Error when generating pdf using script in R

I'm using R to loop through the columns of a data frame and make a graph of the resulting analysis. I don't get any errors when the script runs, but it generates a pdf that cannot be opened.

If I run the content of the script, it works fine. I wondered if there is a problem with how quickly it is looping through, so I tried to force it to pause. This did not seem to make a difference. I'm interested in any suggestions that people have, and I'm also quite new to R so suggestions as to how I can improve the approach are welcome too. Thanks.

for (i in 2:22) {

  # Organise data
  pop_den_z = subset(pop_den, pop_den[i] != "0")  # Remove zeros
  y = pop_den_z[,i]        # Get y col
  x = pop_den_z[,1]        # get x col
  y = log(y)               # Log transform

  # Regression
  lm.0 = lm(formula = y ~ x)                # make linear model
  inter = summary(lm.0)$coefficients[1,1]   # Get intercept
  slop = summary(lm.0)$coefficients[2,1]    # Get slope

  # Write to File
  a = c(i, inter, slop)
  write(a, 开发者_C百科file = "C:/pop_den_coef.txt", ncolumns = 3, append = TRUE, sep = ",")

  ## Setup pdf
  string = paste("C:/LEED/results/Images/R_graphs/Pop_den", paste(i-2), "City.pdf")
  pdf(string, height = 6, width = 9)

  p <- qplot(
    x, y,
    xlab = "Radius [km]",
    ylab = "Population Density [log(people/km)]",
    xlim = x_range,
    main = "Analysis of Cities"
  )

  # geom_abline(intercept,slope)
  p + geom_abline(intercept = inter, slope = slop, colour = "red", size = 1)

  Sys.sleep(5)

  ### close the PDF file
  dev.off()
}

The line should be

print(p + geom_abline(intercept = inter, slope = slop, colour = "red", size = 1))

In pdf devices, ggplot (and lattice) only writes to file when explicitly printed.