How to handle exception in R like in Java? - r

Question

I am writing a server program in R where I upload PDF files and which later extracts data from the tables inside the pdf.
If required table and data is there, it works fine. But if not, it gives me error for files[i][[1]][[3]] and files[i][[1]][[4]].
error: subscript out of bounds
I want to default the value of buy and sell price at NA, if the table is not there.
all_data <- eventReactive(input$done, {
req(input$files)
files = {}
cost_price_list = {}
sell_price_list = {}
df <- data.frame(cost_price_list = character(), sell_price_list = character())
files <- lapply(input$files$datapath, extract_tables)
for (i in 1:length(input$files$datapath))
{
tryCatch(
{
cost_price_list <- files[i][[1]][[3]]
sell_price_list <- files[i][[1]][[4]]
},
error=function(cond) {
cost_price_list[i] = NA
sell_price_list[i] = NA
}
)
df[nrow(df) + 1,] <- c(cost_price_list[i],sell_price_list[i])
}
#return dataframe as table
df
})
But the above code doesn't work for me if the table is not there in pdf.
What am i doing wrong?
Please help.

Answer 1

There are multiple problems with your code.
First of, files = {} works but almost certainly doesn’t do what you intended. Did you mean files = list()? Otherwise, the idiomatic way of expressing what your code does is to write files = NULL in R. A more idiomatic way would be not to assign an empty object at all. In fact, your code overrides this initial value of files anyway, so files = {} is entirely redundant; and so are the next two assignments.
Next, since files is a list, you need to use double brackets to extract single elements from it: files[[i]], not files[i].
Another problem with your code is that assignment in R is always local. So cost_price_list[i] = NA creates a local variable inside the error handler function, even though cost_price_list exists in an outer scope.
If you want to assign to the outer variable, you need to explicitly specify the scope (or use <<-, but I recommend against this practice):
…
outer = environment()
for (i in 1:length(input$files$datapath))
{
tryCatch(
{
cost_price_list <- files[i][[1]][[3]]
sell_price_list <- files[i][[1]][[4]]
},
error=function(cond) {
outer$cost_price_list[i] = NA
outer$sell_price_list[i] = NA
}
)
df[nrow(df) + 1,] <- c(cost_price_list[i],sell_price_list[i])
}
…
But this still won’t work, because these variables do not have a value that can be meaningfully subset into (and it is not entirely clear what your code is attempting to do). Still, I hope the above gives you a foundation to work with.