Select a variable based on two inputs in Rshiny - r

Question

I have a dataset with variables for the ID of patients, different tests (MMT), and the treatment.
ID
MMT_II_week15_change
MMT_II_Week20_change
MMT_Tot_week15_change
MMT_Tot_Week20_change
Treatment
As you can see, we have two different tests (MMT_II_change and MMT_Tot_change), for two different timepoints (week15, week20).
What I want is the user to be able to select, first, the test, and then, the timepoint.
In reality, he would be picking just one of the variables, but in two different steps.
Something like:
**Select test:**
MMT_II
MMT_III
**Select timepoint:**
Week15
Week20
And after this, the variable selected would be:
e.g: MMT_II_Week20_change
I though of using regex for this, but it seems quite complicated and coulnd't find of a way of doing it.
Any help really appreciated, as I've been stuck with this for a while.

Answer 1

Would something like this work?
VAR = paste0(test,"_",timepoint,"_change")
...
# then later to use the variable...
.data[[VAR]]

Answer 2

You can wrap the checking of changess occured in a single reactive function in the server section of your code.
uptodateChoice <- reactive({
paste0(input$firstcontrol, "_", input$secondcontrol, "_change")
})
This function will be called once any of the two controls state change.
You can also add any validate(need(...)) checks inside the function if required or simply return() if some conditions are not satisfied.
You can access the string value calling uptodateChoice().

Answer 3

I'm thinking about pivoting the data to longer format, filter it and then pivot again to wider. This way we can filter using filter function directly.
library(tidyverse)
library(shiny)
# create some data
df <- tibble(
ID = 1:5, MMT_II_week15_change = sample(seq(0.01, 0.2, 0.01), 5), MMT_II_week20_change = sample(seq(0.01, 0.2, 0.01), 5),
MMT_Tot_week15_change = sample(seq(0.01, 0.2, 0.01), 5), MMT_Tot_week20_change = sample(seq(0.01, 0.2, 0.01), 5)
)
# pivot wider capturing MMT_* for the first column and the number of week in the second.
df_pivot <- pivot_longer(df, -ID, names_to = c("test", "week"), values_to = "change", names_pattern = "(MMT_.*)_week(\\d+)_change$")
## APP
library(shiny)
ui <- fluidPage(
selectInput("test", "Select Test", choices = unique(df_pivot$test)),
selectInput("timepoint", "Select Timepoint", choices = NULL),
tableOutput("table")
)
server <- function(input, output, session) {
table <- reactiveVal(NULL)
observeEvent(input$test, {
choices <- filter(.data = df_pivot, test == input$test) %>%
{
unique(.$week)
}
updateSelectInput(inputId = "timepoint", choices = choices)
})
# this could also be a reactive.
observe({
table(filter(df_pivot, test == input$test, week == input$timepoint) %>%
pivot_wider(names_from = "test", values_from = "change"))
})
output$table <- renderTable({
table()
})
}
shinyApp(ui, server)