How can I remove an element from a list

Question

I have a list and I want to remove a single element from it   How can I do this   I ve tried looking up what I think the obvious names for this function would be in the reference manual and I haven t found anything appropriate

User · Answer

You can use which   x lt -c 1 5  x   1  1 2 3 4 5 x lt -x -which x  4   x   1  1 2 3 5

User · Answer

Using lapply and grep   lst  lt - list a   1 4  b   4 8  c   8 10    say you want to remove a and c toremove lt -c  a   c   lstnew lt -lst -unlist lapply toremove  function x  grep x  names lst          or pattern lt - a c  lstnew lt -lst -grep pattern  names lst

User · Answer

I don t know R at all  but a bit of creative googling led me here  http   tolstoy newcastle edu au R help 05 04 1919 html  The key quote from there      I do not find explicit documentation for R on how to remove elements from lists  but trial and error tells me      myList  5    lt - NULL      will remove the 5th element and then  close up  the hole caused by deletion of that element  That suffles the index values  So I have to be careful in dropping elements  I must work from the back of the list to the front    A response to that post later in the thread states      For deleting an element of a list  see R FAQ 7 1   And the relevant section of the R FAQ says          Do not set x i  or x  i   to NULL  because this will remove the corresponding component from the list     Which seems to tell you  in a somewhat backwards way  how to remove an element   Hope that helps  or at least leads you in the right direction

User · Answer

You can also negatively index from a list using the extract function of the magrittr package to remove a list item  a  lt - seq 1 5  b  lt - seq 2 6  c  lt - seq 3 7  l  lt - list a b c   library magrittr   extract l -1   simple one-function method   1    1  2 3 4 5 6    2    1  3 4 5 6 7

User · Answer

Don t know if you still need an answer to this but I found from my limited  3 weeks worth of self-teaching R  experience with R that  using the NULL assignment is actually wrong or sub-optimal especially if you re dynamically updating a list in something like a for-loop   To be more precise  using  myList  5    lt - NULL   will throw the error     myList  5    lt - NULL   replacement has length zero   or     more elements supplied than there are to replace   What I found to work more consistently is  myList  lt - myList  -5

User · Answer

If you have a named list and want to remove a specific element you can try   lst  lt - list a   1 4  b   4 8  c   8 10   if  b   in  names lst   lst  lt - lst  - which names lst      b      This will make a list lst with elements a  b  c  The second line removes element b after it checks that it exists  to avoid the problem  hjv mentioned    or better   lst b  lt - NULL   This way it is not a problem to try to delete a non-existent element  e g  lst g  lt - NULL

User · Answer

How about this  Again  using indices   gt  m  lt - c 1 5   gt  m  1  1 2 3 4 5   gt  m 1 length m -1   1  1 2 3 4   or   gt  m - length m     1  1 2 3 4

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There s the rlist package  http   cran r-project org web packages rlist index html  to deal with various kinds of list operations   Example  http   cran r-project org web packages rlist vignettes Filtering html    library rlist  devs  lt -    list      p1 list name  Ken  age 24        interest c  reading   music   movies          lang list r 2 csharp 4 python 3        p2 list name  James  age 25        interest c  sports   music          lang list r 3 java 2 cpp 5        p3 list name  Penny  age 24        interest c  movies   reading          lang list r 1 cpp 4 python 2     list remove devs  c  p1   p2      Results in      p3    p3 name    1   Penny        p3 age    1  24       p3 interest    1   movies    reading        p3 lang    p3 lang r    1  1       p3 lang cpp    1  4       p3 lang python    1  2

User · Answer

Use -  Negative sign  along with position of element  example if 3rd element is to be removed use it as your list -3   Input     my list  lt - list a   3  b   3  c   4  d    Hello   e   NA  my list     a     1  3     b    1  3     c    1  4     d    1   Hello      e    1  NA   Remove single element from list   my list -3       a      1  3      b     1  3      d     1   Hello       e   1  NA   Remove multiple elements from list   my list c -1 -3 -2        d      1   Hello       e     1  NA      my list c -3 -5        a      1  3      b     1  3      my list -seq 1 2        c      1  4      d     1   Hello       e     1  NA

User · Answer

if you d like to avoid numeric indices  you can use  a  lt - setdiff names a  c  name1         namen      to delete names namea   namen from a   this works for lists   gt  l  lt - list a 1 b 2   gt  l setdiff names l   a     b  1  2   as well as for vectors   gt  v  lt - c a 1 b 2   gt  v setdiff names v   a    b  2

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In the case of named lists I find those helper functions useful  member  lt - function list names          return the elements of the list with the input names     member  names  lt - names list      index  lt - which member  names  in  names      list index          exclude  lt - function list names           return the elements of the list not belonging to names      member  names  lt - names list       index  lt - which   member  names  in  names       list index          aa  lt - structure list a   1 10  b   4 5  fruits   c  apple    orange       Names   c  a    b    fruits      gt  aa     a      1   1  2  3  4  5  6  7  8  9 10      b     1  4 5      fruits     1   apple    orange     gt  member aa  fruits       fruits     1   apple    orange     gt  exclude aa  fruits       a      1   1  2  3  4  5  6  7  8  9 10      b     1  4 5

User · Answer

Just wanted to quickly add  because I didn t see it in any of the answers  that  for a named list  you can also do l  name    lt - NULL  For example   l  lt - list a   1  b   2  cc   3  l  b    lt - NULL

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If you don t want to modify the list in-place  e g  for passing the list with an element removed to a function   you can use indexing  negative indices mean  don t include this element    x  lt - list  a    b    c    d    e      example list  x -2           without 2nd element  x -c 2  3      without 2nd and 3rd   Also  logical index vectors are useful   x x     b      without elements that are  b    This works with dataframes  too   df  lt - data frame number   1 5  name   letters 1 5    df df name     b            rows without  b   df df number    2    1      rows with odd numbers only

User · Answer

I would like to add that if it s a named list you can simply use within    l  lt - list a   1  b   2       gt  within l  rm a    b  1  2   So you can overwrite the original list  l  lt - within l  rm a      to remove element named a from list l

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Here is how the remove the last element of a list in R   x  lt - list  a    b    c    d    e   x length x    lt - NULL   If x might be a vector then you would need to create a new object   x  lt - c  a    b    c    d    e   x  lt - x -length x      Work for lists and vectors

User · Answer

Removing Null elements from a list in single line     x x - which sapply x is null  arr ind TRUE     Cheers

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