Dynamic row index object not found in user-defined function R - r

Question

I am trying to pass an integer ("i") to a function in which "i" is used as a row index for a data frame. However, doing this...
user_definedFUN <- function (i){
...
result <- df[i, "col_name"]
...
}
x <- user_definedFUN(1)
...yields the following error:
Error in `[.data.frame`(df, i, "col_name") :
object 'i' not found
I'm certain this is a simple issue of how I am referencing "i" within the brackets (even if not simple enough for me to find a solution); however, I have provided additional details below if necessary.
The data.frame:
gen_name <- c("Boomers","Gen X","Millenials","Gen Z")
gen_years <- c("1946 to 1964","1965 to 1980","1981 to 1996", "1997 to 2011")
gen_xmin <- c(11, 9, 5, 2)
gen_xmax <- c(15, 11, 8, 5)
GEN_G.labels <- data.frame(gen_name, gen_years, gen_xmin, gen_xmax)
The data.frame contains information for four generations that will be used to plot rectangles as layers on a ggplot bar chart of populations by age.
The rectangles will be created by the following function that will be called from a loop and is provided the row index for specific generation (1 = "Boomers", 2 = "Gen X", etc.)
genlabelsFUN <- function(i){
# return a geom_rect()
rv <- geom_rect(aes(
xmin = GEN_G.labels[i, "gen_xmin"],
xmax = GEN_G.labels[i, "gen_xmax"],
ymin = 1000,
ymax = 1100)
, fill = "red")
return(rv)
}
ggplot(...snip...) +
...snip... +
genlabelsFUN(1)
The function works if a static index value is used. For example, 'GEN_G.labels[1, "gen_xmin"]' instead of 'GEN_G.labels[i, "gen_xmin"]' places a red rectangle between 11 and 15 on the x-axis at 1,000 on the y-axis with a height of 100. Although, the function is pointless without the dynamic aspect of "i".
The following image shows the output when using a static index value (Note: I'm using a different y-axis scale in my example above for simplicity). The final code will loop through each row of GEN_G.labels and run genlabelsFUN() to create a similar rectangle for each generation.
Thanks
EDIT:
Full ggplot
scaleFUN <- function(x) formatC( x / 1000, format = "f", big.mark = ",", digits = 0) #format as thousands with comma
ggplot(data = GEN_G.data_frame, aes(x = range, y = persons)) +
geom_bar(stat = "identity") +
theme_classic() +
theme(
axis.text.x = element_text(angle = 90, hjust = 1)) +
scale_y_continuous(
name = "Persons (thousands)",
labels = scaleFUN) +
genlabelsFUN(1)
EDIT 2:
Reproducible example (functioning based on MrFlick comment below)
GEN_G.dataframe <- data.frame(
range = c(1:21),
persons = abs(rnorm(21))*50)
GEN_G.labelsx <- data.frame(
gen_name = c("Group A","Group B","Group C","Group D"),
gen_xmin = c(11, 9, 5, 2),
gen_xmax = c(15, 11, 9, 5))
GEN_G.labelsx$gen_name <- factor(
GEN_G.labelsx$gen_name,
levels = GEN_G.labelsx$gen_name)
ggplot() +
geom_bar(data=GEN_G.dataframe,aes(x=range, y=persons),stat="identity") +
theme_classic() +
theme(
axis.text.x = element_text(angle = 90, hjust = 1)) +
geom_rect(aes(
xmin = gen_xmin,
xmax = gen_xmax,
ymin = 175,
ymax = 180,
fill = gen_name),
data = GEN_G.labelsx)
Output from Edit 2 example.

Answer 1

You can't use variables like i inside an aes(). Symbols inside the aes() are not evaluated until the plot is actually drawn. There's no way for R to properly capture the environment were i is defined that way so the value would have changed by the time the plot is drawn.
However, I don't really think a loop/function is even necessary. You should just be able to do
geom_rect(aes(xmin=gen_xmin, xmax=gen_xmax), ymin=1000, ymax=1000, data=GEN_G.labels)
to use a different data.frame for that layer. Then all the boxes are drawn at once without a loop.