[R] readLines() and unz() and non-empty final line

[Marttila Mikko]

Thanks Iris, Bert, and Tim.

Whether unz() is blocking or not by default doesn’t seem to be documented. Indeed, thank you Iris for finding out that explicitly opening it as blocking would work. That made me wonder if it’s non-blocking by default then, which would have been surprising. However, explicitly opening it as non-blocking seems to lead to problems as well:

> local({
+   con <- unz("hello.zip", "hello.txt")
+   open(con, blocking = FALSE)
+   on.exit(close(con))
+   res <- readLines(con)
+   res
+ })
Error in readLines(con) : seek not enabled for this connection
Calls: local ... eval.parent -> eval -> eval -> eval -> eval -> readLines
Execution halted
So, the behaviour of unz() seems to be different depending on whether it was explicitly opened before passed to readLines(). Should this be fixed or documented?

Best,

Mikko

From: Bert Gunter <bgunter.4567 using gmail.com>
Sent: Thursday, 24 October 2024 18:13
To: Iris Simmons <ikwsimmo using gmail.com>
Cc: Marttila Mikko <mikko.marttila using orionpharma.com>; r-help using r-project.org
Subject: Re: [R] readLines() and unz() and non-empty final line

You don't often get email from bgunter.4567 using gmail.com<mailto:bgunter.4567 using gmail.com>. Learn why this is important<https://aka.ms/LearnAboutSenderIdentification>
But note:

> zip("hello.zip", "hello.txt")
updating: hello.txt (stored 0%)
> readChar(unz("hello.zip","hello.txt"),100)
[1] "hello"

I leave it to you and other wiser heads to figure out.

Cheers,
Bert

On Thu, Oct 24, 2024 at 8:57 AM Iris Simmons <ikwsimmo using gmail.com<mailto:ikwsimmo using gmail.com>> wrote:
Hi Mikko,


I tried running a few different things, and it seems as though
explicitly using `open()` and opening a blocking connection works.

```R
cat("hello", file = "hello.txt")
zip("hello.zip", "hello.txt")
local({
    conn <- unz("hello.zip", "hello.txt")
    on.exit(close(conn))
    ## you can use "r" instead of "rt"
    ##
    ## 'blocking = TRUE' is the default, so remove if desired
    open(conn, "rb", blocking = TRUE)
    readLines(conn)
})
```

A blocking connection might be undesirable for you, in which case
someone else might have a better solution.

On Thu, Oct 24, 2024 at 10:58 AM Marttila Mikko via R-help
<r-help using r-project.org<mailto:r-help using r-project.org>> wrote:
>
> Dear list,
>
> I'm seeing a strange interaction with readLines() and unz() when reading
> a file without an empty final line. The final line gets dropped silently:
>
> > cat("hello", file = "hello.txt")
> > zip("hello.zip", "hello.txt")
>   adding: hello.txt (stored 0%)
> > readLines(unz("hello.zip", "hello.txt"))
> character(0)
>
> The documentation for readLines() says if the final line is incomplete for
> "non-blocking text-mode connections" the line is "pushed back, silently"
> but otherwise "accepted with a warning".
>
> My understanding is that the unz() here is blocking so the line should be
> accepted. Is that incorrect? If so, how would I go about reading such
> lines from a zip file?
>
> Best,
>
> Mikko
>
>
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