Data frame changes from numeric to character

I open my csv file and I control the class of each of my data:

mydataP<-read.csv("Energy_protein2.csv", stringsAsFactors=F) 

apply(mydataP, 2, function(i) class(i))
#[1] "numeric"

I add a column and check the class of the data:

mydataP[ ,"ID"] <-rep(c("KOH1", "KOH2", "KOH3", "KON1", "KON2", "KON3", "WTH1", "WTH2", "WTH3","WTN1", "WTN2", "WTN3"), each=2)

apply(mydataP, 2, function(i) class(i))

Here it changes to a "character"

as.numeric(as.factor(mydataP))
#Error in sort.list(y) : 'x' must be atomic for 'sort.list'
#Have you called 'sort' on a list?

as.numeric(as.character(mydataP))

I get a vector with 117 NA

I have no idea what to do now, as soon I touch the frame it changes to character, can somebody help me? Thanks

That happens because apply converts your data.frame to matrix and those can only have one class in them.

Try this instead:

sapply(mydataP, class)

This is the reason you should normally try to avoid using apply on data.frames.

This behavior is documented in the help file (?apply):

If X is not an array but an object of a class with a non-null dim value (such as a data frame), apply attempts to coerce it to an array via as.matrix if it is two-dimensional (e.g., a data frame) or via as.array.

Here's a reproducible example with the built-in iris dataset:

> apply(iris, 2, function(i) class(i))
#Sepal.Length  Sepal.Width Petal.Length  Petal.Width      Species 
# "character"  "character"  "character"  "character"  "character" 

> sapply(iris, class)
#Sepal.Length  Sepal.Width Petal.Length  Petal.Width      Species 
#   "numeric"    "numeric"    "numeric"    "numeric"     "factor" 

> str(iris)
#'data.frame':  150 obs. of  5 variables:
# $ Sepal.Length: num  5.1 4.9 4.7 4.6 5 5.4 4.6 5 4.4 4.9 ...
# $ Sepal.Width : num  3.5 3 3.2 3.1 3.6 3.9 3.4 3.4 2.9 3.1 ...
# $ Petal.Length: num  1.4 1.4 1.3 1.5 1.4 1.7 1.4 1.5 1.4 1.5 ...
# $ Petal.Width : num  0.2 0.2 0.2 0.2 0.2 0.4 0.3 0.2 0.2 0.1 ...
# $ Species     : Factor w/ 3 levels "setosa","versicolor",..: 1 1 1 1 1 1 1 1 1 1 ...

As you can see, apply converts all columns to the same class.