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      Hi,<br>
      <br>
      What's the warning?<br>
      <br>
      Matthew<br>
      <br>
      <br>
      On 20/09/13 14:48, Ricardo Saporta wrote:<br>
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    <blockquote
cite="mid:CAE7Aa4TyhNFbpZ7_J9nseNgwW-N0k9QzgyOBbrprnRxi4umcDw@mail.gmail.com"
      type="cite">
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          <div>I've encountered the following issue iterating over a
            list of data.tables. </div>
          <div>The issue is only with mapply, not with lapply .</div>
          <div><br>
          </div>
          <div> </div>
          <div>Given a list of data.table's, mapply'ing over the list
            directly </div>
          <div>cannot modify in place. </div>
          <div><br>
          </div>
          <div>Also if attempting to add a new column, we get an
            "Invalid .internal.selfref" warning. </div>
          <div>Modifying an existing column does not issue a warning,
            but still fails to modify-in-place</div>
          <div><br>
          </div>
          <div>WORKAROUND: </div>
          <div>----------</div>
          <div>The workaround is to iterate over an index to the list,
            then to </div>
          <div>  modify each data.table via list.of.DTs[[i]][ .. ]</div>
          <div><br>
          </div>
          <div>**Interestingly, this issue occurs with `mapply`, but not
            `lapply`.**</div>
          <div><br>
          </div>
          <div> </div>
          <div>EXAMPLE:</div>
          <div>-------- </div>
          <div>  # Given a list of DT's and two lists of vectors, </div>
          <div>  #   we want to add the corresponding vectors as columns
            to the DT. </div>
          <div><br>
          </div>
          <div>## ---------------- ##</div>
          <div>##   SAMPLE DATA:   ##</div>
          <div>## ---------------- ##</div>
          <div>  # list of data.tables</div>
          <div>  list.DT <- list(</div>
          <div>    DT1=data.table(Col1=111:115, Col2=121:125),</div>
          <div>    DT2=data.table(Col1=211:215, Col2=221:225)</div>
          <div>    )</div>
          <div><br>
          </div>
          <div>  # lists of columns to add</div>
          <div>  list.Col3 <- list(131:135, 231:235)</div>
          <div>  list.Col4 <- list(141:145, 241:245)</div>
          <div><br>
          </div>
          <div><br>
          </div>
          <div>## ------------------------------------ ##</div>
          <div>##   Iterating over the list elements   ##</div>
          <div>##     adding a new column              ##</div>
          <div>## ------------------------------------ ##</div>
          <div>##   Will issue warning and             ##</div>
          <div>##     will fail to modify in place     ##</div>
          <div>## ------------------------------------ ##</div>
          <div>  mapply (</div>
          <div>      function(DT, C3, C4)</div>
          <div>
                     DT[, c("Col3", "Col4") := list(C3, C4)],</div>
          <div>      </div>
          <div>      list.DT,  # iterating over the list</div>
          <div>      list.Col3, list.Col4,</div>
          <div>      SIMPLIFY=FALSE</div>
          <div>    ) </div>
          <div><br>
          </div>
          <div>  ## Note the lack of change</div>
          <div>  list.DT</div>
          <div><br>
          </div>
          <div><br>
          </div>
          <div>## ------------------------------------ ##</div>
          <div>##   Iterating over an index            ##</div>
          <div>## ------------------------------------ ##</div>
          <div>  mapply (</div>
          <div>      function(i, C3, C4)</div>
          <div>         list.DT[[i]] [, c("Col3", "Col4") := list(C3,
            C4)],</div>
          <div>     </div>
          <div>      seq(list.DT),   # iterating over an index to the
            list</div>
          <div>      list.Col3, list.Col4,</div>
          <div>      SIMPLIFY=FALSE</div>
          <div>    )</div>
          <div><br>
          </div>
          <div>  ## Note each DT _has_ been modified</div>
          <div>  list.DT</div>
          <div><br>
          </div>
          <div>## ------------------------------------ ##</div>
          <div>##   Iterating over the list elements   ##</div>
          <div>##     modifying existing column        ##</div>
          <div>## ------------------------------------ ##</div>
          <div>##   No warning issued, but             ##</div>
          <div>##     Will fail to modify in place     ##</div>
          <div>## ------------------------------------ ##</div>
          <div>  mapply (</div>
          <div>      function(DT, C3, C4)</div>
          <div>         DT[, c("Col3", "Col4") := list(Col3*1e3,
            Col4*1e4)],</div>
          <div><br>
          </div>
          <div>
                  list.DT,  # iterating over the list</div>
          <div>      list.Col3, list.Col4,</div>
          <div>      SIMPLIFY=FALSE</div>
          <div>    ) </div>
          <div><br>
          </div>
          <div>  ## Note the lack of change (compare with output from
            `mapply`)</div>
          <div>  list.DT</div>
          <div><br>
          </div>
          <div>## ------------------------------------ ##</div>
          <div>##                                      ##</div>
          <div>##   `lapply` works as expected.        ##</div>
          <div>##                                      ##</div>
          <div>## ------------------------------------ ##</div>
          <div>  </div>
          <div>  ## NOW WITH lapply</div>
          <div>  lapply(list.DT, </div>
          <div>    function(DT)</div>
          <div>      DT[, newCol := LETTERS[1:5]]</div>
          <div>  )</div>
          <div><br>
          </div>
          <div>  ## Note the new column: </div>
          <div>  list.DT</div>
          <div><br>
          </div>
          <div><br>
          </div>
          <div><br>
          </div>
          <div># ========================== # </div>
          <div><br>
          </div>
          <div>##   NON-WORKAROUNDS   ## </div>
          <div>##</div>
          <div>## I also tried all of the following alternatives</div>
          <div>##   in hopes of being able to iterate over the list </div>
          <div>##   directly, using `mapply`.  </div>
          <div>## None of these worked. </div>
          <div><br>
          </div>
          <div># (1) Creating the DTs First, then creating the list from
            them</div>
          <div>    DT1 <- data.table(Col1=111:115, Col2=121:125)</div>
          <div>    DT2 <- data.table(Col1=211:215, Col2=221:225)</div>
          <div><br>
          </div>
          <div>    list.DT <- list(DT1=DT1,DT2=DT2 )</div>
          <div><br>
          </div>
          <div><br>
          </div>
          <div># (2) Same as 1, and using `copy()` in the call to
            `list()`</div>
          <div>    list.DT <- list(DT1=copy(DT1), </div>
          <div>                    DT2=copy(DT2) )</div>
          <div><br>
          </div>
          <div># (3) lapply'ing `copy` and then iterating over that list</div>
          <div>    list.DT <- lapply(list.DT, copy)</div>
          <div><br>
          </div>
          <div># (4) Not naming the list elements</div>
          <div>    list.DT <- list(DT1, DT2)</div>
          <div>    # and tried</div>
          <div>    list.DT <- list(copy(DT1), copy(DT2))</div>
          <div><br>
          </div>
          <div>## All of the above still failed to modify in place</div>
          <div>##   (and also issued the same warning if trying to add a
            column)</div>
          <div>##    when iterating using mapply</div>
          <div><br>
          </div>
          <div>  mapply(function(DT, C3, C4)</div>
          <div>    DT[, c("Col3", "Col4") := list(C3, C4)],</div>
          <div>    list.DT, list.Col3, list.Col4,</div>
          <div>    SIMPLIFY=FALSE)</div>
          <div><br>
          </div>
          <div><br>
          </div>
          <div># ========================== # </div>
        </div>
        <div><br>
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          <div
style="color:rgb(34,34,34);font-family:arial,sans-serif;font-size:13px;background-color:rgb(255,255,255)">
            <div style="font-size:13px">Ricardo Saporta</div>
            <div style="font-size:13px">
              Rutgers University, New Jersey<br>
            </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>
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