How to access secondary inputs with variable names in R Shiny? - r

Question

I'm trying to have the user enter a numeric input, then generate a number of input boxes equal to that first numeric input. I would then like to find the sum of the responses to these secondary numeric inputs. However, I am having trouble accessing these variables in a comprehensive way, since their names are created used numeric variables. Right now when I run it I get this error:
Warning: Error in get: object 'inp21' not found
Thanks
library(shiny)
ui <- fluidPage(
# Application title
titlePanel("Test"),
sidebarLayout(
sidebarPanel(
numericInput("inp1",
"Enter num:",1)
),
mainPanel(
uiOutput("more_inp"),
textOutput("num_inps")
)
)
)
server <- function(input, output) {
counter <- reactiveValues(countervalue = 0)
counter2 <- reactiveValues(counter2value = 0)
output$more_inp <- renderUI({
#Generates number of numeric inputs equal to original numeric input
mylist <- lapply(1:input$inp1, function(i) {
inp_identifier <- paste("inp2", i, sep="")
inp_name<- paste("Input2",i,sep=" ")
list(
numericInput(inp_identifier,inp_name, 5)
)
})
do.call(tagList, unlist(mylist, recursive = FALSE))
})
#Display number of secondary inputs
#Count number of secondary inputs
observeEvent(input$inp1, {
counter$countervalue <- counter$countervalue + 1
})
#Find sum of secondary inputs
output$num_inps<-renderText(input$inp1)
observeEvent(input$inp1,{
for (i in 1:counter$countervalue) {
counter2$counter2value <- counter2$counter2value + get(paste("inp2", i, sep=""))
print(counter2$counter2value)
}
})
}
Run the application
shinyApp(ui = ui, server = server)

Answer 1

Additionaly to r2evans suggestion simply "filter" out the case when an input field cannot be found. It seems that this observeEvent is called before the other input fields can be created. As a consequence, you receive an empty vector (numeric(0)) when you try to access one of them.
observeEvent(input$inp1,{
for (i in 1:counter$countervalue) {
if (isTruthy(input[[paste0("inp2", i)]])) {
counter2$counter2value <- counter2$counter2value + input[[paste0("inp2", i)]]
print(counter2$counter2value)
}
}
})

Answer 2

You aren't checking for the presence before adding. The observeEvent block is firing aggressively, so even though counter$countervalue is 1, there are not additional input fields present, so input[[paste("inp2", i, sep="")]] returns NULL. Anything plus null is numeric(0).
How to find this
observeEvent(input$inp1,{
browser()
for (i in 1:counter$countervalue) {
counter2$counter2value <- counter2$counter2value + get(paste("inp2", i, sep=""))
print(counter2$counter2value)
}
})
Run your app. When it hits the debugger and shows you Browse[2]>, then
counter$countervalue
# [1] 1
counter2$counter2value
# [1] 0
i <- 1L
get(paste("inp2", i, sep=""))
# Error in get(paste("inp2", i, sep = "")) : object 'inp21' not found
input[[ paste("inp2", i, sep="") ]]
# NULL
names(input)
# [1] "inp1"
A quick check could be to look for that paste(.) name in names(input).
Tangent
for (i in 1:n) works fine as long as you are 100% certain that n here will always be 1 or greater. If there is the remote possibility that it will be <1, then the results will be rather unintuitive.
That is, if n is 0, then I would expect the for loop to do nothing. As a vector example,
for (nm in c('a','b')) ... # executes twice GOOD
for (nm in c()) ... # does not execute GOOD
for (i in 1:2) ... # executes twice GOOD
for (i in 1:0) ... # executes twice BAD
The reason is that 1:0 resolves to a reverse sequence, so seq(1, 0) and therefore c(1L, 0L).
A safer alternative if you expect a possible zero is seq_len:
seq_len(2)
# [1] 1 2
seq_len(0)
# integer(0)
(seq(1, length.out=n) is equivalent.)
An even safer alternative if you cannot guarantee "positive only", then
seq_len(max(0, n))
(since seq_len(-1) is an error).
Suggested solution
observeEvent(input$inp1,{
for (i in seq_len(counter$countervalue)) { # or max(0, ...)
nm <- paste("inp2", i, sep="")
if (nm %in% names(input)) {
counter2$counter2value <- counter2$counter2value + input[[nm]]
print(counter2$counter2value)
}
}
})