Conditional Control

All survey configuration settings are defined within the server function in the app.R file. The server() function is a standard Shiny server function that takes input, output, and session as arguments.

If you create a new survey using a template, the server() function looks like this:

server <- function(input, output, session) {

  # Define conditional skip logic (skip to page if a condition is true)
  sd_skip_if()

  # Define conditional display logic (show a question if a condition is true)
  sd_show_if()

  # Main server to control the app
  sd_server(db = db)

}

The sd_skip_if() and sd_show_if() functions are used to define conditional skip and show logic for the survey. This page details how to use each of these functions to control many aspects of the survey flow logic.

Conditional display

It is often useful to have a question display based on some condition, such as the respondent choosing a particular value in a multiple choice question.

For example, let’s say we have a choice question about people’s favorite penguin type, and the last option is “other”. If the respondent chose it, you may want a second question to display that allows them to specify the “other” penguin type, like this:


To implement this, you first need to define both the conditional question and the target question in the survey.qmd file, like this:

```{r}
# Conditional question
sd_question(
  type  = "mc",
  id    = "penguins",
  label = "Which is your favorite type of penguin?",
  option = c(
    "Adélie"    = "adelie",
    "Chinstrap" = "chinstrap",
    "Gentoo"    = "gentoo",
    "Other"     = "other"
  )
)

# Target question
sd_question(
  type  = "text",
  id    = "penguins_other", 
  label = "Please specify the other penguin type:"
)
```

Then in the server function in the app.R file, you can use the sd_show_if() function to define that the "penguins_other" question would only be shown if the respondent chose the "other" option in the "penguins" question, like this:

server <- function(input, output, session) {

  sd_show_if(
    input$penguins == "other" ~ "penguins_other" 
  )

  sd_server(db = db)

}

You can provide multiple conditions to the sd_show_if() function, each separated by a comma. The structure for each condition in the sd_show_if() function is always:

<condition> ~ "target_question_id"

In the example above, input$penguins == "other" is the condition, and "penguins_other" is the target question that will be shown if the condition is met. The ~ symbol is used to separate the condition from the target question.

Note

The input object is a Shiny object that stores each question id defined by sd_question() in your survey.qmd file, so whenever referring to a question in a condition, you must use the format input$question_id.

Conditional skipping

Often times you’ll want to send respondents to different parts of the survey based on some condition, such as the respondent choosing a particular value in a multiple choice question.

For example, let’s say you want to screen out people who do not own a vehicle. To do this, you would first define a question in your survey.qmd file about their vehicle ownership, e.g.:

```{r}
sd_question(
  type  = 'mc',
  id    = 'vehicle_ownership',
  label = "Do you own your vehicle?",
  option = c(
    'Yes' = 'yes',
    'No'  = 'no'
  )
)
```

You would also need to define a screenout page to send respondents to, like this:

::: {#screenout .sd-page}

Sorry, but you are not qualified to take our survey.

:::

Then in the server function in the app.R file, you can use the sd_skip_if() function to define the condition under which the respondent will be sent to the target screenout page, like this:

server <- function(input, output, session) {

  sd_skip_if(
    input$vehicle_ownership == "no" ~ "screenout"
  )

  sd_server(db = db)

}

Just like the sd_show_if() function, you can provide multiple conditions to the sd_skip_if() function, each separated by a comma. The structure for each condition in the sd_skip_if() function is always:

<condition> ~ "target_page_id"

In the example above, input$vehicle_ownership == "no" is the condition, and "screenout" is the target page that the respondent will be sent to if the condition is met.

Common conditions

Both the sd_show_if() and sd_skip_if() functions require a condition that returns a logical value (TRUE or FALSE). The condition can be defined in a number of ways. In this section, we’ll highlight some of the most common types of conditions you might need.

Note

The show_if demo showcases how to use the sd_show_if() function with a variety of different conditions, but the same principles apply to conditions used in the sd_skip_if() function.

Conditioning on question answers

One of the most common situations is conditioning on the value of a single question or multiple questions, like this:

sd_show_if(

 # Simple condition based on single question choice
 input$penguins1 == "other" ~ "penguins1_other",

 # Multiple condition based on multiple question choices
 input$penguins2 == "other" & input$show_other == "show" ~ "penguins2_other"

)

In the first condition, the penguins1 question is checked to see if the respondent chose the "other" option. If they did, the penguins1_other question will be shown.

In the second condition, the penguins2 question is checked to see if the respondent chose the "other" option, and the show_other question is checked to see if the respondent chose the "show" option. With this condition, the penguins2_other question will only be shown if both conditions are TRUE.

Numeric values

Another common condition is checking the value of a numeric question. To do so, you need to wrap the input$question_id in the as.numeric() function because all question values are stored as strings, like this:

sd_show_if(
 as.numeric(input$car_number) > 1 ~ "car_ownership"
)

In the condition above, the car_number question is checked to see if the respondent chose a number greater than 1. If they did, the car_ownership question will be shown.

Multiple response questions

For multiple response question types (e.g. mc_multiple), the question returns a vector storing all the chosen values. You can use this vector to check for different conditions, such as whether the chosen values are in some set of values using the %in% operator, or whether the respondent chose a number of options using the length() function, like this:

sd_show_if(

 # Check if the respondent chose "apple", "banana", or both
 all(input$fav_fruits %in% c("apple", "banana")) ~ "apple_or_banana",

 # Check if the respondent chose more than 3 fruits
 length(input$fav_fruits) > 3 ~ "fruit_number"

)

In the first example, the fav_fruits question is checked to see if the respondent chose "apple", "banana", or both; if so, the apple_or_banana question will be shown.

In the second example, the fav_fruits question is checked to see if the respondent chose more than 3 fruits; if so, the fruit_number question will be shown.

Conditioning on answered status

You may want to show a target question if a question is answered at all or not. To do this, we created the sd_is_answered() function that returns TRUE if a question is answered and FALSE otherwise.

For example, let’s say you had a multiple choice question fav_fruit that asked you to choose your favorite fruit from a list of options, and a target question num_fruit that asked how many fruit you eat per day. If we wanted to show the num_fruit question so long as the fav_fruit question is answered, we can use sd_is_answered("fav_fruit") in the sd_show_if() function, like this:

sd_show_if(
  sd_is_answered("fav_fruit") ~ "num_fruit" 
)

This way, as long as the fav_fruit question is answered, no matter which option the user picks the num_fruit question will appear.

For "matrix" type questions, sd_is_answered() will only be TRUE if all sub-questions (matrix rows)in it are answered.

Conditions with custom functions

For situations where the conditional logic is more complex, we recommend defining a custom function that will return a logical value (TRUE or FALSE). You can then pass this function to the sd_show_if() or sd_skip_if() functions as a condition.

For example, let’s say we had a mc type question where we asked how many cars the respondent owned, and we included numeric options 1 through 5 as well as a final option "6 or more". If we wanted to set a condition that would return TRUE if the user had more than one car, we could not use as.numeric(input$question_id) > 1 because this would return NA if the respondent chose the "6 or more" option.

To address this, we could create a custom function to handle this special condition:

server <- function(input, output, session) {

  more_than_one_car <- function(input) {
    num_cars <- as.numeric(input$car_number)
    if (is.na(num_cars)) { return(TRUE) }
    return(num_cars > 1)
  }

  sd_show_if(
    more_than_one_car(input) ~ "car_ownership"
  )

  sd_server(db = db)

}

In the more_than_one_car() function, we first obtain the numeric value of the car_number question using as.numeric(input$car_number). If this value is NA it indicates that the respondent chose the "6 or more" option, in which case we return TRUE. If not, then we can be sure that num_cars is a number, and we can return the simple condition num_cars > 1.

Note that here the more_than_one_car function is defined in the condition without () after it. This is because we are passing the function itself, not the result of the function.

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