Access global variable within function

Question

I have two data.frames df1 and errors and a function add_to_errors(). The argument of the function is a vector of row-numbers (of df1) and the function should add add the row selected to a new data.frame called errors.

df1 <- data.frame(id=1:10, var1=rep(NA, 2), var2=rep(NA, 2)) errors <- data.frame() add_to_errors <- function(ids){ rows_to_add <- df1[ids,] errors <- rbind(errors, rows_to_add) return(errors) } add_to_errors(c(1,2)) add_to_errors(c(6,7))

When I execute add_to_errors(1, 2) and add_to_errors(6,7), it looks as if errors was overwritten each time the function is called. How can I access errors as a global variable within the function?

The output should look like this:

 id var1 var2 1 1 NA NA 2 2 NA NA 3 6 NA NA 4 7 NA NA

Tim Biegeleisen · Accepted Answer · 2019-06-21 10:07:42Z

I recommend not using the parent scope operator <<-, and instead using your function with proper design and encapsulation:

add_to_errors <- function(df1, ids, errors) { rows_to_add <- df1[ids,] errors <- rbind(errors, rows_to_add) return(errors) } df1 <- data.frame(id=1:10, var1=rep(NA, 2), var2=rep(NA, 2)) errors <- data.frame() errors <- add_to_errors(df1, c(1,2), errors) errors <- add_to_errors(df1, c(6,7), errors) errors id var1 var2 1 1 NA NA 2 2 NA NA 6 6 NA NA 7 7 NA NA

The above functions has been refactored to accept all needed parameters as inputs, and the output is the data frame generated using your logic. When calling it, we clobber (overwrite) the input data frame with the return value.

I was going to suggest this but you got there! Definitely this is the way to go. Side effects are to be avoided.
@LyzandeR As a one-off script, your answer is totally fine. But when building larger codes, <<- can get hard to control :P

LyzandeR · Accepted Answer · 2019-06-21 10:11:27Z

One way would be to use <<-, like this:

add_to_errors <- function(ids){ rows_to_add <- df1[ids,] errors <<- rbind(errors, rows_to_add) return(errors) } add_to_errors(c(1,2)) # id var1 var2 #1 1 NA NA #2 2 NA NA add_to_errors(c(6,7)) # id var1 var2 #1 1 NA NA #2 2 NA NA #6 6 NA NA #7 7 NA NA

Changing the global environment through <<- within a function is not considered good practice (this is called side effect and it's better to be avoided - if possible - as it creates bugs).

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