12

I successfully create a plot using the following:

# suppose I have a p <- ggplot(data=df, ...) then the following works 
# I get those two segments plotted correctly
p <- p + geom_segment(aes(x=1,y=103,xend=1,yend=107))
p <- p + geom_segment(aes(x=5,y=103,xend=5,yend=107))

However if I do:

values <- c(1, 5)
for (i in values) {
   p <- p + geom_segment(aes(x=i,y=103,xend=i,yend=107))
}

It doesn't work, only the last segment is created. Can anyone advice what's wrong here?

SkyWalker
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2 Answers2

20

It has to do with the lazy evaluation of the aes() values. You are binding to the variable i but not actually doing anything with it in the loop. The mappings aren't resolved till you actually print(p). Essentially this means they are all being bound to i and after the loop exits, i will have the value it had during the final loop.

So the problem really is you shounld't be using aes() here as you don't really want active binding. Just set the x and xend values outside the aes(). (And since the y's are constant they should be outside the aes() as well).

values <- c(1, 5)
for (i in values) {
   p <- p + geom_segment(x=i, y=103, xend=i, yend=107)
}
MrFlick
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  • What i posted will fix it. Don't use `aes()` in the `geom_segment()` in the loop. – MrFlick Jul 07 '14 at 19:16
  • That's what I thought. Do you know why adding a `print` statement or using `force` in the for loop doesn't *fix* the problem? – csgillespie Jul 07 '14 at 19:53
  • @csgillespie Because in this case you're still binding to the *variable* `i`, not the value. You can't force `aes` to resolve the name till it actually draws the plot. `force` is sometimes useful in situations like this, but with the non-standard evaluation of `aes` it won't help in this case. – MrFlick Jul 07 '14 at 19:56
18

An alternative approach would be to avoid using a loop at all. You can pack your segment data up in a separate data.frame from your main data and use aes() to plot everything at once like so: 

segment_data = data.frame(
    x = c(1, 5),
    xend = c(1, 5), 
    y = c(103, 103),
    yend = c(107, 107)
)

p = ggplot(df, ...) +
geom_segment(data = segment_data, aes(x = x, y = y, xend = xend, yend = yend))
Taylor White
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