possible.

Consider the following example:

df <- data.frame("val1" = c("1","2","3","4"), "val2" = c("A", "B", "C", "D"), stringsAsFactors = FALSE)

I obviously want val1 to be numeric and val2 to remain as a character. Therefore, my result should look like this:

'data.frame':   4 obs. of  2 variables:
 $ val1: num  1 2 3 4
 $ val2: chr  "A" "B" "C" "D"

Right now I'm accomplishing this by checking if the coercion would result in NULL and then proceeding in coercing if this isn't the case:

res <- as.data.frame(lapply(df, function(x){

  x <- sapply(x, function(y) {
    if (is.na(as.numeric(y))) {
      return(y)
    } else {
      y <- as.numeric(y)
      return(y)
    }    
  })

  return(x)

}), stringsAsFactors = FALSE)

However, this doesn't strike me as the correct solution because of multiple issues:

I suspect that there is a faster way of accomplishing this
For some reason I receive the warning In FUN(X[[i]], ...) : NAs introduced by coercion, although this isn't the case (see result)
This seems inappropriate when handling other data types, i.e. dates

Is there a general, heuristic approach to this, or another, more sustainable solution? Thanks

回答1:

The recent file readers like data.table::fread or the readr package do a pretty decent job in identifying and converting columns to the appropriate type.

So my first reaction was to suggest to write the data to file and read it in again, e.g.,

library(data.table)
fwrite(df, "dummy.csv")
df_new <- fread("dummy.csv")
str(df_new)

Classes ‘data.table’ and 'data.frame':    4 obs. of  2 variables:
 $ val1: int  1 2 3 4
 $ val2: chr  "A" "B" "C" "D"
 - attr(*, ".internal.selfref")=<externalptr>

or without actually writing to disk:

df_new <- fread(paste(capture.output(fwrite(df, "")), collapse = "\n"))

However, d.b's suggestions are much smarter but need some polishing to avoid coercion to factor:

df[] <- lapply(df, type.convert, as.is = TRUE)
str(df)

'data.frame': 4 obs. of  2 variables:
 $ val1: int  1 2 3 4
 $ val2: chr  "A" "B" "C" "D"

df[] <- lapply(df, readr::parse_guess)

回答2:

You should check dataPreparation package. You will find function findAndTransformNumerics function that will do exactly what you want.

require(dataPreparation)
data("messy_adult")
sapply(messy_adult[, .(num1, num2, mail)], class)
   num1        num2        mail 
"character" "character"    "factor"

messy_adult is an ugly data set to illustrate functions from this package. Here num1 and num2 are strings :/

messy_adult <- findAndTransformNumerics(messy_adult)
[1] "findAndTransformNumerics: It took me 0.18s to identify 3 numerics column(s), i will set them as numerics"
[1] "setColAsNumeric: I will set some columns as numeric"
[1] "setColAsNumeric: I am doing the columnnum1"
[1] "setColAsNumeric: 0 NA have been created due to transformation to numeric."
[1] "setColAsNumeric: I will set some columns as numeric"
[1] "setColAsNumeric: I am doing the columnnum2"
[1] "setColAsNumeric: 0 NA have been created due to transformation to numeric."
[1] "setColAsNumeric: I am doing the columnnum3"
[1] "setColAsNumeric: 0 NA have been created due to transformation to numeric."
[1] "findAndTransformNumerics: It took me 0.09s to transform 3 column(s) to a numeric format."

Here we performed the search and it logged what it found

And know:

sapply(messy_adult[, .(num1, num2, mail)], class)
     num1      num2      mail 
"numeric" "numeric"  "factor"

Hope it helps!

Disclamer: I'm the author of this package.

来源：https://stackoverflow.com/questions/45940855/coerce-variables-in-data-frame-to-appropriate-format

标签

type-conversion

coercion