[R] sparseMatrix image not working in for loop

[Shunran Zhang]

Hi all,

I have just encountered a weird behavior of image(sparseMatrix) when 
trying to output plots to files in a for loop.

A sample code is provided below at the end of this mail as well as in 
the attachment. In the code, calling image(sparseMatrix) works outside 
of a for loop - the image file is written. Calling 
image(as.matrix(sparseMatrix)) also works inside a for loop thus 
eliminating file access problem. However when using image(sparseMatrix) 
inside for loop, no image file is produced, nor is there any error message.

The execution environment is on 2 different distributions of Linux 
running R 3.6.3 and 4.2.1, Matrix library 1.2-18 and 1.5-3 respectively, 
and all showing the same behavior.

Can anyone point me a direction on where to look for?

Sincerely,

Shunran Zhang

--- Sample code ---

# Initialize dataset library(Matrix) set.seed(0) filler <- 
list(sample(1:1000,1000), sample(1:1000,1000), runif(1000,0,100)) 
sampledata <- sparseMatrix(i = filler[[1]], j = filler[[2]], x = 
filler[[3]], dims = c(1000,1000)) # Works: image on a sparse matrix 
outside for loop item <- "a" png(paste("img/out_of_for_",item,".png", 
sep = "", collapse = ""), width = 1000, height = 1000) image(sampledata) 
dev.off() remove(item) for(item in c("a")){  # Does not work: image on a 
sparse matrix inside for loop  png(paste("img/in_for_img_",item,".png", 
sep = "", collapse = ""), width = 1000, height = 1000) 
  image(sampledata)  dev.off()  # Works: image on a dense matrix inside 
for loop  png(paste("img/in_for_hist_",item,".png", sep = "", collapse = 
""), width = 1000, height = 1000)  image(as.matrix(sampledata)) 
  dev.off() }

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