Where does dput write when diverting output using sink?

Question

I would like to see the result of dput in the console while redirecting output to a file using sink.

> sink(file = 'test.txt', split = TRUE)
> x <- 2^(1:4)
> x # test.txt now contains: [1] 2 4 8 16
[1]  2  4  8 16
> dput(x) # where does this return value go?
> dput(x, file = 'test.txt') # test.txt is overwritten with: c(2, 4, 6, 8)

Why does x print its value to the console (as expected), but dput(x) does not?

(I am using R 3.4.3 with RStudio Version 1.1.423 on Windows 7)

Answer 1

dput is actually writing the output where expected, but it's not writing it when expected. Running the following code shows that dput output remains pending until the next normal output:

sink(file = 'test.txt', split = TRUE)
x <- 2^(1:4)
x
dput(2*x,file="")
3*x

...gives a test.txt with:

[1]  2  4  8 16
c(4, 8, 16, 32)
[1]  6 12 24 48

Alternatively, running the sink() function to close the file will also force the pending output (but will close the connection).

sink(file = 'test.txt', split = TRUE)
x <- 2^(1:4)
x
dput(2*x,file="")
sink()

Answer 2

It only goes where you tell it to. That's what "returns invisibly" means. You can change this behavior though...

sink(file = "text.txt", split = TRUE)
x <- 2^(1:4)
x
# [1]  2  4  8 16
dput(x)
# returned invisibly, which is why you don't see it.
# but you can assign it to a variable

my_var <- dput(x)
my_var
# [1]  2  4  8 16

# But if you want to make it noisy, wrap it in ()
# When assigning it to a variable
(my_var <- dput(x))
# [1]  2  4  8 16

# This works even without assigning it to a variable 
(dput(x))
# [1]  2  4  8 16


# do this with your dput to sink command
(dput(x, file = "test.txt"))
# [1]  2  4  8 16

So, in summary, if you want to see something happen in the console that is happening behind the scenes (whether as the result of an invisible(<return statement>) or an assignment), wrap it in an outer layer of ()