R: round() can find object, sprintf() cannot, why?

Question

I have a function that takes a dataframe and plots a number of columns from that data frame using ggplot2. The aes() function in ggplot2 takes a label argument and I want to use sprintf to format that argument - and this is something I have done many times before in other code. When I pass the format string to sprintf (in this case "%1.1f") it says "object not found". If I use the round() function and pass an argument to that function it can find it without problems. Same goes for format(). Apparently only sprintf() is unable to see the object.

At first I thought this was a lazy evaluation issue caused by calling the function rather than using the code inline, but using force() on the format string I pass to sprintf does not resolve the issue. I can work around this, but I would like to know why it happens. Of course, it may be something trivial that I have overlooked.

Q. Why does sprintf() not find the string object?

Code follows (edited and pruned for more minimal example)

require(gdata)
require(ggplot2)
require(scales)
require(gridExtra)
require(lubridate)
require(plyr)
require(reshape)

set.seed(12345)
# Create dummy time series data with year and month
monthsback <- 64
startdate <- as.Date(paste(year(now()),month(now()),"1",sep = "-")) - months(monthsback)
mydf <- data.frame(mydate = seq(as.Date(startdate), by = "month", length.out = monthsback), myvalue5 = runif(monthsback, min = 200, max = 300))
mydf$year <- as.numeric(format(as.Date(mydf$mydate), format="%Y"))
mydf$month <- as.numeric(format(as.Date(mydf$mydate), format="%m"))

getchart_highlight_value <- function(
                          plotdf,
                          digits_used = 1
                          )
{
    force(digits_used)
    #p <- ggplot(data = plotdf, aes(x = month(mydate, label = TRUE), y = year(mydate), fill = myvalue5, label = round(myvalue5, digits_used))) +
    # note that the line below using sprintf() does not work, whereas the line above using round() is fine
    p <- ggplot(data = plotdf, aes(x = month(mydate, label = TRUE), y = year(mydate), fill = myvalue5, label = sprintf(paste("%1.",digits_used,"f", sep = ""), myvalue5))) +
      scale_x_date(labels = date_format("%Y"), breaks = date_breaks("years")) +
      scale_y_reverse(breaks = 2007:2012, labels = 2007:2012, expand = c(0,0)) +
      geom_tile() + geom_text(size = 4, colour = "black") +
      scale_fill_gradient2(low = "blue", high = "red", limits = c(min(plotdf$myvalue5), max(plotdf$myvalue5)), midpoint = median(plotdf$myvalue5)) +
      scale_x_discrete(expand = c(0,0)) +
      opts(panel.grid.major = theme_blank()) +
      opts(panel.background = theme_rect(fill = "transparent", colour = NA)) +
      png(filename = "c:/sprintf_test.png", width = 700, height = 300, units = "px", res = NA)
      print(p)
      dev.off()
}

getchart_highlight_value (plotdf <- mydf,
                          digits_used <- 1)

Answer 1

Using the minimal example of Martin (that is a minimal example, see also this question), you can make the code work by specifying the environment ggplot() should use. For that, specify the argument environment in the ggplot() function, eg like this:

require(ggplot2)

getchart_highlight_value <- function(df)
{
  fmt <- "%1.1f"
  ggplot(df, aes(x, x, label=sprintf(fmt, lbl)),
         environment = environment()) + 

  geom_tile(bg="white") + 
  geom_text(size = 4, colour = "black")
}

df <- data.frame(x = 1:5, lbl = runif(5))
getchart_highlight_value (df)

The function environment() returns the current (local) environment, which is the environment created by the function getchart_highlight_value(). If you don't specify this, ggplot() will look in the global environment, and there the variable fmt is not defined.

Nothing to do with lazy evaluation, everything to do with selecting the right environment.

The code above produces following plot: