How to use reactives in loops. What works, what doesn't and why.

reactives-in-loops.R

# -------------------------------------------------------------------
# ------------------ REACTIVES INSIDE FOR LOOPS ---------------------
# -------------------------------------------------------------------

# -------------------------------------------------------------------
# --- EXAMPLE 1: this works fine, because there are no reactives in -
# --- the for lopp --------------------------------------------------
# -------------------------------------------------------------------
library(shiny)

ui <- fluidPage(
  verbatimTextOutput("txt")
)

server <- function(input, output, session) {
  rv <- reactiveValues(one = 1, two = 2, three = 3)
  
  output$txt <- renderPrint({
    vals <- list()
    for (val in reactiveValuesToList(rv)) {
      vals[[val]] <- val
    }
    for (i in seq_len(length(vals))) {
      print(vals[[i]])
    }
  })
}

shinyApp(ui, server)
#> [1] 1
#> [1] 2
#> [1] 3


# -------------------------------------------------------------------
# --- EXAMPLE 2: this works fine, because even though there is a ----
# --- reactive in the for lopp, it is invoked immediately (inside ---
# --- the for loop) -------------------------------------------------
# -------------------------------------------------------------------
library(shiny)

ui <- fluidPage(
  verbatimTextOutput("txt")
)

server <- function(input, output, session) {
  rv <- reactiveValues(one = 1, two = 2, three = 3)
  
  output$txt <- renderPrint({
    vals <- list()
    for (val in reactiveValuesToList(rv)) {
      vals[[val]] <- reactive({ val })()
    }
    for (i in seq_len(length(vals))) {
      print(vals[[i]])
    }
  })
}

shinyApp(ui, server)
#> [1] 1
#> [1] 2
#> [1] 3


# -------------------------------------------------------------------
# --- EXAMPLE 3: this does not work, because there is a reactive in -
# --- the for lopp that is only invoked outside the for loop --------
# -------------------------------------------------------------------
library(shiny)

ui <- fluidPage(
  verbatimTextOutput("txt")
)

server <- function(input, output, session) {
  rv <- reactiveValues(one = 1, two = 2, three = 3)
  
  output$txt <- renderPrint({
    vals <- list()
    for (val in reactiveValuesToList(rv)) {
      vals[[val]] <- reactive({ val })
    }
    for (i in seq_len(length(vals))) {
      print(vals[[i]]())
    }
  })
}

shinyApp(ui, server)
#> [1] 3
#> [1] 3
#> [1] 3

# --- Joe on this behavior:
# --- > It's because all the iterations of the for loop share the same
# --- > reference to el. So when any of the created reactive expressions 
# --- > execute, they're using whatever the final value of el was.


# -------------------------------------------------------------------
# --- EXAMPLE 4: this works, because even though there is a reactive 
# --- in the for lopp that is only invoked outside of it, it is: ----
# ---                                                            ----  
# ---    * wrapped in a call to `local({ })`;                    ----
# ---                                                            ----
# ---    * the iter variable (`val`) is assigned to `local({ })` ----
# ---      specific variable (`myVal`);                          ----
# ---                                                            ----
# ---    * the super-assign operator (`<-`) is used              ----
# -------------------------------------------------------------------
library(shiny)

ui <- fluidPage(
  verbatimTextOutput("txt")
)

server <- function(input, output, session) {
  rv <- reactiveValues(one = 1, two = 2, three = 3)
  
  output$txt <- renderPrint({
    vals <- list()
    for (val in reactiveValuesToList(rv)) {
      local({
        myVal <- val
        vals[[myVal]] <<- reactive({ myVal })
      })
    }
    for (i in seq_len(length(vals))) {
      print(vals[[i]]())
    }
  })
}

shinyApp(ui, server)
#> [1] 1
#> [1] 2
#> [1] 3

# --- Joe on this behavior:
# --- > You can fix this by using a for loop but introducing a 
# --- > local({...}) inside of there, and creating a local variable 
# --- > in there whose value is assigned to el outside of the reactive.


# -------------------------------------------------------------------
# --- EXAMPLE 5: this works, because we use lapply instead of a for -
# --- loop ----------------------------------------------------------
# -------------------------------------------------------------------
library(shiny)

ui <- fluidPage(
  verbatimTextOutput("txt")
)

server <- function(input, output, session) {
  rv <- reactiveValues(one = 1, two = 2, three = 3)
  
  output$txt <- renderPrint({
    vals <- lapply(
      reactiveValuesToList(rv),
      function(x) { reactive(x) }
    )

    # --- Note that this alternative would not work because if doesn't
    # --- get its own reference to the iter variable, so we always ---
    # --- need to create a wrapper for the reactive ------------------
    # vals <- lapply(
    #   reactiveValuesToList(rv), 
    #   reactive
    # )
    
    for (i in seq_len(length(vals))) {
      print(vals[[i]]())
    }
  })
}

shinyApp(ui, server)
#> [1] 1
#> [1] 2
#> [1] 3

# --- Joe on this behavior:
# --- > You can fix this by using lapply instead of a for loop; since 
# --- > each iteration executes as its own function call, it gets its 
# --- > own reference to el.

Great overview! There is a typo that was copied to many of the comments:

lopp

I also recommend reading this answer on Stack Overflow: https://stackoverflow.com/a/54331019/1445572