How to use loop to generate the data in a table in Shiny? - r

Question

I just started to learn shiny few days, and I have been troubled by this problem for a long time.
I need to generate a table(Two-column table), and the data in the table needs to be calculated based on the input (then I can use this table to generate a scatter plot in ggplot()).
I try to make the code more visible, so I want to use for loop to replace potentially hundreds of lines of highly repetitive code. Otherwise, it will look like (input$meansy1)-1)^2, (input$meansy1)-2)^2......(input$meansy1)-100)^2.
I don't know why it can't be used correctly in data.frame().
This is part of the code,
shinyUI(fluidPage(
numericInput("y1", "y1:", sample(1:100,1), min = 1, max = 100)),
tableOutput("tb")
))
shinyServer(function(input, output,session) {
list <-c()
for (i in 1:100) {
local({
list[[i]] <-reactive(((input$y1)-i)^2)}
)}
dt = data.frame(y_roof = 1:100, B=list)
output$tb <- renderTable({
dt
})
})

Answer 1

When developing a feature for a shiny app it makes sense to look at the underlying operation separately from the shiny context. That way you can figure out if you have a shiny specific issue or not.
Let's look at the operation you want to do first: Iteratively subtracting the values 1 to 100 from x and squaring the result.
You can do this in base R, like this:
x <- 1
dt1 <- data.frame(y_roof = 1:100)
(x - dt1$y_roof)^2
#> [1] 0 1 4 9 16 25 36 49 64 81 100 121 144 169 196
#> [16] 225 256 289 324 361 400 441 484 529 576 625 676 729 784 841
#> [31] 900 961 1024 1089 1156 1225 1296 1369 1444 1521 1600 1681 1764 1849 1936
#> [46] 2025 2116 2209 2304 2401 2500 2601 2704 2809 2916 3025 3136 3249 3364 3481
#> [61] 3600 3721 3844 3969 4096 4225 4356 4489 4624 4761 4900 5041 5184 5329 5476
#> [76] 5625 5776 5929 6084 6241 6400 6561 6724 6889 7056 7225 7396 7569 7744 7921
#> [91] 8100 8281 8464 8649 8836 9025 9216 9409 9604 9801
To store the results in a dataframe change the last line to:
dt1$col2 <- (x - dt1$y_roof)^2
head(dt1)
#> y_roof col2
#> 1 1 0
#> 2 2 1
#> 3 3 4
#> 4 4 9
#> 5 5 16
#> 6 6 25
Doing the same in the tidyverse would look like this:
library(dplyr)
dt2 <-
data.frame(y_roof = 1:100) %>%
mutate(col2 = (x - y_roof)^2)
head(dt2)
#> y_roof col2
#> 1 1 0
#> 2 2 1
#> 3 3 4
#> 4 4 9
#> 5 5 16
#> 6 6 25
Now we can work this into the shiny app:
library(shiny)
library(dplyr)
ui <-
shinyUI(fluidPage(
numericInput("y1", "y1:", sample(1:100, 1), min = 1, max = 100),
tableOutput("tb")
))
server <-
shinyServer(function(input, output, session) {
output$tb <- renderTable({
data.frame(y_roof = 1:100) %>%
mutate(col2 = (input$y1 - y_roof) ^ 2)
})
})
shinyApp(ui, server, options = list(launch.browser = TRUE))