How to save previous interaction using reactiveValues - shiny

Question

I am developing a table which does some calculations each time that an action button is used. One column of my table depends on its previous value,
C_new <- C_old + B_new - A_new
For instance, if A=4, B=A+2 and C= C(-1) + B - A my expected results are
A B C
1 2 3
4 6 5
I have tried to save the previous value of column C using reactiveValue, as mentioned in How to “remember” information from the previous iteration when using reactiveTimer in Shiny?, but it doesn't work. I don't know where I am getting wrong.
Here is my code
library(shiny)
ui <- fluidPage(
sidebarPanel(textInput("c1","Example"),
actionButton("update", "Update Table")),
mainPanel(tableOutput("example"))
)
server <- function(input, output) {
C_old <- reactive(x=3)
values <- reactiveValues(df = data.frame(A=1, B=2, C=3))
newEntry <- observeEvent(input$update,{
A_new <- as.numeric(input$c1)
B_new <- A_new + 2
C_new <- isolate (C_old$x + B_new - A_new)
C_old$x <<- C_new
new <- data.frame(A=A_new,B=B_new, C=C_new)
# attach the new line to the old data frame here:
values$df <- rbind(values$df, new)
})
# Print the content of values$df
output$example <- renderTable({
return(values$df)
})
}
shinyApp(ui = ui, server = server)

Answer 1

Important to know, observeEvents (similar to observes) don't have outputs. You just observe a change and do something in their body, but nothing is supposed to be returned (this is different to reactive({ }), which also observes changes but has return values), useful link.
Isolate is not needed in observeEvent , because nothing triggers an update except for input$update (this is different to observe and reactive, where all changeable items in the body trigger an update).
Below is the solution to your problem. I used reactiveVal which stores one single updateable value (see ?reactiveVal for help). After clicking the action button, I retrieve the old table by calling values(), calculate all new values (beware, I need to use tail to get only the last C value) and attach the result to the old value before storing the extended table into values by calling values(new_df):
library(shiny)
ui <- fluidPage(
sidebarPanel(numericInput("c1","Example", 0),
actionButton("update", "Update Table")),
mainPanel(tableOutput("example"))
)
server <- function(input, output) {
# stores the current data frame, called by values() and set by values(new_data_table)
values <- reactiveVal(data.frame(A=1, B=2, C=3))
# update values table on button click
observeEvent(input$update,{
old_values <- values()
A_new <- input$c1
B_new <- A_new + 2
C_new <- tail(old_values$C, 1) + B_new - A_new # tail to get the last C value
new_values <- data.frame(A=A_new, B=B_new, C=C_new)
# attach the new line to the old data frame here:
new_df <- rbind(old_values, new_values)
#store the result in values variable
values(new_df)
})
# Print the content of values$df
output$example <- renderTable({
return(values())
})
}
shinyApp(ui = ui, server = server)