reactive environment within reactiveValues (RShiny) - shiny

Question

I am trying to build a matrix with reactive dimensions that I want to update using an observeEvent expression. My idea was the following:
First, I am creating a reactiveValues object with a matrix of dimensions input$length (--> reactive) and input 0. Then I am using observeEvent with actionButton to trigger an update in the matrix. This entails updating a specific cell in the matrix, indicated by a reactive index vector (ind()), with a reactive value (value()).
I understand the problem: within mat = ... I cannot use another reactive expression, however I don't have an alternative solution and would very much appreciate any input on this.
Thanks in advance!
Kind regards,
Julian
ui <- fluidPage(
numericInput("length", "Dimensions of the matrix", value = 5),
numericInput("a", "value for a", value = 2),
numericInput("b", "value for b", value = 2),
numericInput("ind1", "value for index vector 1", value = 1),
numericInput("ind2", "value for index vector 2", value = 1),
actionButton("go", "Update"),
tableOutput("matrix")
)
server <- function(input, output) {
### Calculate the value that will be used for the update
value <- reactive(
mean(rbeta(100, input$a, input$b))
)
### Create a reactive index vector used to determine the position of the cell in the matrix
ind <- reactive(
c(input$ind1, input$ind2)
)
### Create reactiveValues matrix with dimensions specified in length
beta.matrix <- reactiveValues(
mat = matrix(0, input$length, input$length)
)
### Update matrix at positon ind with new value
observeEvent(input$go, {
beta.matrix$mat[ind()[1], ind()[2]] <- value()
}
)
### Render matrix
output$matrix <- renderTable({
mat <- beta.matrix$mat
mat
})
}
# Run the application
shinyApp(ui = ui, server = server)

Answer 1

I think you'll need two reactive "stages" here.
initialise an empty matrix when the dimensions change
react on changes to the content of the matrix
Please check the following:
library(shiny)
ui <- fluidPage(
numericInput("length", "Dimensions of the matrix", value = 5),
numericInput("a", "value for a", value = 2),
numericInput("b", "value for b", value = 2),
numericInput("ind1", "value for index vector 1", value = 1),
numericInput("ind2", "value for index vector 2", value = 1),
actionButton("go", "Update"),
tableOutput("matrix")
)
server <- function(input, output) {
### Calculate the value that will be used for the update
value <- reactive(mean(rbeta(100, input$a, input$b)))
### Create a reactive index vector used to determine the position of the cell in the matrix
ind <- reactive(c(input$ind1, input$ind2))
beta.matrix <- reactiveValues(mat = NULL)
beta.matrix.ini <- reactive({
mat = matrix(0, input$length, input$length)
})
observe({
beta.matrix$mat <- beta.matrix.ini()
})
### Update matrix at positon ind with new value
observeEvent(input$go, {
beta.matrix$mat[ind()[1], ind()[2]] <- value()
})
### Render matrix
output$matrix <- renderTable({
mat <- beta.matrix$mat
mat
})
}
# Run the application
shinyApp(ui = ui, server = server)