[datatable-help] What is the fastest way to determine that data.table is empty

[Matthew Dowle]

I can't beat that. nrow isn't primitive (unlike length and $), so just
searching for and finding nrow in base is going to take time in a loop.
Having said that, even dim(DT) is relatively slow and dim is primitive, so
that can't be the whole reason.

> DT=data.table(a=1:3,b=4:6)
> system.time(for (i in 1:10000) nrow(DT))
   user  system elapsed
  1.128   0.004   1.138
> system.time(for (i in 1:10000) NROW(DT))
   user  system elapsed
  1.112   0.008   1.123
> system.time(for (i in 1:10000) dim(DT))
   user  system elapsed
  1.056   0.000   1.056
> system.time(for (i in 1:10000) length(DT[[1L]]))
   user  system elapsed
  0.568   0.000   0.572
> system.time(for (i in 1:10000) length(DT$a))  # yours is indeed fastest
   user  system elapsed
  0.168   0.000   0.169

However, are you 100% sure that Rprof() is telling you that's really the
bottleneck? The branch itself, or whatever is done inside the branch,
perhaps. And, you know to vectorize I assume. I usually roll my eyes when
people find significant differences of insignificant times (like this)
unless they really do need to loop for some reason.

My only thought to go faster than length(DT$a), is a direct .External call
to a C function that returns LENGTH(VECTOR_ELT(DT,0)).  As soon as that's
wrapped up in a function call, though, it'll go slower than the primitive
calls in length(DT$a) I guess.

You call also drop the != 0 part.

    if ( length(...) != 0 )

is the same as :

    if ( length(...) )

Matthew


> Hi,
>
> I have found that if(nrow(d.table) != 0) is significantly reduces
> performance of my application.
>
> Could you please advise on faster ways to figure that data.table doesn't
> contains any rows. The fastest I have found is length(d.table$column) !=
> 0.
>
>
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