In this post, I make an Shiny app that allows the user to choose the quantity of the terms that will be made part of some exemplar operation with data as follows:

The challenge is to cover in some operation the new variables made by the user.
Firstly, I built a basic structure of the app in the User Interface (UI) part, following the Sidebar Layout with two columns. Just for perfumery, I put a theme provided by bslib package:

ui <- fluidPage( theme =  bslib::bs_theme(bootswatch = "litera"),
    sidebarLayout(
    sidebarPanel(
        "sidebar content..."
    ),
    mainPanel(
        fluidRow(
            column(width = 4, "Column content..."),
            column(width = 6, "Column content..."))
    )
))  # end of the sidebarLayout and fluidPage

Now, create the quantities’ selector of terms by the user with selectInput( inputId="qntt_terms", .... It needs to be chosen before the terms’ value because it restarts the field without saving the value if it is filled. So, in this example, the user can’t write the value and click to add one more field. In my example, I will make the first term independent of the selector, understanding the idea, you can put it in the selector’s function of creation of terms. So, the first input is numericInput( inputId="term_1", ... and the space to the new terms is the dynamic UI uiOutput("new_terms").

ui <- fluidPage( theme =  bslib::bs_theme(bootswatch = "litera"),
    sidebarLayout(
    sidebarPanel(
        selectInput("qntt_terms",
                        label = "Number of Terms",
                        choices = 1:7,
                        selected = 1),
        numericInput("term_1", "Put a Number (1):", 0),
        uiOutput("new_terms")
    ),
    mainPanel(
        fluidRow(
            column(width = 4, "Column content..."),
            column(width = 6, "Column content..."))
    )
))  # end of the sidebarLayout and fluidPage

Now, make the server part. The named earlier dynamic UI new_terms will be made by new_t() reactive operation:

server <- function(input, output, session) {

    new_t <- reactive({
        "counter and terms' creator"
    })       # end of the reactive

    output$new_terms <- renderUI({ new_t() })
}

The new_t() takes qntt_terms as the number of times that will be printed the same numeric input that the term_1 but making new names to this new fields with the lapply(2:n, function(i){ ...inputId=paste0("term_",i)... } structure. The if(n>1){...} is because I let the first term independent and it will run only since two:

server <- function(input, output, session) {

new_t <- reactive({
        n <- input$qntt_terms |> as.numeric()
        if(n>1){
            lapply(2:n, function(i) {
                br()
                numericInput(inputId = paste0("term_",i),
                                 label = paste0("Put a Number (", i,"):"), 0)
            }) # end of the function and lapply
        }     # end of the if(n>1)
    }) 

    output$new_terms <- renderUI({ new_t() })
}

I create this new example data to make part of the operations with the new terms:

data_ex <- tibble("col_a" = rnorm(7),
                      "col_b" = rnorm(7),
                      "col_c" = rnorm(7))

Now, still in server part, make the function operation(wich_col) that handle all the i= 1,…, n terms by input[[paste0("term_",i)]]. This is the core of the idea of operations with dynamically created variables.

for (i in 1:req(input$qntt_terms)) {... input[[paste0("term_",i)]] ...}

In this example, I make a multiplication with the correspondent i-th row of the wich_col data column. It returns a vector of results.

operation <- function(which_col){
        temp_vect <- vector()
        for (i in 1:req(input$qntt_terms)) {
            temp_vect[i] <-
                req(input[[paste0("term_",i)]]) *
                data_ex |> pull(which_col) |> nth(i)
        }
        return(temp_vect)
    }

In first column of main panel, I show the data with tableOutput("data_ex_call") in UI part that requires an output$data_ex_call <- renderTable() in server part. In the second column, I print some uses of the created function. Join server, ui, and the data, we have:

library(shiny)
library(bslib)
library(tidyverse)

data_ex <- tibble("col_a" = rnorm(7),
                      "col_b" = rnorm(7),
                      "col_c" = rnorm(7))

ui <- fluidPage( theme =  bslib::bs_theme(bootswatch = "litera"),
    sidebarLayout(
    sidebarPanel(
        selectInput("qntt_terms",
                        label = "Number of Terms",
                        choices = 1:7,
                        selected = 1),
        numericInput("term_1", "Put a Number (1):", 0),
        uiOutput("new_terms")
    ),
    mainPanel(
        fluidRow(
            column(width = 4, "The data to be multiplied row by term:" |> h3(),
                     tableOutput("data_ex_call")),
            column(width = 6,
                     "The Sum with A col results:" |> h3(),
                     textOutput("sum_a_terms"),
                     br(),
                     "The Prod with B col results:" |> h3(),
                     textOutput("prod_b_terms"),
                     br(),
                     markdown("Vector of Output of the `operation(which_col = 'col_a')`:") |> h3(),
                     textOutput("test_of_func")
    ))
    )
))  # end of the sidebarLayout and fluidPage

server <- function(input, output, session) {

    new_t <- reactive({
        n <- input$qntt_terms |> as.numeric()
        if(n>1){
            lapply(2:n, function(i) {
                br()
                numericInput(inputId = paste0("term_",i),
                                 label = paste0("Put a Number (", i,"):"), 0)
            }) # end of the function and lapply
        }     # end of the if(n>1)
    })       # end of the reactive

    output$new_terms <- renderUI({ new_t() })
    
    operation <- function(which_col){
        temp_vect <- vector()
        for (i in 1:req(input$qntt_terms)) {
            temp_vect[i] <-
                req(input[[paste0("term_",i)]]) *
                data_ex |> pull(which_col) |> nth(i)
        }
        return(temp_vect)
    }

    output$sum_a_terms <- renderText({
        operation(which_col = "col_a") |> sum()
    })
    output$prod_b_terms <- renderText({
        operation(which_col = "col_b") |> prod()
    })
    output$test_of_func <- renderText({
        operation(which_col = "col_a")
    })
    
    output$data_ex_call <- renderTable({data_ex},
                                                  striped = TRUE,
                                                  digits = 2)  
    
}

shinyApp(ui, server)

This is the app:

I thanks any commentary and helps. Finally, I thanks to the socialscientist to help me with this issue.