How can I populate a vector efficiently if each time I modify one element the whole vector gets copied and moved

Asked Dec 05 '18 at 21:39

Active Dec 05 '18 at 21:48

Viewed 33 times

y <- rep(0, 10)
pryr::address(y)
y[3] <- 4
pryr::address(y)

output:

[1] "0x1fa4d8e0"
...
[1] "0x20cc70c0"

The address of y, the entire vector, is different after modifying just one element. This means that the whole vector was copied and moved. I was told that pre-allocating was the efficient way to fill a vector of unknown size in R. Actually, this is worst than growing a vector.

Am I missing something

EDITS:

I am using RStudio

edited Dec 05 '18 at 21:48

asked Dec 05 '18 at 21:39

Big Cat Public Safety Act

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Maybe I'm misunderstanding you, but aren't those two addresses the same? – joran Dec 05 '18 at 21:40
I fixed it. I copied and pasted the same address twice. But they are in fact different. You can try it on your own. – Big Cat Public Safety Act Dec 05 '18 at 21:45
So, that's the thing, I did, and the addresses I get are the same. The rules R uses for when to copy objects are complicated and can depend on what other objects you have floating around and how they may/may not be connected. – joran Dec 05 '18 at 21:47
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If you are using Rstudio, these are relevant: [Operator “[ – Henrik Dec 05 '18 at 21:47
I am using RStudio – Big Cat Public Safety Act Dec 05 '18 at 21:48
I am using latest 3.5.1. And if I run all the lines at once then the address is the same, but if I run it line by line the address is different. Also, if use "< – Big Cat Public Safety Act Dec 05 '18 at 21:55

How can I populate a vector efficiently if each time I modify one element the whole vector gets copied and moved

0 Answers0