How to store plots in a list when inside a loop?

Question

I'm working inside a loop and I'd like to save plots in a list so I can plot them together in a .pdf. The problem is that the list is not filled up properly and re-updates with the results of the last run. So, in the end what I get is a list with five elements that are exactly the same.

I'm aware the loops may seem useless, but I just have them to create a test code (with a reproducible error) as close to the real one as possible. So, I need to leave the loops as they are.

library (ggplot)
library (gridExtra)

plist <- list()

for (z in 1:5){
  n <- 100
  k <- seq(0, to=4500+z*2000, length=n)
  tmp <- numeric(n)
  for (i in 1:n){
    tmp[i] <- (5*(i*3)^2)}

  plist[[z]] <- ggplot() +
    geom_line(aes(x = k, y = tmp)) +
    theme_bw()

  pdf(sprintf("p%s.pdf", z),
      width = 6, height = 4, onefile = T)
  plot(plist[[z]])
  dev.off()
}

do.call(grid.arrange, c(plist, ncol = 5))

Yes, why not save the plots into a list? Then you don't need to get them into a list later for plotting together. — aosmith, Jan 26 '17 at 21:40
In one place you have assign to sprintf("p%s", z) but then you use "p_%s" when you try to `get`. — IRTFM, Jan 26 '17 at 21:44
Thant's what I'm try to do, when I create listp... Is it there a better way? — user3262756, Jan 26 '17 at 21:44
The first time through the loop you try to get a non-existent object, namely `p2` as the error message states. Do you really want two loops: one for creation and the other for accessing the plots? Also: `grid.arrange` is not in pkg-ggplot2. — IRTFM, Jan 26 '17 at 21:50
I know the loops seem useless here but they are needed in my real code... — user3262756, Jan 26 '17 at 21:51
Now you are being obtuse (failing to read the error message or my comment pointing you to it.) When z=1 you try to use `get` with an argument of "p2" which throws an error. You try to access p2 before it is created because there an implicit inner loop with the `lapply` call. — IRTFM, Jan 26 '17 at 21:53
Sorry, yes, just edited the code, I didn't see that error message before. grid.arrage (package gridExtra). Thanks! — user3262756, Jan 26 '17 at 22:19
@aosmith - thanks for your comment, you're right it's easier to create the list from the beginning (code updated). However, I keep having the same problem, all plots are the same one. Is it there anything very obvious I'm missing? — user3262756, Jan 26 '17 at 23:02
One minor note: your inner loop is unnecessary. You can just do `tmp <- (5*((1:n)*3)^2)`. All the arithmetic operations are vectorized. — Gregor Thomas, Jan 26 '17 at 23:05

score 4 · Accepted Answer · edited May 23 '17 at 11:53

This answer is based on: Storing plot objects in a list

library(ggplot2)
library(gridExtra)

plist <- list()

for (z in 1:5){
  n <- 100
  k <- seq(0, to=4500+z*2000, length=n)
  tmp <- numeric(n)
  for (i in 1:n){
    tmp[i] <- (5*(i*3)^2)}

  data <- data.frame(n, k, tmp)

  plist[[z]] <- ggplot(data = data) + #data needs to be given!!
    geom_line(aes(x = k, y = tmp)) +
    theme_bw()

  pdf(sprintf("p%s.pdf", z),
      width = 6, height = 4, onefile = T)
  plot(plist[[z]])
  dev.off()
}

do.call(grid.arrange, c(plist, ncol = 5))

How to store plots in a list when inside a loop?

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