Shiny - assigning argument to function - function

Question

I am trying to create a shiny app that applies a self-made function to an uploaded dataset, then allows to download the modified results. Here is my code:
library(shiny)
library(tidyverse)
namkurz <- function(data, a_spalte) {
kuerzel <- vector(length = length(data$a_spalte))
for (i in 1:length(data$a_spalte)){
spez = data$Art[i]
s = unlist(strsplit(spez, " ", fixed = TRUE))
s = substr(s, 1, 2)
s = paste(s, collapse = ' ')
kuerzel[[i]] = s
}
data <- data %>%
mutate(kurz = kuerzel)
}
ui <- fluidPage(
fileInput('upload','Deine Kartierungsdaten'),
textInput('art', 'Wie heißt die Spalte mit Artnamen?'),
downloadButton('analyse','Artenkürzel hinzufügen')
)
server <- function(input, output, session) {
data <- reactive({
req(input$upload)
ext <- tools::file_ext(input$upload$name)
switch(ext,
csv = vroom::vroom(input$upload$datapath, delim = ";"),
validate("Invalid file; Please upload a .csv file")
)
})
art <- reactive(input$art)
output$analyse <- downloadHandler(
filename = function() {
paste0('mit_kuerzel', ".csv")
},
content = function(file) {
ergebnis <- reactive(namkurz(data(), art()))
vroom::vroom_write(ergebnis(), file)
}
)
}
shinyApp(ui, server)
When trying to save the output I get a 'Warning: Unknown or uninitialised column:' error. I think my problem is in the assignment of argument 'art' to the 'ergebnis' object, but I can't find the way to fix it.

Answer 1

I recommend a few things:
(Required) In your function, a_spalte is a character vector and not the literal name of column in the frame, so you need to use [[ instead of $, see The difference between bracket [ ] and double bracket [[ ]] for accessing the elements of a list or dataframe and Dynamically select data frame columns using $ and a character value.
Change all references of data$a_spalte to data[[a_spalte]].
namkurz <- function(data, a_spalte) {
kuerzel <- vector(length = length(data[[a_spalte]]))
for (i in 1:length(data[[a_spalte]])){
spez = data$Art[i]
s = unlist(strsplit(spez, " ", fixed = TRUE))
s = substr(s, 1, 2)
s = paste(s, collapse = ' ')
kuerzel[[i]] = s
}
data <- data %>%
mutate(kurz = kuerzel)
}
Your function is a bit inefficient doing things row-wise, we can vectorize that operation.
namkurz <- function(data, a_spalte) {
spez <- strsplit(data$Art, " ", fixed = TRUE)
data$kurz <- sapply(spez, function(z) paste(substr(z, 1, 2), collapse = " "))
data
}
(Optional) The content= portion of downloadHandler is already reactive, you do not need to wrap namkurz in reactive. Because of this, you also don't need to treat ergebnis as reactive.
output$analyse <- downloadHandler(
filename = ...,
content = function(file) {
ergebnis <- namkurz(data(), art())
vroom::vroom_write(ergebnis, file)
}
)
(Optional) Your output filename is fixed, so two things here: if it's always going to be "mit_kuerzel.csv", then there's no need for paste0, just use function() "mit_kuerzel.csv".
However, if you are intending to return a file named something based on the original input filename, one could do something like:
filename = function() {
paste0(tools::file_path_sans_ext(basename(input$upload$name)),
"_mit_kuerzel.",
tools::file_ext(input$upload$name))
},
to add _mit_kuerzel to the base portion of the uploaded filename. Note that the file in the content= section is never this name, the new_mit_kuerzel.csv is the filename offered to the downloading browser as a suggestion, that is all.
(Optional) You are using a .csv file extension in the downloadHandler, but the default for vroom::vroom_write is to use delim = "\t", which is not a CSV. I suggest either adding delim = ";" (or similar), or changing the returned filename extension to .tsv instead.