R shiny error: Problems with a reactive function

Question

I am making an interactive graph in R studio with the new shiny markdown. What the idea is to create an active page where you have two input fields to determine the size of a, the size of b and what is left is the size of c. The input is in percentage. The output is a graph with a line and bars, which works. However, I have two problems.

1. The second input has to be dependent of the first input, because you can never have more than 100 percent.
2. The size of a and b is determined after the input is given. If the input changes, a, b and c has to be determined again.

I am quite new to R and searched to other questions but could not solve my problems. This is my code and how I tried to solve it:

inputPanel(
  numericInput("aa", label = "a:",
              min = 1 , max =100 , value = 80 , step = 1),

  numericInput("bb", label = "b:",
              min = 1 , max = 100-input$aa , value = 15 , step = 1)
)

This is my first problem. I want to be the second numeric input to be reactive on the first input. I tried the following options for this problem:

  numericInput("bb", label = "b:",
              min = 1 , max = observe(100-input$aa,suspended= TRUE) , value = 15 , step = 1)

This did not work. The next thing I tried is:

  numericInput("bb", label = "b:",
              min = 1 , reactive({max = 100-input$aa}) , value = 15 , step = 1)

This as well gave an error. After trying this options, I find it quite logic it does not work, however, still can not find it how to solve this.

The second problem is in the next code.

###change the column b
 for(i in nrow(df)){reactive({
  if(input$aa>df$a[i]){
  df$b[i] = "a"
} else {if(input$aa+input$bb>df$a[i]){
  df$b[i] = "b"
}else {df$b[i] = "c"}}})}

I want to change the value of the column b in df. This piece of code does not give any errors, however, it is not active, it does not change the column b. I tried the following:

###Change the column b
shinyServer(function(input, output) {
for(i in nrow(df)){reactive({
  if(input$large>df$a[i]){
  df$b[i] = "a"
} else {if(input$aa+input$bb>df$a[i]){
  df$b[i] = "b"
}else {df$b[i] = "c"}}})}})

This seems to ignore the code even more. This is what I tried and I am quite new to R. I hope someone can help me with this issue.

Thanks in advance, EDIT After comment

Shortest program (in my opinion)

---
title: "a"
author: "b"
date: "6/24/2014"
output: html_document
runtime: shiny
---

```{r, echo=FALSE}

library(ggplot2)
library(shiny)
df= data.frame(a=c(1:20),b=c(1:20))

df$c=1 #just to make sure column c exist.
inputPanel(
  numericInput("aa", label="a", min = 1, max = 100, value =50)
              ,

  numericInput("bb",label= "b", min = 1, max = 100-input$aa, value = 10)
)

  df$c <- reactive({
  df$c <- input$aa
})

renderPlot({
  ggplot(df) +
  geom_line(aes(a, b, colour= c ))
})
```

Eddy

Answer 1

Here is an example using renderUI. I havent looked at using shiny in markdown documents but im guessing it parses the document in a clever manner to allocate the appropriate quantities to a shinyUI and server function:

---
title: "a"
author: "b"
date: "6/24/2014"
output: html_document
runtime: shiny
---

```{r, echo=FALSE}
library(ggplot2)
library(shiny)
df= data.frame(a=c(1:20),b=c(1:20))

df$c=1 #just to make sure column c exist.
inputPanel(
  numericInput("aa", label="a", min = 1, max = 100, value = 50)
  , uiOutput('test')
)

output$test <- renderUI({
  numericInput("bb",label= "b", min = 1, max = 100-as.integer(input$aa), value = 10)
})
myReact <- reactive({
  out <- df
  out$b <- out$b + input$aa
  out
})
renderPlot({
  ggplot(myReact()) + geom_line(aes(a, b, colour= c ))
})

```