Using geom_segment() within a for loop?

Question

My question is an exact duplicate of this one [1], but the provided answer isn't working on my system.

Question: How can I add geom_segment() layers within a for loop? If I use aes, as the OP of the referenced question does, I also get only the last layer. If, as the answerer suggests, I don't use aes, I don't get any segments at all.

Here is a minimal example:

  ga <- 2.39996322972865332
  p <- ggplot()
  p <- p + scale_y_continuous(limits = c(-1, 1))
  p <- p + scale_x_continuous(limits = c(-1, 1))
  for (i in 0:2) {
    #     p <- p + geom_segment(x = -cos(i*ga), y = -sin(i*ga), xend = cos(i*ga), yend = sin(i*ga)) # No segments
    p <- p + geom_segment(aes(x = -cos(i*ga), y = -sin(i*ga), xend = cos(i*ga), yend = sin(i*ga))) # Only last segment
  }
  p

I'm using R version 3.1.2 and (apparently) ggplot2 version 1.0.0 on Ubuntu.

(Note: I get the same result if I use repeat or while.)

[1] Enriching a ggplot2 plot with multiple geom_segment in a loop?

IRTFM · Accepted Answer · 2015-03-13 17:17:24Z

4

Skip the loop. Pass a vector to the i variable:

 p <- p + geom_segment(
                aes(x = -cos((0:2)*ga), y = -sin((0:2)*ga),
                    xend = cos((0:2)*ga), yend = sin((0:2)*ga))
                       )

Just a check to see that you can use a variable as well as a numeric constant:

 i <- 0:2
 p <- p + geom_segment(aes(x = -cos(i*ga), y = -sin(i*ga), 
                           xend = cos(i*ga), yend = sin(i*ga)) )

enter image description here

answered Mar 13, 2015 at 17:17

IRTFM

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2 Comments

sudo make install Over a year ago

Ahhh brilliant! I need to start vectorizing things in my head by default. Thanks so much!

Garini Over a year ago

This the cleanest and useful solution. I found so many melting and data.frame "casting" and they were always suboptimal and often broken (e.g. if you use a different data for the main plotting).

Roman Marusyk · Accepted Answer · 2015-12-03 18:27:36Z

1

I had a very similar problem, but could not avoid using a for loop. I solved it by

adding inherit.aes = TRUE

after the aes() statement within the geom_segment() statement.

edited Dec 3, 2015 at 18:27

Roman Marusyk

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answered Dec 3, 2015 at 18:06

Hanne

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Comments

sudo make install · Accepted Answer · 2015-03-13 19:02:57Z

-1

I just wanted to add one point to @BondedDust's answer.

If you try to put this into a function, something like this...

  draw.golden.angle <- function(n) {

  ga <- 2.39996322972865332
  p <- ggplot()
  p <- p + scale_y_continuous(limits = c(-1, 1))
  p <- p + scale_x_continuous(limits = c(-1, 1))
  i <- 0:n
  p <- p + geom_segment(aes(x = -cos(i*ga), y = -sin(i*ga), xend = cos(i*ga), yend = sin(i*ga))) # Only last segment
  p

  }

  draw.golden.angle(7)

...you will get an error saying that the variable i isn't recognized. You can fix this by assigning i to a global variable as follows:

i <<- 0:n

answered Mar 13, 2015 at 19:02

sudo make install

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6 Comments

Gregor Thomas Over a year ago

Assigning to a global variable is a very dangerous way to go about this.

sudo make install Over a year ago

Thanks for the warning, @Gregor--I will read up on global variables. Is there another way you can think to do this? If you'd like I can make it a separate question.

Gregor Thomas Over a year ago

I can't think of a really easy way. The robust solution would be to add new data columns for each value of i, melt, and use geom_segment with a grouping variable, aes(group = i). It's quite a bit more work, but that's the price we pay to have stable and reliable functions.

Gregor Thomas Over a year ago

Also, fortunes::fortune(236)

Gregor Thomas Over a year ago

And Circle 6 of The R Inferno is about global assignment (and why you should never do it).

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