Converting data frame column from character to numeric

Question

I have a data frame that I construct as such:

> yyz <- data.frame(a = c("1","2","n/a"), b = c(1,2,"n/a"))

> apply(yyz, 2, class)
      a           b 
"character" "character"

I am attempting to convert the last column to numeric while still maintaining the first column as a character. I tried this:

> yyz$b <- as.numeric(as.character(yyz$b))
> yyz
  a  b
  1  1
  2  2
  n/a NA

But when I run the apply class it is showing me that they are both character classes.

> apply(yyz, 2, class)
      a           b 
"character" "character"

Am I setting up the data frame wrong? Or is it the way R is interpreting the data frame?

Answer 1

If we need only one column to be numeric

yyz$b <- as.numeric(as.character(yyz$b))

But, if all the columns needs to changed to numeric , use lapply to loop over the columns and convert to numeric by first converting it to character class as the columns were factor .

yyz[] <- lapply(yyz, function(x) as.numeric(as.character(x)))

Both the columns in the OP's post are factor because of the string "n/a" . This could be easily avoided while reading the file using na.strings = "n/a" in the read.table/read.csv or if we are using data.frame , we can have character columns with stringsAsFactors=FALSE (the default is stringsAsFactors=TRUE )

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Regarding the usage of apply , it converts the dataset to matrix and matrix can hold only a single class. To check the class , we need

lapply(yyz, class)

Or

sapply(yyz, class)

Or check

str(yyz)