[R] Translating a basic Python script into R

[Ista Zahn]

Hi,

On Sat, Dec 28, 2013 at 10:46 AM, sun shine <phaedrusv at gmail.com> wrote:
> Hi
>
> I am attempting to translate some of the models that Donella Meadows wrote
> about in her book "Thinking in systems" into code. Originally, I had wanted
> to do this in Python, but thought that it would be fun to see if it is
> feasible to do so in R, especially given the plotting capacity of R.
>
> Meadows describes a very simple example of a stock and flow: 50 gallons of
> water in a bath tub - drain out at a rate of 5 gal/ minute and then turn on
> the faucet after five minutes which flows at 5 gal/ min. The outcome is
> obviously that after 5 minutes, the bath tub will maintain a steady stock of
> 25 gal thereafter.
>
> My basic code in Python looks like this:
>
> ====Python code=====
>
> stock = 50
> time = 1
> inflow_a = 0
> inflow_b = 5
> outflow = 5
>
> x = [stock]
> y = [time]
>
> print "Model of inflow and outflow rates of water"
> print "version 3"
> print
>
> print stock
> while time <= 5:
>     stock = (stock - outflow) + inflow_a
>     time += 1
>     y += [time]
>     x += [stock]
>     print stock
>     if stock == 30:
>         print "Faucet turned on"
>
> while time >= 6 and time <= 9:
>     stock = (stock - outflow) + inflow_b
>     time += 1
>     y += [time]
>     x += [stock]
>     print stock
>
> print "Volume in tub stabilises at %d gallons over %d minutes" % (stock,
> time)
> print x
> print y
> ==== end code====
>
> I want to translate this into an equivalent script in R.
>
> After some searching around, I found how to set up a while loop, and
> constructed the first section, like this:
>
> ======R code======
>
> while(time <= 10) {
>   if time <= 5
>   stock <
>   time <- time + 1
>   print(time)
> }
>
> ===== end code =====
>
> However, what I would like to learn how to do is to nest the if conditions
> in a way similar to that given in the Python code.

I don't see any nested conditions in the python code... A direct
translation in R looks almost the same, except that you need to group
using parentheses and brackets instead of whitespace, and there is no
+= in R (at least not that I'm aware of). Making those changes gives

  stock = 50
  time = 1
  inflow_a = 0
  inflow_b = 5
  outflow = 5
  x = stock
  y = time

  print ("Model of inflow and outflow rates of water")
  print ("version 3")

  print (stock)
  while (time <= 9) {

      stock = (stock - outflow) + inflow_a
      time = time + 1
      y = c(y, time)
      x = c(x, stock)
      print (stock)
      if (stock == 30) {
          print ("Faucet turned on")
      }
  }

  while (time >= 6 & time <= 9) {
      stock = (stock - outflow) + inflow_b
      time = time + 1
      y = c(y, time)
      x = c(x, stock)
      print (stock)
  }
      sprintf("Volume in tub stabilises at %d gallons over %d
minutes", stock, time)
      print (x)
      print (y)


>
> I'm sure that there must be some very elegant way to do this, but I cannot
> find out how to do so in any of the books I have, nor do my web searches
> throw back anything useful (I suspect that I'm not phrasing the question
> properly).

In both python and R you can of course use if/else instead of the two
separate while loops. An R version is

  stock = 50
  time = 1
  inflow_a = 0
  inflow_b = 5
  outflow = 5
  x = stock
  y = time

  print ("Model of inflow and outflow rates of water")
  print ("version 3\n")


  print (stock)
  while (time <= 9) {
      if(time <= 5) {
          stock = (stock - outflow) + inflow_a
      } else {
          stock = (stock - outflow) + inflow_b
      }
      time = time + 1
      y = c(y, time)
      x = c(x, stock)
      print (stock)
      if (stock == 30) {
          print ("Faucet turned on")
      }
  }

  sprintf("Volume in tub stabilises at %d gallons over %d minutes", stock, time)
  print (x)
  print (y)
  plot(y, x)


>
> Can someone please offer a few suggestions about ways that I could translate
> the Python script into R so that I can then run a plot as well?

You can plot in python, e.g.,

  from matplotlib.pyplot import *
  plot(y, x)
  show()

Best,
Ista
>
> Many thanks in anticipation.
>
> Sun
>
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