How to check if dev is not null?

Question

I have a function for ploting like this:

f <- function(x,y){  

  for (i in 1:10){

    png(getwd(), height=1500, width=500)
    plot(x,y)
    dev.off()
  }
}

everything is all right when I use: f(2,3) but if I use f(2,"a") then the function is breaking before dev.off()

I can place on.exit(dev.off(), add=T) on the begening of the function but then in normal execution I will dev.off() too much 1 time.

I think the solution is to put sth like this to the function:

 on.exit(if(dev != NULL) dev.off(), add=T)

but how to check that dev != NULL ?

how to use that dev.list() This doesnt work: on.exit(if(exists(dev.list()) == T){ dev.off()}, add=T) — wBacz
– wBacz, Commented Mar 31, 2017 at 15:23
OK, on.exit(if(is.null(dev.list()) == F){ dev.off()}, add=T) this is OK ? — wBacz
– wBacz, Commented Mar 31, 2017 at 15:25

β.εηοιτ.βε · Accepted Answer · 2020-06-11 20:42:35Z

1

Simply use the following technique:

if (length(dev.list()!=0) {dev.off()}

this is actually an efficient technique to check the returning values of many other functions too, for instance, to check if grep finds something:

if(length(grep("me","I am not here"))==0) {print("not found")}

hope it helps.

edited Jun 11, 2020 at 20:42

β.εηοιτ.βε

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answered Jun 11, 2020 at 20:22

ashkan

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4 Comments

Therkel Over a year ago

Where would you put the missing ) in your first line? It seems to me that if (length(dev.list()!=0)) and if (length(dev.list())!=0) (before and after !=0) gives the same result.

Therkel Over a year ago

In fact, do we need to do the !=0? The if function interprets the output of length as a boolean. E.g. length(1:3) will return TRUE.

ashkan Over a year ago

I missed putting the second close parenthesis in the If section after !=0 which should be exactly!=0))

ashkan Over a year ago

This was a trick litterally independent of the output type of function as it works out by checking the length of output. So, if you need to check an actual value then it won’t work for you.

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