append to dataframe in function - is globalenv really required - r

Question

I am using the following code, which works fine (improvement suggestions very much welcome):
WeeklySlopes <- function(Year, Week){
DynamicQuery <- paste('select DayOfYear, Week, Year, Close from SourceData where year =', Year, 'and week =', Week, 'order by DayOfYear')
SubData = sqldf(DynamicQuery)
SubData$X <- as.numeric(rownames(SubData))
lmfit <- lm(Close ~ X, data = SubData)
lmfit <- tidy(lmfit)
Slope <- as.numeric(sqldf("select estimate from lmfit where term = 'X'"))
e <- globalenv()
e$WeeklySlopesDf[nrow(e$WeeklySlopesDf) + 1,] = c(Year,Week, Slope)
}
WeeklySlopesDf <- data.frame(Year = integer(), Week = integer(), Slope = double())
WeeklySlopes(2017, 15)
WeeklySlopes(2017, 14)
head(WeeklySlopesDf)
Is there really no other way to append a row to my existing dataframe. I seem to need to access the globalenv. On the other hand, why can sqldf 'see' the 'global' dataframe SourceData?

Answer 1

dfrm <- data.frame(a=1:10, b=letters[1:10]) # reproducible example
myfunc <- function(new_a=20){ g <- globalenv(); g$dfrm[3,1] <- new_a; cat(dfrm[3,1])}
myfunc()
20
dfrm
a b
1 1 a
2 2 b
3 20 c # so your strategy might work, although it's unconventional.
Now try to extend dataframe outside a function:
dfrm[11, ] <- c(a=20,b="c")
An occult disaster (conversion of numeric column to character):
str(dfrm)
'data.frame': 11 obs. of 2 variables:
$ a: chr "1" "2" "20" "4" ...
$ b: Factor w/ 10 levels "a","b","c","d",..: 1 2 3 4 5 6 7 8 9 10 ...
So use a list to avoid occult coercion:
dfrm <- data.frame(a=1:10, b=letters[1:10]) # start over
dfrm[11, ] <- list(a=20,b="c")
str(dfrm)
'data.frame': 11 obs. of 2 variables:
$ a: num 1 2 3 4 5 6 7 8 9 10 ...
$ b: Factor w/ 10 levels "a","b","c","d",..: 1 2 3 4 5 6 7 8 9 10 ...
Now try within a function:
myfunc <- function(new_a=20, new_b="ZZ"){ g <- globalenv(); g$dfrm[nrow(dfrm)+1, ] <- list(a=new_a,b=new_b)}
myfunc()
Warning message:
In `[<-.factor`(`*tmp*`, iseq, value = "ZZ") :
invalid factor level, NA generated
str(dfrm)
'data.frame': 12 obs. of 2 variables:
$ a: num 1 2 3 4 5 6 7 8 9 10 ...
$ b: Factor w/ 10 levels "a","b","c","d",..: 1 2 3 4 5 6 7 8 9 10 ...
So it succeeds, but if there are any factor columns, non-existent levels will get turned into NA values (with a warning). You method of using named access to objects in the global environment is rather unconventional but there is a set of tested methods that you might want to examine. Look at ?R6. Other options are <<- and assign which allows one to specify the environment in which the assignment is to occur.