#+TITLE: Datatable package
#+DATE: 2021-10-26
#+OPTIONS: creator:nil timestamp:nil todo:nil num:nil
#+PROPERTY: header-args:R :results output :session *Rc* :cmdline :tangle yes
#+PROPERTY: header-args:R+ :exports both
#+SETUPFILE: https://fniessen.github.io/org-html-themes/org/theme-readtheorg.setup
#+HTML_HEAD: <style> #content{max-width:1800px;}</style>
#+HTML_HEAD: <style> p{max-width:800px;}</style>
#+HTML_HEAD: <style> li{max-width:800px;}</style>
#+HTML_HEAD: <style> pre.src{background: #f5f5f5;color:black;}</style>
#+HTML_HEAD: <style> .example{background: #fefefa;color:black;}</style>


#+begin_export html
  <div id="subtitle" style="float: center; text-align: center;">
    <p>
      Org document with
      <a href="http://www.r-project.org/">R</a> code
    </p>
    <p>
      <a href="http://www.r-project.org/">
	<img src="http://www.r-project.org/Rlogo.jpg"/>
      </a>
    </p>
  </div>
#+end_export

#+begin_src R :exports code
  rm(list = ls())
#+end_src


** Select columns in "j"

This creates a vector

#+begin_src R
  dt_c <- flights[, arr_delay]
  head(dt_c)
#+end_src

To create a data.table, use the function "~list~" or dot "~.~"

#+begin_src R
  dt_c1 <- flights[, list(arr_delay)]
  head(dt_c1)
#+end_src

The selection is wrapped in "~list~", thus, the columns can be
renamed

#+begin_src R
  dt_c2 <- flights[, .(delay2arriv = arr_delay, dist = distance)]
  head(dt_c2)
#+end_src

#+begin_src R
  dt_c2a <- flights[, c("arr_delay", "distance")]
  head(dt_c2a)
#+end_src


#+begin_src R
  dt_c2b <- flights[, year:day]
  head(dt_c2b)
#+end_src

Building new colummns

#+begin_src R
  head(dt_c2b[, `:=`(time = year -1, # name = value,
                     pumpkin = month,
                     tomatoe = day +1)])
#+end_src

Renaming colummns

#+begin_src R
  setnames(dt_c2b,
           c("time", "pumpkin", "tomatoe"),# old names
           c("time_minus1", "pumpkin_month", "day_plus1")) # new names

  names(dt_c2b)
#+end_src

#+begin_src R
  setnames(dt_c2b, toupper) # new names
  names(dt_c2b)
#+end_src

Renaming colummns

#+begin_src R
  setnames(dt_c2b, tolower) # new names
  names(dt_c2b)
#+end_src

#+begin_src R
  dt_c2c <- flights[, -(year:day)]
  head(dt_c2c)
#+end_src

Select columns by patterns, e.g., selecting 5 lines form columns having "del"
characters in the name:

#+begin_src R
  flights[1:5, names(flights) %like% "del", with = FALSE]
#+end_src