Append value to empty vector in R

Question

I m trying to learn R and I can t figure out how to append to a list   If this were Python I would         Python vector      values     a   b   c   d   e   f   g    for i in range 0 len values        vector append values i     How do you do this in R    R Programming  gt  vector   c    gt  values   c  a   b   c   d   e   f   g    gt  for  i in 1 length values      append value i  to empty vector

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In R  you can try out this way   X   NULL X   NULL values   letters 1 10  values    1   a   b   c   d   e   f   g   h   i   j  X   append X values  X    1   a   b   c   d   e   f   g   h   i   j  X   append X letters 23 26   X    1   a   b   c   d   e   f   g   h   i   j   w   x   y   z

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Sometimes we have to use loops  for example  when we don t know how many iterations we need to get the result  Take while loops as an example  Below are methods you absolutely should avoid   a numeric 0  b 1 system time          while b lt  1e5         b b 1       a lt -c a pi                user  system elapsed    13 2     0 0    13 2   a numeric 0  b 1 system time          while b lt  1e5         b b 1       a lt -append a pi                user  system elapsed    11 06    5 72   16 84    These are very inefficient because R copies the vector every time it appends   The most efficient way to append is to use index  Note that this time I let it iterate 1e7 times  but it s still much faster than c   a numeric 0  system time          while length a  lt 1e7         a length a  1  pi               user  system elapsed    5 71    0 39    6 12     This is acceptable  And we can make it a bit faster by replacing   with      a numeric 0  system time          while length a  lt 1e7         a  length a  1   pi               user  system elapsed    5 29    0 38    5 69      Maybe you already noticed that length can be time consuming  If we replace length with a counter   a numeric 0  b 1 system time          while b lt  1e7         a  b   pi       b b 1               user  system elapsed    3 35    0 41    3 76   As other users mentioned  pre-allocating the vector is very helpful  But this is a trade-off between speed and memory usage if you don t know how many loops you need to get the result   a rep NaN 2 1e7  b 1 system time          while b lt  1e7         a  b   pi       b b 1           a a  is na a           user  system elapsed    1 57    0 06    1 63    An intermediate method is to gradually add blocks of results   a numeric 0  b 0 step count 0 step 1e6 system time          repeat        a step rep NaN step        for i in seq len step            b b 1         a step  i   pi         if b gt  1e7             a step a step 1 i            break                         a  step count step 1  b  a step       if b gt  1e7  break       step count step count 1              user  system elapsed   1 71    0 17    1 89

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You have a few options    c vector  values  append vector  values  vector  length vector    1   length vector    length values     lt - values   The first one is the standard approach   The second one gives you the option to append someplace other than the end   The last one is a bit contorted but has the advantage of modifying vector  though really  you could just as easily do vector  lt - c vector  values    Notice that in R you don t need to cycle through vectors   You can just operate on them in whole   Also  this is fairly basic stuff  so you should go through some of the references   Some more options based on OP feedback   for i in values  vector  lt - c vector  i

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Just for the sake of completeness  appending values to a vector in a for loop is not really the philosophy in R  R works better by operating on vectors as a whole  as  BrodieG pointed out  See if your code can t be rewritten as   ouput  lt - sapply values  function v  return 2 v     Output will be a vector of return values  You can also use lapply if values is a list instead of a vector

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FWIW  analogous to python s append     b  lt - 1 b  lt - c b  2

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What you re using in the python code is called a list in python  and it s tottaly different from R vectors  if i get what you wanna do     you can do like this if you ll put them manually   v  lt - c  a    b    c      if your values are in a list  v  lt - as vector your list     if you just need to append v  lt - append v  value  after length v

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gt  vec  lt - c letters 1 3     vec  lt - c  a   b   c     or just empty vector  vec  lt - c     gt  values lt - c 1 2 3    gt  for  i in 1 length values          print paste  length of vec   length vec            vec length vec  1   lt - values i    Appends value at the end of vector       1   length of vec 3   1   length of vec 4   1   length of vec 5    gt  vec  1   a   b   c   1   2   3

User · Answer

Appending to an object in a for loop causes the entire object to be copied on every iteration  which causes a lot of people to say  R is slow   or  R loops should be avoided    As BrodieG mentioned in the comments  it is much better to pre-allocate a vector of the desired length  then set the element values in the loop   Here are several ways to append values to a vector   All of them are discouraged     Appending to a vector in a loop    one way for  i in 1 length values     vector i   lt - values i    another way for  i in 1 length values     vector  lt - c vector  values i     yet another way    for  v in values    vector  lt - c vector  v        more ways   help  append   would have answered your question and saved the time it took you to write this question  but would have caused you to develop bad habits    -   Note that vector  lt - c   isn t an empty vector  it s NULL   If you want an empty character vector  use vector  lt - character     Pre-allocate the vector before looping  If you absolutely must use a for loop  you should pre-allocate the entire vector before the loop   This will be much faster than appending for larger vectors   set seed 21  values  lt - sample letters  1e4  TRUE  vector  lt - character 0    slow system time  for  i in 1 length values   vector i   lt - values i        user  system elapsed     0 340   0 000   0 343  vector  lt - character length values     fast er  system time  for  i in 1 length values   vector i   lt - values i        user  system elapsed     0 024   0 000   0 023

[r] Append value to empty vector in R?

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