Shiny reactive objects logic issue - r

Question

My goal is to implement a Shiny app which shows several "problems" to the user with a list of possible answers, then lets the user choose an answer and stores it. Every problem has a previously stored answer that the user can overwrite.
I can't find a way to satisfy these two constraints at the same time with reactive objects:
let the user select the next problem, with the answer being re-initialized from the previously stored answer
store the answer for the current problem when the user selects a new answer (and only in this case)
Below a simplified code (no data, no loading/writing) which shows my current attempt. In this version the issue is that when a new problem is selected, the selected answer from the previous problem is immediately written.
library(shiny)
maxProblem=10
ui <- fluidPage(
titlePanel("Debugging test"),
fluidRow(
column(12,
verbatimTextOutput("nbProblems"),
uiOutput("ProblemSelection"),
uiOutput("answerSelection")
)
)
)
server <- function(input, output) {
output$ProblemSelection <- renderUI({
numericInput("ProblemSelectionNo",
"Select Problem no",
value = 1, min=1, max=maxProblem)
})
currentProblemData <- reactive({
print("calling loadCurrentProblemData")
if (!is.null(input$ProblemSelectionNo)) {
print("pretending to load data and previously stored answer for problem", input$ProblemSelectionNo)
list( choices=c(1,2,3), answer=1)
}
})
output$answerSelection <- renderUI({
l<-currentProblemData()
choicesList <- l$choices
names(choicesList) <- l$choices
radioButtons("answerInput", label = "Select answer",
choices = choicesList,
selected = l$answer)
})
writeChanges <- observe({
print('calling writeChanges')
l<-currentProblemData()
newAnswer <- input$answerInput
prevAnswer <- l$answer
if (!is.null(prevAnswer) && !is.null(newAnswer) && (newAnswer != prevAnswer)) {
print(paste('Pretending to write new answer :',newAnswer,'for problem', input$ProblemSelectionNo))
l$answer <- newAnswer
}
})
}
# Run the application
shinyApp(ui = ui, server = server)

Answer 1

This might have the functionality you are looking for. I made a workable example to try based on some of what you have.
First, I created a default list choices_answer which could flexibly store your default problem choices and answers. A reactiveValues list lst (rv$lst) will start with this, and then change as new answers are selected to store the new responses.
When a new problem is selected through the numericInput, then the radioButtons are updated based on the current answer for that problem (using the rv$lst). Likewise, when a new answer is chosen (or answer is changed), the rv$lst will be updated with the new answer for storage.
I also added output ListData to show what the storage of answers looks like as you make selections using the radio buttons.
library(shiny)
maxProblem = 5
choices_answer = list()
for (i in seq_along(1:maxProblem)) {
choices_answer[[i]] <- list(
choices = c("1", "2", "3"),
answer = "1"
)
}
ui <- fluidPage(
titlePanel("Debugging test"),
fluidRow(
column(12,
numericInput("ProblemSelectionNo",
"Select Problem no",
value = 1, min = 1, max = maxProblem),
radioButtons("answerInput", label = "Select answer",
choices = choices_answer[[1]][["choices"]]),
verbatimTextOutput("ListData")
)
)
)
server <- function(input, output, session) {
rv <- reactiveValues(lst = choices_answer)
observeEvent(input$ProblemSelectionNo, {
updateRadioButtons(session, "answerInput",
choices = rv$lst[[input$ProblemSelectionNo]][["choices"]],
selected = rv$lst[[input$ProblemSelectionNo]][["answer"]])
})
observeEvent(input$answerInput, {
rv$lst[[input$ProblemSelectionNo]][["answer"]] <- input$answerInput
})
output$ListData <- renderPrint({rv$lst})
}
# Run the application
shinyApp(ui = ui, server = server)