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pull() to collect a single column #2054

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krlmlr opened this issue Aug 5, 2016 · 5 comments
Closed

pull() to collect a single column #2054

krlmlr opened this issue Aug 5, 2016 · 5 comments

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@krlmlr
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@krlmlr krlmlr commented Aug 5, 2016

For regular data sources, I can do the following to obtain a count as an atomic value:

> data_frame(1:3) %>% tally %>% as.numeric
[1] 3

For SQL sources, I have to insert a collect():

> memdb_frame(a=1:3) %>% tally %>% as.numeric
Error in magrittr(.) : (list) object cannot be coerced to type 'double'
3: traceback(1)
2: magrittr(.) at pipe.R#6
1: memdb_frame(a = 1:3) %>% tally %>% as.numeric
> memdb_frame(a=1:3) %>% tally %>% collect %>% as.numeric
[1] 3

Perhaps we can implement as.numeric.tbl_lazy() and others?

@hadley
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@hadley hadley commented Feb 2, 2017

I think we should implement pull() from lplyr: https://github.com/paulponcet/lplyr. It could default to the first column.

@krlmlr
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@krlmlr krlmlr commented Feb 3, 2017

What's special about the first column? Often you want to pull data after applying mutate(), which adds columns at the end.

The verb itself could also have more general mutate semantics: iris %>% pull(Sepal.Length * Sepal.Width) .

@hadley
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@hadley hadley commented Feb 3, 2017

It's just a reasonable default because you often have a single column data frame.

I think I like the idea of allowing it to do inline mutation too.

@krlmlr
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@krlmlr krlmlr commented Feb 3, 2017

If the last column is the default, it will still work for one-column data frames.

@hadley
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@hadley hadley commented Feb 3, 2017

Hmmmm, that just somehow feels less predictable to me. It probably doesn't really matter if we support inline mutate.

@hadley hadley changed the title [SQL] FR: as.numeric() et al. pull() function to collect a single column Feb 14, 2017
@hadley hadley changed the title pull() function to collect a single column pull() to collect a single column Feb 14, 2017
@hadley hadley closed this in 0b9aabf Feb 24, 2017
@lock lock bot locked as resolved and limited conversation to collaborators Jun 8, 2018
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