Adding layers to ggplots works but adding in a loop does not

Question

I want to add to an existing ggplot in a loop. It works fine as shown in a minimal example below when I add points to a list not using a loop (left plot). When I do the same in a loop, the final result only contains the last point that I added (right plot).

library(ggplot2) 

p <- list(); pl <- list()
x0 <- c(-1,1); y0 <- c(-3,3); q <- c(-5,5)

#no loop
p[[1]] <- ggplot() + coord_fixed(xlim = q, ylim = q)
p[[2]] <- p[[1]] +geom_point(aes(x=x0[1], y=y0[1])) 
p[[3]] <- p[[2]] + geom_point(aes(x=x0[2], y=y0[2])) 

#loop
pl[[1]] <- ggplot() + coord_fixed(xlim = q, ylim = q)
for (i in 1:2)
{
  pl[[i+1]] <- pl[[i]] + geom_point(aes(x=x0[i], y=y0[i]))
}

p[[3]]
pl[[3]]

Answer 1

This is due to what is called "lazy evaluation", explained in several posts (like this). You don't need to add the plots into lists you just overwrite and get the same result. As for the loop you need to put your data into a data.frame and feed it to the geom_point() function:

p <- list(); pl <- list()
x0 <- c(-1,1); y0 <- c(-3,3); q <- c(-5,5)

#no loop
p <- ggplot() + coord_fixed(xlim = q, ylim = q)
p <- p +geom_point(aes(x=x0[1], y=y0[1])) 
p <- p + geom_point(aes(x=x0[2], y=y0[2])) 

#loop
pl <- ggplot() + coord_fixed(xlim = q, ylim = q)
for (i in 1:2){
  data<-cbind.data.frame(x=x0[i], y=y0[i])
  pl <- pl + geom_point(data=data,aes(x=x, y=y))
}

p
pl

Answer 2

You're a victim of lazy evaluation. [See, for example, here.] A for loop uses lazy evaluation. Fortunately, lapply does not. So,

p <- ggplot() + coord_fixed(xlim = q, ylim = q)
lapply(
  1:2,
  function(i) p <<- p + geom_point(aes(x=x0[i], y=y0[i]))
)

gives you what you want.

Note the use of <<- as a quick and dirty fix.