How to extract element name, not value, from a list in R Shiny selectInput()?

问题

I would like to extract the element name, and not the specific value, from a list used for the choices argument in selectInput() from R Shiny.

The selectInput function looks like this:

# ...
selectInput("xvar", "What is the predictor variable?",
                        choices = list("MPG" = "mpg",
                                       "Cylinders" = "cyl",
                                       "Engine Displacement" = "disp",
                                       "Horse Power" = "hp",
                                       "Gears" = "gear"),
# ...

In my server.R code I would like to use, for example, "Cylinders" and not "cyl" as an axis label. For example (using ggplot2):

# ...
labs(x = input$xvar, y = input$yvar) +
# ...

names(input$xvar) returns NULL. Is there any way to call input$xvar and return the name?

回答1:

Thanks to Paul's comments, the links he provided, and this SO thread, I was able to answer my question.

Below I provide the old ui.R and server.R scripts which generated axis labels I was not happy with, as well as new ui.R and server.R scripts where the axis labels are improved. (Changes in the new scripts are marked with # diff)

old ui.R:

shinyUI(fluidPage(
    titlePanel("Fit Regression Line for Chosen Variables and Points"),
    sidebarLayout(
        sidebarPanel(
            h2("Model Specifics"), br(),
            selectInput("xvar", "What is the predictor variable?",
                        choices = list("MPG" = "mpg",
                                       "Cylinders" = "cyl",
                                       "Engine Displacement" = "disp",
                                       "Horse Power" = "hp",
                                       "Gears" = "gear"),
                        multiple = FALSE),
            selectInput("yvar", "What is the outcome variable?",
                        choices = list("MPG" = "mpg",
                                       "Cylinders" = "cyl",
                                       "Engine Displacement" = "disp",
                                       "Horse Power" = "hp",
                                       "Gears" = "gear"),
                        multiple = FALSE, selected = "cyl"),
            h4("Intercept"), textOutput("int"),
            h4("Slope"), textOutput("slope")
        ),
        mainPanel(
            br(), h2("Display"), h4("Drag to select which points to include in model"),
            plotOutput("plot", brush = brushOpts(id = "brush1"))
        )
    )
))

old server.R:

shinyServer(function(input, output) {
        model <- reactive({
                points <- brushedPoints(mtcars, brush = input$brush1,
                                        xvar = input$xvar,
                                        yvar = input$yvar)
                if(nrow(points) <= 1) {
                        return(NULL)
                } else {
                        lm(as.formula(paste0(input$yvar,
                                             "~", input$xvar)),
                           data = points)
                }
        })
        output$int <- renderText({
                if(is.null(model())) {
                        "Too few data points selected"
                } else {
                        round(model()[[1]][1], 2)
                }
        })
        output$slope <- renderText({
                if(is.null(model())) {
                        "Too few data points selected"
                } else {
                        round(model()[[1]][2], 2)
                }
        })
        output$plot <- renderPlot({
                library(ggplot2)
                ggplot(mapping = aes(x = mtcars[, input$xvar],
                                     y = mtcars[, input$yvar])) +
                        theme_minimal() +
                        geom_point() +
                        labs(x = input$xvar, y = input$yvar) +
                        coord_cartesian(x = c(0, 1.2*max(mtcars[, input$xvar])),
                                        y = c(0, 1.2*max(mtcars[, input$yvar]))) +
                if(!is.null(model())) {
                        geom_abline(intercept = model()[[1]][1], slope = model()[[1]][2],
                                    colour = "red", lwd = 2, alpha = 0.3)
                }
        })
})

Changes in the scripts are marked with # diff

new ui.R:

shinyUI(fluidPage(
    titlePanel("Fit Regression Line for Chosen Variables and Points"),
    sidebarLayout(
        sidebarPanel(
            h2("Model Specifics"), br(),
            uiOutput("si_xvar"), # diff
            uiOutput("si_yvar"), # diff
            h4("Intercept"), textOutput("int"),
            h4("Slope"), textOutput("slope")
        ),
        mainPanel(
            br(), h2("Display"), h4("Drag to select which points to include in model"),
            plotOutput("plot", brush = brushOpts(id = "brush1"))
        )
    )
))

new server.R:

shinyServer(function(input, output) {
    varlist <- list("MPG" = "mpg",  # diff
                    "Cylinders" = "cyl",
                    "Engine Displacement" = "disp",
                    "Horse Power" = "hp",
                    "Gears" = "gear")
    output$si_xvar <- renderUI(     # diff
        selectInput("xvar", "What is the predictor variable?",
                    choices = varlist,
                    multiple = FALSE)
    )
    output$si_yvar <- renderUI(     # diff
        selectInput("yvar", "What is the outcome variable?",
                    choices = varlist,
                    multiple = FALSE, selected = "cyl")
    )
    model <- reactive({
        points <- brushedPoints(mtcars, brush = input$brush1,
                                xvar = input$xvar,
                                yvar = input$yvar)
        if(nrow(points) <= 1) {
            return(NULL)
        } else {
            lm(as.formula(paste0(input$yvar,
                                 "~", input$xvar)),
               data = points)
        }
    })
    output$int <- renderText({
        if(is.null(model())) {
            "Too few data points selected"
        } else {
            round(model()[[1]][1], 2)
        }
    })
    output$slope <- renderText({
        if(is.null(model())) {
            "Too few data points selected"
        } else {
            round(model()[[1]][2], 2)
        }
    })
    output$plot <- renderPlot({
        library(ggplot2)
        ggplot(mapping = aes(x = mtcars[, input$xvar],
                             y = mtcars[, input$yvar])) +
            theme_minimal() +
            geom_point() +
            labs(x = names(which(input$xvar == varlist)),       # diff
                 y = names(which(input$yvar == varlist))) +     # diff
            coord_cartesian(x = c(0, 1.2*max(mtcars[, input$xvar])),
                            y = c(0, 1.2*max(mtcars[, input$yvar]))) +
            if(!is.null(model())) {
                geom_abline(intercept = model()[[1]][1], slope = model()[[1]][2],
                            colour = "red", lwd = 2, alpha = 0.3)
            }
    })
})

来源：https://stackoverflow.com/questions/62526706/how-to-extract-element-name-not-value-from-a-list-in-r-shiny-selectinput