How can I use a for loop in an R shiny app

Question

Aim

I want to use a for loop in a shiny app. The code below will work without the for loop, but then fails to work with it.

Attempt

library(shiny) library(dplyr) library(lubridate) #create table dates <- seq(as.Date("2020-01-01"), as.Date("2022-04-01"), by="months") supply <- floor(runif(28, min = 80000, max = 100000)) demand <- rep(0, length(dates)) shortfall <- ifelse(demand - supply > 0, demand -supply, 0) df <- bind_cols(dates, supply, demand, shortfall) names(df) <- c("dates", "supply", "demand", "shortfall") # Define UI ---- ui <- navbarPage( titlePanel("Supply & Demand"), tabPanel(title = "Demand", mainPanel(h3("Demand", align = "center"), plotOutput(outputId = "demand")), sidebarPanel(sliderInput(inputId = "A", label = "A", min = 0, max = 50000, value = 5000), sliderInput(inputId = "B", label = "B", min = 0, max = 50000, value = 5000), sliderInput(inputId = "C", label = "C", min = 0, max = 50000, value = 5000))), tabPanel(title = "Table", mainPanel(h3("Table", align = "center"), tableOutput(outputId = "table"))) ) # Define server logic ---- server <- function(input, output) { df_ <- reactive({ df %>% mutate(demand = input$A + input$B + input$C, shortfall = rep(0, 28), returning_shortfall = c(10000, rep(0, 27)), stored_shortfall = rep(0, 28), total_demand = rep(0, 28)) }) df2_ <- reactive({ for (i in 2:length(df_()$dates)) { df_()$total_demand[i] = df_()$demand[i] + df_()$returning_shortfall[i-1] df_()$shortfall[i] = ifelse(df_()$total_demand[i] - df_()$supply[i] > 0, df_()$total_demand[i] - df_()$supply[i], 0) df_()$returning_shortfall[i] = df_()$shortfall[i-1] * 0.15 df_()$stored_shortfall[i] = df_()$stored_shortfall[i-1] + df_()$shortfall[i] * 0.15 } }) #Table output$table <- renderTable(df2_()) } # Run the app ---- shinyApp(ui = ui, server = server)

Error message

Warning: Error in =: invalid (NULL) left side of assignment 113: <reactive:df2_> [C:/Users/user/filename] 97: df2_ 96: renderTable 95: func 82: renderFunc 81: output$table 1: runApp

Outcome Sought

Tha main challenge is creating a column that where each value relies on a previous row's value. "For loops" seems to solve that problem, but it does not work in shiny.

You cannot access reactive components (e.g., df_) outside of a reactive block (such as *reactive, observe*, render*, etc). Add your logic either within the df_ initial-definition reactive, or create a second reactive block that uses df_() and creates a new frame (e.g., df2_ <- reactive(...)). — r2evans
– r2evans, Commented Apr 1, 2021 at 12:30
Thanks r2evans, I've put the for loop in a second reactive and now the app loads. However, I'm now getting the error message: "invalid (NULL) left side of assignment" instead of the table showing up. Any ideas? — Namra
– Namra, Commented Apr 1, 2021 at 12:48
You shiny app here isn't updated, so it could be a number of things. Please update your question with current code. Thanks! — r2evans
– r2evans, Commented Apr 1, 2021 at 12:49

YBS · Accepted Answer · 2021-04-01 13:32:14Z

You just need one reactive object. Try this

library(shiny) library(dplyr) library(lubridate) #create table dates <- seq(as.Date("2020-01-01"), as.Date("2022-04-01"), by="months") supply <- floor(runif(28, min = 8000, max = 20000)) demand <- rep(0, length(dates)) shortfall <- ifelse(demand - supply > 0, demand -supply, 0) df1 <- bind_cols(dates, supply, demand, shortfall) names(df1) <- c("dates", "supply", "demand", "shortfall") # Define UI ---- ui <- navbarPage( titlePanel("Supply & Demand"), tabPanel(title = "Demand", mainPanel(h3("Demand", align = "center"), plotOutput(outputId = "demand")), sidebarPanel(sliderInput(inputId = "A", label = "A", min = 0, max = 50000, value = 5000), sliderInput(inputId = "B", label = "B", min = 0, max = 50000, value = 5000), sliderInput(inputId = "C", label = "C", min = 0, max = 50000, value = 5000))), tabPanel(title = "Table", mainPanel(h3("Table", align = "center"), tableOutput(outputId = "table"))) ) # Define server logic ---- server <- function(input, output) { df2 <- reactive({ df <- df1 %>% mutate(demand = input$A + input$B + input$C, shortfall = rep(0, 28), returning_shortfall = c(10000, rep(0, 27)), stored_shortfall = rep(0, 28), total_demand = rep(0, 28)) for (i in 2:length(df$dates)) { df$total_demand[i] <- df$demand[i] + df$returning_shortfall[i-1] df$shortfall[i] <- ifelse(df$total_demand[i] - df$supply[i] > 0, df$total_demand[i] - df$supply[i], 0) df$returning_shortfall[i] <- df$shortfall[i-1] * 0.15 df$stored_shortfall[i] <- df$stored_shortfall[i-1] + df$shortfall[i] * 0.15 } df }) #Table output$table <- renderTable(df2()) } # Run the app ---- shinyApp(ui = ui, server = server)

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