Data frame changes from numeric to character

Question

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         


        

Answer 1

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.

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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.