How to assign from a function which returns more than one value

Question

Still trying to get into the R logic     what is the  best  way to unpack  on LHS  the results from a function returning multiple values   I can t do this apparently   R gt  functionReturningTwoValues  lt - function     return c 1  2     R gt  functionReturningTwoValues    1  1 2 R gt  a  b  lt - functionReturningTwoValues   Error  unexpected     in  a   R gt  c a  b   lt - functionReturningTwoValues   Error in c a  b   lt - functionReturningTwoValues     object  a  not found   must I really do the following   R gt  r  lt - functionReturningTwoValues   R gt  a  lt - r 1   b  lt - r 2    or would the R programmer write something more like this   R gt  functionReturningTwoValues  lt - function    return list first 1  second 2    R gt  r  lt - functionReturningTwoValues   R gt  r first  1  1 R gt  r second  1  2   --- edited to answer Shane s questions ---  I don t really need giving names to the result value parts   I am applying one aggregate function to the first component and an other to the second component  min and max   if it was the same function for both components I would not need splitting them

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Lists seem perfect for this purpose   For example within the function you would have  x   desired return value 1    vector  matrix  etc   y   desired return value 2    vector  matrix  etc   returnlist   list x y           end of function   main program  x   returnlist  1    y   returnlist  2

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If you want to return the output of your function to the Global Environment  you can use list2env  like in this example   myfun  lt - function x    a  lt - 1 x                        b  lt - 5 x                        df  lt - data frame a a  b b                          newList  lt - list  my obj1    a   my obj2    b   myDF  df                         list2env newList   GlobalEnv                               myfun 3    This function will create three objects in your Global Environment    gt  my obj1    1  1 2 3   gt  my obj2    1  5 4 3   gt  myDF     a b   1 1 5   2 2 4   3 3 3

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With R 3 6 1  I can do the following  fr2v  lt - function     c 5 3    a b  lt - fr2v    a b  1      prints  5   a b  2      prints  3

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I put together an R package zeallot to tackle this problem  zeallot includes a multiple assignment or unpacking assignment operator    lt -   The LHS of the operator is any number of variables to assign  built using calls to c    The RHS of the operator is a vector  list  data frame  date object  or any custom object with an implemented destructure method  see  zeallot  destructure    Here are a handful of examples based on the original post   library zeallot   functionReturningTwoValues  lt - function        return c 1  2       c a  b    lt -  functionReturningTwoValues   a    1 b    2  functionReturningListOfValues  lt - function       return list 1  2  3      c d  e  f    lt -  functionReturningListOfValues   d    1 e    2 f    3  functionReturningNestedList  lt - function       return list 1  list 2  3       c f  c g  h     lt -  functionReturningNestedList   f    1 g    2 h    3  functionReturningTooManyValues  lt - function       return as list 1 20      c i  j     rest    lt -  functionReturningTooManyValues   i       1 j       2 rest    list 3  4  5        Check out the package vignette for more information and examples

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Yes to your second and third questions -- that s what you need to do as you cannot have multiple  lvalues  on the left of an assignment

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Usually I wrap the output into a list  which is very flexible  you can have any combination of numbers  strings  vectors  matrices  arrays  lists  objects int he output   so like   func2 lt -function input       a lt -input 1    b lt -input 2    output lt -list a b     return output     output lt -func2 5   for  i in output       print i      1  6  1  7

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How about using assign   functionReturningTwoValues  lt - function a  b      assign a  1  pos 1    assign b  2  pos 1      You can pass the names of the variable you want to be passed by reference    gt  functionReturningTwoValues  a    b    gt  a  1  1  gt  b  1  2   If you need to access the existing values  the converse of assign is get

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To obtain multiple outputs from a function and keep them in the desired format you can save the outputs to your hard disk  in the working directory  from within the function and then load them from outside the function   myfun  lt - function x                          df1  lt -                           df2  lt -                           save df1  file    myfile1                         save df2  file    myfile2     load  myfile1   load  myfile2

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There s no right answer to this question   I really depends on what you re doing with the data   In the simple example above  I would strongly suggest    Keep things as simple as possible  Wherever possible  it s a best practice to keep your functions vectorized   That provides the greatest amount of flexibility and speed in the long run    Is it important that the values 1 and 2 above have names   In other words  why is it important in this example that 1 and 2 be named a and b  rather than just r 1  and r 2    One important thing to understand in this context is that a and b are also both vectors of length 1   So you re not really changing anything in the process of making that assignment  other than having 2 new vectors that don t need subscripts to be referenced    gt  r  lt - c 1 2   gt  a  lt - r 1   gt  b  lt - r 2   gt  class r   1   numeric   gt  class a   1   numeric   gt  a  1  1  gt  a 1   1  1   You can also assign the names to the original vector if you would rather reference the letter than the index    gt  names r   lt - c  a   b    gt  names r   1   a   b   gt  r  a   a  1     Edit  Given that you will be applying min and max to each vector separately  I would suggest either using a matrix  if a and b will be the same length and the same data type  or data frame  if a and b will be the same length but can be different data types  or else use a list like in your last example  if they can be of differing lengths and data types       gt  r  lt - data frame a 1 4  b 5 8   gt  r   a b 1 1 5 2 2 6 3 3 7 4 4 8  gt  min r a   1  1  gt  max r b   1  8

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functionReturningTwoValues  lt - function        results  lt - list     results first  lt - 1   results second  lt -2   return results     a  lt - functionReturningTwoValues     I think this works

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I somehow stumbled on this clever hack on the internet     I m not sure if it s nasty or beautiful  but it lets you create a  magical  operator that allows you to unpack multiple return values into their own variable  The    function is defined here  and included below for posterity         lt - function lhs  rhs      frame  lt - parent frame     lhs  lt - as list substitute lhs     if  length lhs   gt  1      lhs  lt - lhs -1    if  length lhs     1        do call      list lhs  1    rhs   envir frame      return invisible NULL          if  is function rhs     is rhs   formula        rhs  lt - list rhs    if  length lhs   gt  length rhs       rhs  lt - c rhs  rep list NULL   length lhs  - length rhs      for  i in 1 length lhs       do call      list lhs  i    rhs  i     envir frame    return invisible NULL        With that in hand  you can do what you re after   functionReturningTwoValues  lt - function       return list 1  matrix 0  2  2      c a  b     functionReturningTwoValues   a   1  1 b         1    2     1      0    0    2      0    0   I don t know how I feel about that  Perhaps you might find it helpful in your interactive workspace  Using it to build  re- usable libraries  for mass consumption  might not be the best idea  but I guess that s up to you       you know what they say about responsibility and power

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A   If each of foo and bar is a single number  then there s nothing wrong with c foo bar   and you can also name the components  c Foo foo Bar bar   So you could access the components of the result  res  as res 1   res 2   or  in the named case  as res  Foo    res  BAR       B   If foo and bar are vectors of the same type and length  then again there s nothing wrong with returning cbind foo bar  or rbind foo bar   likewise nameable  In the  cbind  case  you would access foo and bar as res  1   res  2  or as res   Foo    res   Bar    You might also prefer to return a dataframe rather than a matrix    data frame Foo foo Bar bar    and access them as res Foo  res Bar  This would also work well if foo and bar were of the same length but not of the same type  e g  foo is a vector of numbers  bar a vector of character strings      C   If foo and bar are sufficiently different not to combine conveniently as above  then you shuld definitely return a list    For example  your function might fit a linear model and  also calculate predicted values  so you could have   LM lt -lm         foo lt -summary LM   bar lt -LM fit   and then you would return list Foo foo Bar bar  and then access the summary as res Foo  the predicted values as res Bar  source  http   r 789695 n4 nabble com How-to-return-multiple-values-in-a-function-td858528 html

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1  list      lt - I had posted this over a decade ago on r-help   Since then it has been added to the gsubfn package  It does not require a special operator but does require that the left hand side be written using list      like this   library gsubfn     need 0 7-0 or later list a  b   lt - functionReturningTwoValues     If you only need the first or second component these all work too   list a   lt - functionReturningTwoValues   list a     lt - functionReturningTwoValues   list   b   lt - functionReturningTwoValues      Of course  if you only needed one value then functionReturningTwoValues    1   or functionReturningTwoValues    2   would be sufficient    See the cited r-help thread for more examples    2  with  If the intent is merely to combine the multiple values subsequently and the return values are named then a simple alternative is to use with    myfun  lt - function   list a   1  b   2   list a  b   lt - myfun   a   b    same with myfun    a   b     3  attach Another alternative is attach   attach myfun    a   b   ADDED  with and attach

[r] How to assign from a function which returns more than one value?

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