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I think this is a simple question but I haven't found a suitable solution. To begin with a set of simplified data :

df <- as.data.frame(matrix(1:20, 5, 4))
str(df)

# 'data.frame': 5 obs. of 4 variables:
# $ V1: int 1 2 3 4 5
# $ V2: int 6 7 8 9 10
# $ V3: int 11 12 13 14 15
# $ V4: int 16 17 18 19 20

We can see that all the classes are the integer. What I wanna achieve is converting the 4 classes to integer, numeric, character, and factor respectively. Of course, I can use

df$V1 <- as.XXX(df$V1)

for each column, but I think it's inefficient.

Expected Output

# 'data.frame': 5 obs. of 4 variables:
# $ V1: int 1 2 3 4 5
# $ V2: num 6 7 8 9 10
# $ V3: chr "11" "12" "13" "14" ...
# $ V4: Factor w/ 5 levels "16","17","18",..: 1 2 3 4 5

Question 2

I reference @joran's answer in R Assign (or copy) column classes from a data frame to another and run the following code :

myclass <- c("integer", "numeric", "character", "factor")
df.2 <- df
df.2 <- mapply(FUN = as, df.2, myclass, SIMPLIFY = F)

When I call df.2, an error appears :

Error in as.character.factor(x) : malformed factor

However, It's okay to call str(df.2), and apparently only V1 and V3 reach my request.

str(df.2)

# 'data.frame': 5 obs. of 4 variables:
# $ V1: int 1 2 3 4 5
# $ V2: int 6 7 8 9 10
# $ V3: chr "11" "12" "13" "14" ...
# $ V4:Formal class 'factor' [package "methods"] with 3 slots
# .. ..@ .Data : int 16 17 18 19 20
# .. ..@ levels : chr 
# .. ..@ .S3Class: chr "factor"

Why cannot as function deal with classes numeric and factor?

We can use mapply and provide the functions as a list to convert the columns.

df <- as.data.frame(matrix(1:20, 5, 4))

df <- mapply(function(x, FUN) FUN(x),
 df, 
 list(as.integer, as.numeric, as.character, as.factor), 
 SIMPLIFY = FALSE)
str(df)
# 'data.frame': 5 obs. of 4 variables:
# $ V1: int 1 2 3 4 5
# $ V2: num 6 7 8 9 10
# $ V3: chr "11" "12" "13" "14" ...
# $ V4: Factor w/ 5 levels "16","17","18",..: 1 2 3 4 5

If you don't exclude the for loop method, try this :

df <- as.data.frame(matrix(1:20, 5, 4))
type <- c("integer", "numeric", "character", "factor")
for(i in 1:ncol(df))
 call <- paste("as", type[i], sep = ".")
 df[[i]] <- do.call(call, list(df[[i]]))


str(df)

# 'data.frame': 5 obs. of 4 variables:
# $ V1: int 1 2 3 4 5
# $ V2: num 6 7 8 9 10
# $ V3: chr "11" "12" "13" "14" ...
# $ V4: Factor w/ 5 levels "16","17","18",..: 1 2 3 4 5