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traducción datatatble-faq #2

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9 changes: 8 additions & 1 deletion vignettes/datatable-faq.Rmd
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---
title: "Frequently Asked Questions about data.table"
title: "Preguntas Frecuentes sobre data.table"
date: "`r Sys.Date()`"
output:
markdown::html_format:
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This document has been quickly revised given the changes in v1.9.8 released Nov 2016. Please do submit pull requests to fix mistakes or improvements. If anyone knows why the table of contents comes out so narrow and squashed when displayed by CRAN, please let us know. This document used to be a PDF and we changed it recently to HTML.






# Beginner FAQs

## Why do `DT[ , 5]` and `DT[2, 5]` return a 1-column data.table rather than vectors like `data.frame`? {#j-num}
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Another compelling reason to make the change was that previously, there was no efficient way to achieve what `X[Y, sum(foo*bar)]` does now. You had to write `X[Y][ , sum(foo*bar)]`. That was suboptimal because `X[Y]` joined all the columns and passed them all to the second compound query without knowing that only `foo` and `bar` are needed. To solve that efficiency problem, extra programming effort was required: `X[Y, list(foo, bar)][ , sum(foo*bar)]`. The change to `by = .EACHI` has simplified this by allowing both queries to be expressed inside a single `DT[...]` query for efficiency.




# General Syntax

## How can I avoid writing a really long `j` expression? You've said that I should use the column _names_, but I've got a lot of columns.
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