In R/Shiny Understanding map function inside shiny Server

This is my code:

letters <-letters[1:5]
ui <- fluidPage(
  
  do.call(tabsetPanel,
          letters %>%
            purrr::map(~tabPanel(title = .x,
                                 uiOutput(outputId = .x)))),
)
server <- function(input, output, session) {
  
  1:5 %>% purrr::map( ~ {
    output[[paste0(letters[.x])]] <-
      renderUI({
        h1(letters[.x])
      })
    
  })
}

shinyApp(ui, server)

This is my code. The main question here is to understand the syntax of the code inside the server function.

Why this code bellow doesn’t work:

1:5 %>% purrr::map(
    ~ output[[paste0(letters[.x])]] <-
      renderUI({
        h1(letters[.x])
      })
  )

And also, for this code that works why do I need to use { } inside map

1:5 %>% purrr::map( ~ {
    output[[paste0(letters[.x])]] <-
      renderUI({
        h1(letters[.x])
      })

  })

Any help to understand this syntax?

>Solution :

To understand this let’s take a simpler example as this question is not related to shiny specifically.

out <- list(1:3, 5:7)
out

#[[1]]
#[1] 1 2 3

#[[2]]
#[1] 5 6 7

To take sum of each list element we can either do

purrr::map(out, sum)
#[[1]]
#[1] 6

#[[2]]
#[1] 18

Or using the ~ notation

purrr::map(out, ~sum(.x))

Now, if you want to do something "more" i.e if the function that you want to apply takes more than 1 step. So let’s say, you want to take square of the summed values you need to use the curly braces ({})

purrr::map(out, ~{
  y <- sum(.x)
  y^2
})

#[[1]]
#[1] 36

#[[2]]
#[1] 324

In this simple example, I know you can do this in 1 step as purrr::map(out, ~sum(.x)^2) but this is just to demonstrate the use of {} here.

If it get’s too complex as a general rule you can always use {} in your functions. So in the first case for taking sum you may do purrr::map(out, ~{sum(.x)}).