Apply function with multiple parameters and output variable - r

Question

I am trying to apply a function with two arguments. The first argument is a dataframe, the second is an integer that defines a row of the df.
col_1 <- c("A", "B", "C")
col_2 <- c("red", "blue", "black")
df <- data.frame(col_1, col_2)
f <- function(x, arg1) {
x[arg1, 1]
x[arg1, 2]
}
apply(df, 1, f)
Looks like the second argument is not passed to the function. Here is the error
Error in x[arg1, 1] : incorrect number of dimensions
when I put arg1=1 like this
apply(df, arg1=1, f)
it gives me a FUN error
Error in match.fun(FUN) : argument "FUN" is missing, with no default
the desired output is "A" and "red", i.e. in my real code I need to operate with the values of each row.
I also want to add an output variable to be able to save a plot that I am making in my real analysis in a file. Can I just add an "output" variable in function(x, arg1) and then do apply(df, arg1=1, f, output="output_file")?

Answer 1

As #Greg mentions, the purpose of this code isn't clear. However, the question seems to relate to how apply() works so here goes:
Basically, when any of the apply family of functions is used, the user-enetered function (f(), in this case) is applied to the subset of the data produced by apply. So here, you've asked apply to evaluate each row then call f() - the first argument to f() would then be a vector rather than the data frame your function requires.
Here's some functioning code:
col_1 <- c("A", "B", "C")
col_2 <- c("red", "blue", "black")
df <- data.frame(col_1, col_2)
f <- function(x) {
x[1]
x[2]
}
apply(df, 1, f)
This generates all of the values of the second column as a vector since x[2] is returned from the function and for each row, will represent the value in the second column.
If you want the arg1 row of results, you could simply use the following:
find_row <- function(df, row) {
df[row, ]
}
find_row(df, 1)
apply() isn't required. Using a single function makes the code simpler to read and should be faster too.