Efficient instant creation functions

[hadley]

i.e. if I have year, month, and day, how do I efficiently create a date (or date time)?

[vspinu]

ISOdate and ISOdatetime is what you are looking for. Not the most user friendly names though.

[vspinu]

How about renaming them into make_date and make_datetime? We already have make_difftime to overcome the existent base::difftime constructor.

Even better, we can use date and datetime and fall back to base::date when the function is called with no arguments.

[hadley]

Sorry, the key was "efficient" - ISOdatetime() works by pasting together the values and then parsing. That's clearly a suboptimal approach. I like make_datetime(), but I think they should raise an error when called without arguments (since we already have now() and it's better for each function to do one job well)

[vspinu]

the key was "efficient" - ISOdatetime()

That's pretty efficient. Any other operation with dates will likely be much more time consuming. We can think of implementing it from scratch but I would like to see a use case when the current situation would be a bottleneck and the speed improvements would actually matter.

(somewhat related #324)

[hadley]

I'd say ISODateTime is at least 100x slower than it could be. It's definitely a problem when you're working with data frame with millions of row.

We could do this much more efficiently in C.

[vspinu]

I'd say ISODateTime is at least 100x slower than it could be.

Oh. You are right. I was confused. I though that ISOdate and ISOdatetime are our functions and they are based on our C parser.

Indeed, they are very slow and I actually replaced myself all occurrences of ISOdatetime in our code a year or so ago. We already can get 100x speedup with our fast_strptime function.

[hadley]

Yeah, and it should be even faster if we don't paste and parse and instead create the time object in C directly.

[vspinu]

Hm. I have added fast_datetime. Unfortunately pasting takes 99.9% of the time so it's not a great improvement overall. I am afraid we will have to do it directly.

[vspinu]

I have ported make_datetime to C. it resulted in 100x speedup indeed.