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    <div class="moz-cite-prefix"><br>
      That was my thought too.  I don't know what str_match_all is,  but
      given the unlist() in getUrls(),  it seems to return a list.  
      Rather than unlist(),  leave it as list,  and data.table should
      happily make a `list` column where each cell is itself a vector. 
      In fact each cell can be anything at all,  even embedded
      data.table, function definitions, or any type of object.<br>
      You might need a list(list(str_match_all(...))) in j to do that.<br>
      <br>
      Or what Rick has suggested here might work first time.  It's hard
      to visualise it without a small reproducible example, so we're
      having to make educated guesses.<br>
      <br>
      Many thanks for the kind words about data.table.<br>
      <br>
      Matthew<br>
      <br>
      <br>
      On 27/09/13 07:44, Ricardo Saporta wrote:<br>
    </div>
    <blockquote
cite="mid:CAE7Aa4R-_DOkzC3JuJ-nbMFSQ5GLTij5rX3jEHHyjp8wL_YCwg@mail.gmail.com"
      type="cite">
      <div dir="ltr">In fact, you should be able to skip the function
        altogether and just use: 
        <div><br>
        </div>
        <div>   db[ (has_url), str_match_all(text, url_pattern), by=id]<br>
        </div>
        <div><br>
        </div>
        <div><br>
        </div>
        <div>(and now, my apologies to all for the email clutter)</div>
        <div>good night</div>
        <div class="gmail_extra"><br>
          <div class="gmail_quote">On Fri, Sep 27, 2013 at 2:41 AM,
            Ricardo Saporta <span dir="ltr"><<a
                moz-do-not-send="true"
                href="mailto:saporta@scarletmail.rutgers.edu"
                target="_blank">saporta@scarletmail.rutgers.edu</a>></span>
            wrote:<br>
            <blockquote class="gmail_quote" style="margin:0 0 0
              .8ex;border-left:1px #ccc solid;padding-left:1ex">
              <div dir="ltr">sorry, I probably should have elaborated
                 (it's late here, in NJ)
                <div><br>
                </div>
                <div>The error you are seeing is most likely coming from
                  your getURL function in that you are adding several
                  ids to a data.frame of varying rows, and `R` cannot
                  recycle it correctly.   </div>
                <div><br>
                </div>
                <div>If you instead breakdown by id, then each time you
                  are only assigning one id and R will be able to
                  recycle appropriately, without issue. </div>
                <div><br>
                </div>
                <div>good luck! </div>
                <div>Rick</div>
                <div>
                  <br>
                </div>
              </div>
              <div class="gmail_extra">
                <div class="im"><br clear="all">
                  <div>
                    <div
                      style="color:rgb(34,34,34);font-size:13px;font-family:arial,sans-serif">
                      <div style="font-size:13px">Ricardo Saporta</div>
                      <div style="font-size:13px">
                        Graduate Student, Data Analytics</div>
                      <div style="font-size:13px"><span
                          style="font-size:13px">Rutgers University, New
                          Jersey</span></div>
                      <div style="font-size:13px"><span
                          style="font-size:13px">e: </span><a
                          moz-do-not-send="true"
                          href="mailto:saporta@rutgers.edu"
                          style="color:rgb(17,85,204);font-size:13px"
                          target="_blank">saporta@rutgers.edu</a></div>
                      <div><br>
                      </div>
                    </div>
                  </div>
                  <br>
                  <br>
                </div>
                <div>
                  <div class="h5">
                    <div class="gmail_quote">On Fri, Sep 27, 2013 at
                      2:37 AM, Ricardo Saporta <span dir="ltr"><<a
                          moz-do-not-send="true"
                          href="mailto:saporta@scarletmail.rutgers.edu"
                          target="_blank">saporta@scarletmail.rutgers.edu</a>></span>
                      wrote:<br>
                      <blockquote class="gmail_quote" style="margin:0 0
                        0 .8ex;border-left:1px #ccc
                        solid;padding-left:1ex">
                        <div dir="ltr">Hi there, 
                          <div><br>
                          </div>
                          <div>Try inserting a `by=id` in </div>
                          <div><br>
                          </div>
                          <div>   <span
                              style="font-family:arial,sans-serif;font-size:13px">a
                              <- db[(has_url), getUrls(text, id),
                              by=id]</span></div>
                          <div>
                            <span
                              style="font-family:arial,sans-serif;font-size:13px"><br>
                            </span></div>
                          <div><span
                              style="font-family:arial,sans-serif;font-size:13px">Also,
                              no need for "</span><span
                              style="font-family:arial,sans-serif;font-size:13px">has_url
                              == T"</span></div>
                          <div><span
                              style="font-family:arial,sans-serif;font-size:13px">instead,
                              use </span></div>
                          <div><span
                              style="font-family:arial,sans-serif;font-size:13px"> 
                              (</span><span
                              style="font-family:arial,sans-serif;font-size:13px">has_url) </span></div>
                          <div><span
                              style="font-family:arial,sans-serif;font-size:13px">If
                              the variable is alread logical.
                               (Otherwise, you are just slowing things
                              down ;) </span></div>
                          <div><span
                              style="font-family:arial,sans-serif;font-size:13px"><br>
                            </span></div>
                          <div><span
                              style="font-family:arial,sans-serif;font-size:13px"><br>
                            </span></div>
                        </div>
                        <div class="gmail_extra"><br clear="all">
                          <div>
                            <div
                              style="color:rgb(34,34,34);font-size:13px;font-family:arial,sans-serif">
                              <div style="font-size:13px">Ricardo
                                Saporta</div>
                              <div style="font-size:13px">Graduate
                                Student, Data Analytics</div>
                              <div style="font-size:13px"><span
                                  style="font-size:13px">Rutgers
                                  University, New Jersey</span></div>
                              <div style="font-size:13px">
                                <span style="font-size:13px">e: </span><a
                                  moz-do-not-send="true"
                                  href="mailto:saporta@rutgers.edu"
                                  style="color:rgb(17,85,204);font-size:13px"
                                  target="_blank">saporta@rutgers.edu</a></div>
                              <div><br>
                              </div>
                            </div>
                          </div>
                          <br>
                          <br>
                          <div class="gmail_quote">
                            <div>
                              <div>On Thu, Sep 26, 2013 at 11:16 PM,
                                Stian Håklev <span dir="ltr"><<a
                                    moz-do-not-send="true"
                                    href="mailto:shaklev@gmail.com"
                                    target="_blank">shaklev@gmail.com</a>></span>
                                wrote:<br>
                              </div>
                            </div>
                            <blockquote class="gmail_quote"
                              style="margin:0 0 0 .8ex;border-left:1px
                              #ccc solid;padding-left:1ex">
                              <div>
                                <div>
                                  <div dir="ltr">I'm trying to run a
                                    function on every row fulfilling a
                                    certain criterium, which returns a
                                    data frame - the idea is then to
                                    take the list of data frames and
                                    rbindlist them together for a
                                    totally separate data.table. (I'm
                                    extracting several URL links from
                                    each forum post, and tagging them
                                    with the forum post they came
                                    from). 
                                    <div>
                                      <br>
                                    </div>
                                    <div>I tried doing this with a
                                      data.table</div>
                                    <div><br>
                                    </div>
                                    <div>a <- db[has_url == T,
                                      getUrls(text, id)]</div>
                                    <div><br>
                                    </div>
                                    <div>and get the message</div>
                                    <div><br>
                                    </div>
                                    <div>
                                      <div>Error in
                                        `$<-.data.frame`(`*tmp*`,
                                        "id", value = c(1L, 6L, 1L, 2L,
                                        4L,  : </div>
                                      <div>  replacement has 11007 rows,
                                        data has 29787 </div>
                                    </div>
                                    <div><br>
                                    </div>
                                    <div>Because some rows have several
                                      URLs... However, I don't care that
                                      these rowlengths don't match, I
                                      still want these rows :) I thought
                                      J would just let me execute
                                      arbitrary R code in the context of
                                      the rows as variable names, etc. </div>
                                    <div><br>
                                    </div>
                                    <div>Here's the function it's
                                      running, but that shouldn't be
                                      relevant</div>
                                    <div><br>
                                    </div>
                                    <div>
                                      <div>getUrls <- function(text,
                                        id) {</div>
                                      <div>  matches <-
                                        str_match_all(text, url_pattern)</div>
                                      <div>  a <-
                                        data.frame(urls=unlist(matches))</div>
                                      <div>  a$id <- id</div>
                                      <div>  a</div>
                                      <div>}</div>
                                      <div><br>
                                      </div>
                                      <div><br>
                                      </div>
                                      <div>Thanks, and thanks for an
                                        amazing package - data.table has
                                        made my life so much easier. It
                                        should be part of base, I think.</div>
                                      <div>Stian Haklev, University of
                                        Toronto</div>
                                    </div>
                                    <span><font color="#888888">
                                        <div>
                                          <div><br>
                                          </div>
                                          -- <br>
                                          <a moz-do-not-send="true"
                                            href="http://reganmian.net/blog"
                                            target="_blank">http://reganmian.net/blog</a>
                                          -- Random Stuff that Matters<br>
                                        </div>
                                      </font></span></div>
                                  <br>
                                </div>
                              </div>
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                            </blockquote>
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