# Simple Shiny App for Gift Exchange Drawing

By Eric Burden | November 26, 2021

‘Tis the season! In this article, I’d like to share how I used a smattering of R and some free online services to overcome a surprisingly tricky holiday speedbump.

## The “Problem”

Every year, I, my brother, my sister, and our spouses draw names for Christmas gifts (mostly because we’d all rather buy presents for each others’ kids). This process has just a few requirements:

• Each person should draw the name of another person in the “gift pool” randomly.
• No one should know the name drawn by anyone else, until gifts are exchanged.

Our first attempt at this involved writing names on slips of paper, dropping the paper in a hat, then drawing names. The problem with this approach was that we may need to repeat the process in case the last name in the hat was the last person to draw or their spouse. And, in a pool of only six married couples, this happened fairly regularly.

## Automate It!

Well, being who I am and doing what I do, it fell to me to sort this whole thing out (plus, I’m the oldest sibling, so if it’s not obviously someone else’s responsibility, then it’s probably mine). Thankfully, this particular problem was well within my wheelhouse. The first, and probably trickiest, part was to randomly assign names without assigning anyone their own or their spouse’s name for gifting. The following bit of R solves this pretty nicely:

# How this works isn't important for now. What is important is to know that
# names is a dataframe with two columns:
# - family: Number used to group individuals who should not be given each
#           others' names for the gift exchange. Spouses, for example.
# - name:   The name of the individual. Each name should be unique. Include
#           last and middle names, if necessary.
names <- get_names_data()

# This is not a clever algorithm. We create a copy of names for the person
# who will draw a name (the "choosers") and for the person whose name will
# be drawn (the "choices"). Then the rows are randomly joined  and checked
# to see if any row contains a chooser_family and choice_family that
# are the same. If so, do it again.
choosers <- dplyr::rename_all(names, stringr::str_replace, "^", "chooser_")
choices  <- dplyr::rename_all(names, stringr::str_replace, "^", "choice_")

try_again <- T
while (try_again) {
a <- dplyr::mutate(choosers, join_key = sample(seq(name_count)))
b <- dplyr::mutate(choices,  join_key = sample(seq(name_count)))
selections <- dplyr::full_join(a, b, by = "join_key")
try_again <- with(selections, any(chooser_family == choice_family))
}
selections <- dplyr::mutate(selections, viewed = FALSE)

# Write the selections somewhere (TBD)
write_selections_data(selections)


Note: I’ll include the library name on all library function calls, just to make it clear which library provides which function. This isn’t strictly necessary, but it is helpful for these self-contained examples.

Ignoring where we get the names data from and where we save the selections data to for now, let’s walk through the “algorithm” for randomly assigning gift givers to gift receivers:

1. Take the dataframe names and make a copy of it, pre-pending “chooser_” to all the column names.
2. Make another copy of names, pre-pending “choice_” to all the column names.
3. Add a column named join_key to both choosers and choices containing a random number from 1 to the number of rows.
4. Join choosers and choices on that join_key.
5. If any record has a chooser_family and choice_family that are the same, do it again.
6. When every “chooser” is assigned a “choice” from another family, add a viewed column with all values defaulted to FALSE.

We’ll use that viewed column later to indicate which people have used our process to view which name they’ve “drawn”. This is a super inefficient way to do this, and it would be more efficient the more names/families are included, but since it only needs to run once a year, it’s efficient enough.

## Deploy It?

The next tricky problem is to provide this information to each person in a way that prevents them (and preferably me, too) from checking to see who drew their name before Christmas. Also, because we all live in different places, it would be nice to have a way to do this at any time, not just when we’re all together for Thanksgiving. Since I’m solving the “random selection” problem in R, and because Shiny apps are so darn easy to create (for simple apps) and deploy, I decided to host a super simple app on shinyapps.io.

This also means I need a persistence layer for this application, because apps hosted on the free tier are prone to being “put to sleep”, causing them to potentially lose state (and thus the permutation of names) between accesses. No problem, there’s at least one really well supported way to persistently save and access data from a Shiny app: Google Drive. Now, this isn’t the most sophisticated approach, but for something that only gets used by six people once a year, it’s sophisticated enough.

### Step One: Setup

The final project setup looks like this:

root
├── app.R
├── .secrets
│   └── <OAUTH TOKEN>
├── christmas_drawing.Rproj
└── .Renviron


app.R contains the code for a super simple shiny application. .secrets is a folder containing an OAuth token for authenticating with Google Drive (more on that in a second). There’s the RStudio *.Rproj and a .Renviron file for environment variables, and that’s pretty much it. Not shown are a renv directory used to manage project dependencies and an rsconnect folder auto-generated by RStudio when deploying the app. I’ve gotten into the habit of using renv for even super small projects like this one, mostly because it’s pretty frictionless.

Because I’m hosting this application on shinyapps.io, I’m using a dedicated Google account for this project, and this app only lives for a couple of days a year, I feel comfortable including my environment variables in a .Renviron file in my project root. .Renviron contains two entries:

2. DRIVE_FOLDER_ID: The ID for the Drive folder used.

In order to access Google Drive, I needed to include the following snippet in the app.R file in order to get and cache the OAuth token in the project directory and upload it to shinyapps.io:

googledrive::drive_auth(
cache = "./.secrets/"
)


This code needs to be run at least once prior to deploying the app, so that the token will be there. Again, this is a small application being deployed for a limited amount of time, using a Google account that’s made specifically for this. This is not how you should handle credentials “in real life”. For a more responsible take on authenticating with Google in a more permanent application, see this documentation.

## Step Two: The “Database”

As shown above, our “data layer” consists of two files: a “names” flat file that lists participants and their groups and a “selections” binary file that holds the random pairings between gift givers and receivers. The names are stored in a “*.csv” file because it’s easy and I can add to it over time as needed. The pairings are stored in a binary file mostly because that makes it harder to just double-click and peek at the contents. This helps with the third requirement listed above. It doesn’t make it impossible to peek, but so far it adds enough overhead that I haven’t been tempted to casually scope out who drew my name so I can drop hints…

To make this happen, I need to be able to read the “names” file and read/write the “selections” file from Google Drive. The following functions help with this:

#' Given a file name, return a Drive ID
#'
#' This function will raise an error if the requested file name does not exist
#' in the folder identified by DRIVE_FOLDER.
#'
#' @param file_name the name of the Drive file
#'
#' @return the ID of the Drive file
get_file_as_id <- function(file_name) {
(found_file_id
|> dplyr::filter(name == file_name)
|> dplyr::pull(id)

if (length(found_file_id) == 0) stop("No file with that name!")
found_file_id
}

#' Gets the list of names, stored in "names.csv"
#'
#' This file should contain two fields:
#' - family: Number used to group individuals who should not be given each
#'           others' names for the gift exchange. Spouses, for example.
#' - name:   The name of the individual. Each name should be unique. Include
#'           last and middle names, if necessary.
#'
#' @return a dataframe of the "names.csv" data
get_names_data <- function() {
(get_file_as_id("names.csv")
}

#' Gets the list of gift-giving pairs, stored in "selections.rds"
#'
#' This file will contain the following fields:
#' - chooser_family: The family value of the individual giving a gift
#' - chooser_name:   The name of the individual giving a gift
#' - choice_family:  The family value of the individual receiving a gift
#' - choice_name:    The name of the individual receiving a gift
#' - viewed:         Logical indicating if the person identified by
#'                   chooser_name has viewed their choice in the app
#'
#' @return a dataframe of the "selections.rds" data
get_selections_data <- function() {
(get_file_as_id("selections.rds")
|> base::rawConnection()
|> base::gzcon()
}

#' Write to "selections.rds"
#'
#' Saves selections as a local RDS file, then uploads it to Google Drive
#'
#' @param selections a dataframe to write to "selections.rds"
write_selections_data <- function(selections) {
base::saveRDS(selections, "selections.rds")

"selections.rds",
DRIVE_FOLDER,
name = "selections.rds",
overwrite = TRUE
)
}


I opted to leave the doc comments in for clarity. I found it clearer to break out the get_file_as_id() function for error-checking and retrieving files by name. DRIVE_FOLDER is a global variable that contains the ID of the Google Drive folder that contains the “names” and “selections” files. This should also clear up the mystery of where the code that generates the pairs gets data from and writes data to.

### Step Three: The User Interface

There isn’t much to say here. The final application UI basically consists of a dropdown and a button, and when you choose your name and click the button, both are hidden and the name of the person you “drew” is revealed.

ui <- fluidPage(
shinyjs::useShinyjs(),
shiny::titlePanel("Draw a name for Christmas!"),

shiny::fluidRow(
shiny::column(
width = 6,
offset = 3,
shiny::h2("Who are you?"),
)
),

shiny::fluidRow(
shiny::column(
width = 6,
offset = 3,
shiny::div(
id = "you-drew",
shiny::h2("You drew..."),
shiny::htmlOutput("drew_name")
),
shiny::div(
id = "draw-button",
shiny::actionButton(
"show_chosen_name",
"Draw a Name!",
class = "btn btn-success btn-block btn-lg"
)
)
)
)
)


This was the simplest setup I could come up with that helps enforce the “no peeking” rule, because there is one added twist: Once someone has “drawn” a name, the viewed boolean in the “selections” file is switched to TRUE, and their name is removed from the dropdown. This means that, if someone peeks to see who person A drew, when person A tries to draw a name they’ll be missing from the dropdown list, at which point they will complain and the process can be restarted.

The most complex part of this is including shinyjs to allow the app to show/hide UI elements when the button is clicked.

### Step Four: The Server

The server code for this app is similarly straightforward. It contains code to fetch the “selections” data, populate the dropdown with the names of people who haven’t drawn yet, and an observer for the button that marks the person who drew, hides the button, then displays the name drawn. The “selections” data is fetched in a tryCatch() function to either fetch the data from Google Drive or generate it from scratch from the “names” data if there is no “selections” file. init_selections() is basically just a wrapper function around the code shown earlier that generates the pairs.

server <- function(input, output, session) {
# Hide this div on server start
shinyjs::hide("you-drew")

# Get the saved "selections" data, or generate it if it doesn't already exist.
selections <- tryCatch(
get_selections_data(),
error = \(e) init_selections()
)

# Get a list of "choosers" and populate the dropdown in the UI.
(choosers
<- selections
|> dplyr::filter(!viewed)
|> dplyr::pull(chooser_name))

# Observer for the button. When clicked, indicate that the "chooser" has
# already viewed their "choice" and update the saved "selections" data.
# Then, display the "choice".
shiny::observeEvent(input$show_chosen_name, { # Mark chooser selections$viewed[selections$chooser_name == input$chooser] <- T
write_selections_data(selections)

# Get choice name
(choice
<- selections
|> dplyr::filter(chooser_name == input$chooser) |> dplyr::pull(choice_name)) # Populate the display of the drawn name (output$drew_name
<- choice
|> shiny::h1(class = "display text-danger")
|> shiny::renderUI())

# Hide the button and show the drawn name
shinyjs::show("you-drew")
shinyjs::hide("draw-button")
}
)
}


Step Five: Deploy It!

With all the code and environment variables in place, all that’s left is to deploy the application. I needed to run it at least one time locally to generate the OAuth token (in reality I ran it locally several times for debugging while I made it). Deployment was super straightforward thanks to the shinyapps.io integration with RStudio. I’ll refer you to the official documentation for a step-by-step set of instructions, just know there aren’t actually that many steps. Importantly, usage of this app fits well within the free usage tier.

## All Together

Just to wrap this up in a neat bow, here’s the full contents of app.R all in one place, for context:

# Library Imports --------------------------------------------------------------

library(dplyr)
library(shiny)
library(shinyjs)
library(stringr)
library(tibble)
library(tidyr)

# Global Application Setup -----------------------------------------------------

#' The .Renviron file stores two important environment variables
#' - DRIVE_FOLDER_ID: The ID for the Drive folder used

#' Authenticate with Google Drive and store the OAuth token in a cache folder
#' in the project directory. This needs to be run at least once before uploading
#' to shinyapps.io, to create the cached OAuth token.
cache = "./cache/"
)

#' Using the DRIVE_FOLDER_ID, mark DRIVE_FOLDER as a Drive folder ID
(DRIVE_FOLDER
<- Sys.getenv("DRIVE_FOLDER_ID")

#' Given a file name, return a Drive ID
#'
#' This function will raise an error if the requested file name does not exist
#' in the folder identified by DRIVE_FOLDER.
#'
#' @param file_name the name of the Drive file
#'
#' @return the ID of the Drive file
get_file_as_id <- function(file_name) {
(found_file_id
|> dplyr::filter(name == file_name)
|> dplyr::pull(id)

if (length(found_file_id) == 0) stop("No file with that name!")
found_file_id
}

#' Gets the list of names, stored in "names.csv"
#'
#' This file should contain two fields:
#' - family: Number used to group individuals who should not be given each
#'           others' names for the gift exchange. Spouses, for example.
#' - name:   The name of the individual. Each name should be unique. Include
#'           last and middle names, if necessary.
#'
#' @return a dataframe of the "names.csv" data
get_names_data <- function() {
(get_file_as_id("names.csv")
}

#' Gets the list of gift-giving pairs, stored in "selections.rds"
#'
#' This file will contain the following fields:
#' - chooser_family: The family value of the individual giving a gift
#' - chooser_name:   The name of the individual giving a gift
#' - choice_family:  The family value of the individual receiving a gift
#' - choice_name:    The name of the individual receiving a gift
#' - viewed:         Logical indicating if the person identified by
#'                   chooser_name has viewed their choice in the app
#'
#' @return a dataframe of the "selections.rds" data
get_selections_data <- function() {
(get_file_as_id("selections.rds")
|> base::rawConnection()
|> base::gzcon()
}

#' Write to "selections.rds"
#'
#' Saves selections as a local RDS file, then uploads it to Google Drive
#'
#' @param selections a dataframe to write to "selections.rds"
write_selections_data <- function(selections) {
base::saveRDS(selections, "selections.rds")

"selections.rds",
DRIVE_FOLDER,
name = "selections.rds",
overwrite = TRUE
)
}

#' Generate the data for "selections.rds"
#'
#' @return a dataframe of selections
init_selections <- function() {
names <- get_names_data()
name_count <- nrow(names)

# This is not a clever algorithm. We create a copy of names for the person
# who will draw a name (the "choosers") and for the person whose name will
# be drawn (the "choices"). Then the rows are randomly joined  and checked
# to see if any row contains a chooser_family and choice_family that
# are the same. If so, do it again.
choosers <- dplyr::rename_all(names, stringr::str_replace, "^", "chooser_")
choices  <- dplyr::rename_all(names, stringr::str_replace, "^", "choice_")

try_again <- T
while (try_again) {
a <- dplyr::mutate(choosers, join_key = sample(seq(name_count)))
b <- dplyr::mutate(choices,  join_key = sample(seq(name_count)))
selections <- dplyr::full_join(a, b, by = "join_key")
try_again <- with(selections, any(chooser_family == choice_family))
}
selections <- dplyr::mutate(selections, viewed = FALSE)

# Write the selections data to Google Drive and return the dataframe
write_selections_data(selections)
selections
}

# Shiny App UI -----------------------------------------------------------------

ui <- fluidPage(
shinyjs::useShinyjs(),
shiny::titlePanel("Draw a name for Christmas!"),

shiny::fluidRow(
shiny::column(
width = 6,
offset = 3,
shiny::h2("Who are you?"),
)
),

shiny::fluidRow(
shiny::column(
width = 6,
offset = 3,
shiny::div(
id = "you-drew",
shiny::h2("You drew..."),
shiny::htmlOutput("drew_name")
),
shiny::div(
id = "draw-button",
shiny::actionButton(
"show_chosen_name",
"Draw a Name!",
class = "btn btn-success btn-block btn-lg"
)
)
)
)
)

# Shiny App Server -------------------------------------------------------------

server <- function(input, output, session) {
shinyjs::hide("you-drew")

# Get the saved "selections" data, or generate it if it doesn't already exist.
selections <- tryCatch(
get_selections_data(),
error = \(e) init_selections()
)

# Get a list of "choosers" and populate the dropdown in the UI.
(choosers
<- selections
|> dplyr::filter(!viewed)
|> dplyr::pull(chooser_name))

# Observer for the button. When clicked, indicate that the "chooser" has
# already viewed their "choice" and update the saved "selections" data.
# Then, display the "choice".
shiny::observeEvent(input$show_chosen_name, { # Mark chooser selections$viewed[selections$chooser_name == input$chooser] <- T
write_selections_data(selections)

(choice
<- selections
|> dplyr::filter(chooser_name == input$chooser) |> dplyr::pull(choice_name)) (output$drew_name
<- choice
|> shiny::h1(class = "display text-danger")
|> shiny::renderUI())

shinyjs::show("you-drew")
shinyjs::hide("draw-button")
}
)
}

# Run the application
shiny::shinyApp(ui = ui, server = server)


In total, that’s about 200 lines of code, with a hefty fraction of it being comments so I’ll remember what I did if I need to make any changes to it next year.

## Wrap Up

When I talk to folks who are learning to code, I often recommend finding small problems that can be solved either in their workplace or lives outside of work. This project fits squarely into that category, because it represents a small amount of code with a somewhat larger amount of problem-solving that touches a few different areas of application development. Shiny apps in particular are nice learning tools, in part because the tooling around them is so well documented and supported in RStudio.

A final word about code style: I’ve recently adopted the convention shown here:

(choice
<- selections
|> dplyr::filter(chooser_name == input\$chooser)
|> dplyr::pull(choice_name))


Using the parentheses this way allows me to put the |> pipe operator at the beginning of the line instead of at the end, which is more consistent with the coding style found in languages like Haskell and Elm, which I find really helps line up these operations and makes for cleaner looking code. Your mileage may vary, but I like it.