Convert a printed message into a character vector

Question

Suppose I have a file "data.txt", which can be created with the following code

m <- matrix(c(13, 14, 4950, 20, 50, 4949, 22, 98, 4948, 
              30, 58, 4947, 43, 48, 4946), 5, byrow = TRUE)
write.table(m, "data.txt", row.names = FALSE, col.names = FALSE)

Reading the file into R with scan, a message is delivered along with the data.

( s <- scan("data.txt") )
# Read 15 items
#  [1]   13   14 4950   20   50 4949   22   98 4948
# [10]   30   58 4947   43   48 4946

I'd like to retrieve the message Read 15 items as a character vector. I know I can get the last recorded warning by typing last.warning, and can turn it into a character vector with names(last.warning), but there is no such object as last.message.

Is there a way to convert an outputted message to a character vector? The desired result would be

[1] "Read 15 items"

My fault, I edited the question. I'd like to know in general how to turn messages into character vectors — Rich Scriven, Jun 24 '14 at 21:51
@RichardScriven As you probably know, `capture.output` or `sink` would normally do the trick. I don't quite understand why they don't work here. — Anders Ellern Bilgrau, Jun 24 '14 at 22:00
sink doesn't work because message has a custom call to `cat` that specifies `file=stderr()` — Señor O, Jun 24 '14 at 22:06

score 9 · Accepted Answer · answered Jun 24 '14 at 22:06

9

The default hander for message() sends the result to stderr via cat(). You can capture that with

tc <- textConnection("messages","w")
sink(tc, type="message")
s <- scan("data.txt")
sink(NULL, type="message")
close(tc)

messages
# [1] "Read 5 items"

answered Jun 24 '14 at 22:06

MrFlick

195,160
17
277
295

score 2 · Answer 2 · answered Dec 23 '17 at 17:59

2

There's a convenient wrapper for sink in utils::capture.output()

z<-capture.output(s<-scan("data.txt"), type="message")
z
#> [1] "Read 15 items"

answered Dec 23 '17 at 17:59

dmi3kno

2,943
17
31

Convert a printed message into a character vector

2 Answers2

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