Question

In R, I have a loop. I would like to subset a dataframe by the variable in the loop. I would like my code to look like this:

library(maptools)
for(theMonth in 600: 0)
{
  Inspections.mp <- readShapeLines("InspDates2")
  counties.mp <- readShapePoly("Boundary")
  plot(counties.mp, axes=FALSE, border="gray")
  data1 <-Inspections.mp[Inspections.mp$MonthInt == theMonth]
  lines(data1, col="blue", lwd=1)
}

Unfortunately, this returns all of the records in data1, and using the line

data1 <-Inspections.mp[Inspections.mp$MonthInt == theMonth,]

causes the following error: Error in bb[1, ] : incorrect number of dimensions

I can, however, get the records I want when just using a constant integer, but I need a variable

library(maptools)
for(theMonth in 600: 0)
{
  Inspections.mp <- readShapeLines("InspDates2")
  counties.mp <- readShapePoly("Boundary")
  plot(counties.mp, axes=FALSE, border="gray")
  data1 <-Inspections.mp[Inspections.mp$MonthInt == 60,]
  lines(data1, col="blue", lwd=1)
}

Answer 1

The problem here seems to be a null subset. This is avoided by using a try/catch statement.

library(maptools)

Inspections.mp <- readShapeLines("InspDates2")
counties.mp <- readShapePoly("Boundary")
for(theMonth in 600: 0)
{
  plot(counties.mp, axes=FALSE, border="gray")

  result <- tryCatch({
    data1 <-Inspections.mp[Inspections.mp$MonthInt == theMonth,]
    lines(data1, col="blue", lwd=1)
    },warning = function(war){
      print("WARNING")
    },error = function (err)
    {
      print("Error")
    }, finally = {
      print("Finally")
    })
}