Bruna Wundervald - 6 months ago 22

R Question

there.

Im building an shiny app thats plots some functions. The user can modify the parameters. The problem appears when I have a restricted function, specially when its related to x. This is one example:

Sliders:

`sliderInput("th19",`

HTML("$$ \\theta_1 $$"),

min = 1,

max = 10,

value = 2)

sliderInput("thB9",

HTML("$$ \\theta_b $$"),

min = 1,

max = 10,

value = 2)

sliderInput("vthB9",

HTML("$$ \\vartheta_b $$"),

min = 1,

max = 20,

value = 2)

Plot:

`mForm9.1 <- as.formula("Y ~ vthB9 + th19*(x - thB9)")`

mExpr9.1 <- mForm9.1[[3]]

output$Curve9 <- renderPlot({

th19 <- input$th19

vthB9 <- input$vthB9

thB9 <- input$thB9

eval(call("curve", mExpr9.1, col = 2, ylab = "", main =

expression(vartheta[b] + theta[1]*(x - theta[b]))))

}, height = 400, width = 600)

mainPanel(

tabsetPanel(type = "tabs",

tabPanel("Gráfico", plotOutput("Curve9"))

))

What happens here is that when 'x' is bigger than 'vthB9', the equation resumes itself to only 'vthB9', and this is only one of the cases I have. Anybody knows what to do?

*Hope I have been clear

*Im using

`flexdashboard`

Answer

There are possibly multiple ways to plot a piecewise function in `R`

. I am going to suggest probably the easiest way of doing it **in this case**:

We first define the piecewise function, say, `fun`

```
fun <- function(x) {
ifelse(test = x <= vthB9,
yes = vthB9 + th19 * (x - thB9),
no = vthB9)
}
```

which we then pass to `curve`

```
curve(expr = fun, from = 0, to = 10, col = 2, ylab = "",
main = expression(vartheta[b] + theta[1] * (x - theta[b])))
```

`curve`

is going to input a **vector** as a parameter to the function `fun`

. The usual `if-else`

statements are not going to work because they can test only one value at a time - unless we write a for-loop or vectorise it somehow with a function `Vectorize`

. Instead of that, we pick `ifelse`

which is already vectorized.

Since you have used a `shiny`

tag I've prepared a `shiny`

app instead of `flexdashboard`

:)

**Full shiny example**

```
ui <- fluidPage(
sidebarLayout(
sidebarPanel(
sliderInput("th19",
HTML("θ <sub>1</sub>"),
min = 1,
max = 10,
value = 2),
sliderInput("thB9",
HTML("θ <sub>b</sub>"),
min = 1,
max = 10,
value = 2),
sliderInput("vthB9",
HTML("ϑ <sub>b</sub>"),
min = 1,
max = 20,
value = 2)
),
mainPanel(
tabsetPanel(type = "tabs",
tabPanel("Gráfico", plotOutput("Curve9")))
))
)
server <- function(input, output) {
output$Curve9 <- renderPlot({
th19 <- input$th19
vthB9 <- input$vthB9
thB9 <- input$thB9
# Define a piece wise function
fun <- function(x) {
ifelse(test = x <= vthB9,
yes = vthB9 + th19 * (x - thB9),
no = vthB9)
}
# x-axis goes now from 0 to 10
curve(expr = fun, from = 0, to = 10, col = 2, ylab = "",
main = expression(vartheta[b] + theta[1] * (x - theta[b])))
}, height = 400, width = 600)
}
shinyApp(ui, server)
```