How to map and remove nil values in Ruby

Question

I have a map which either changes a value or sets it to nil  I then want to remove the nil entries from the list  The list doesn t need to be kept   This is what I currently have     A simple example function  which returns a value or nil def transform n    rand  gt  0 5   n   10   nil   end  items map     x  transform x       1  2  3  4  5    gt   10  nil  30  40  nil  items reject     x  x nil       10  nil  30  40  nil    gt   10  30  40    I m aware I could just do a loop and conditionally collect in another array like this   new items      items each do  x      x   transform x      new items append x  unless x nil  end items   new items   But it doesn t seem that idiomatic  Is there a nice way to map a function over a list  removing excluding the nils as you go

User · Answer

You could use compact    1  nil  3  nil  nil  compact   gt   1  3       I d like to remind people that if you re getting an array containing nils as the output of a map block  and that block tries to conditionally return values  then you ve got code smell and need to rethink your logic    For instance  if you re doing something that does this    1 2 3  map   i    if i   2    0     i   end       gt   nil  2  nil    Then don t  Instead  prior to the map  reject the stuff you don t want or select what you do want    1 2 3  select   i  i   2    0   map   i    i       gt   2    I consider using compact to clean up a mess as a last-ditch effort to get rid of things we didn t handle correctly  usually because we didn t know what was coming at us  We should always know what sort of data is being thrown around in our program  Unexpected unknown data is bad  Anytime I see nils in an array I m working on  I dig into why they exist  and see if I can improve the code generating the array  rather than allow Ruby to waste time and memory generating nils then sifting through the array to remove them later     Just my   0 2f      2 to f 100

User · Answer

Try using reduce or inject    1  2  3  reduce        memo  i    if i   2    0     memo  lt  lt  i   end    memo     I agree with the accepted answer that we shouldn t map and compact  but not for the same reasons   I feel deep inside that map then compact is equivalent to select then map  Consider  map is a one-to-one function  If you are mapping from some set of values  and you map  then you want one value in the output set for each value in the input set  If you are having to select before-hand  then you probably don t want a map on the set  If you are having to select afterwards  or compact  then you probably don t want a map on the set  In either case you are iterating twice over the entire set  when a reduce only needs to go once   Also  in English  you are trying to  reduce a set of integers into a set of even integers

User · Answer

Ruby 2 7  There is now  Ruby 2 7 is introducing filter map for this exact purpose  It s idiomatic and performant  and I d expect it to become the norm very soon  For example  numbers    1  2  5  8  10  13  enum filter map    i  i   2 if i even        gt   4  16  20   In your case  as the block evaluates to falsey  simply  items filter map    x  process x url     quot Ruby 2 7 adds Enumerable filter map quot  is a good read on the subject  with some performance benchmarks against some of the earlier approaches to this problem  N   100 000 enum   1 upto 1 000  Benchmark bmbm do  x    x report  quot select   map quot      N times   enum select    i  i even    map   i  i   1         x report  quot map   compact quot     N times   enum map    i  i   1 if i even    compact       x report  quot filter map quot        N times   enum filter map    i  i   1 if i even        end    Rehearsal -------------------------------------------------   select   map    8 569651   0 051319   8 620970    8 632449    map   compact   7 392666   0 133964   7 526630    7 538013    filter map      6 923772   0 022314   6 946086    6 956135    --------------------------------------- total  23 093686sec                          user     system      total        real   select   map    8 550637   0 033190   8 583827    8 597627    map   compact   7 263667   0 131180   7 394847    7 405570    filter map      6 761388   0 018223   6 779611    6 790559

User · Answer

Definitely compact is the best approach for solving this task  However  we can achieve the same result just with a simple subtraction    1  nil  3  nil  nil  -  nil     gt   1  3

User · Answer

One more way to accomplish it will be as shown below  Here  we use Enumerable each with object to collect values  and make use of Object tap to get rid of temporary variable that is otherwise needed for nil check on result of process x method   items each with object       x  obj   process x  tap   r  obj  lt  lt  r unless r nil        Complete example for illustration   items    1 2 3 4 5  def process x     rand 10   gt  5   nil   x end  items each with object       x  obj   process x  tap   r  obj  lt  lt  r unless r nil        Alternate approach   By looking at the method you are calling process x url  it is not clear what is the purpose of input x in that method   If I assume that you are going to process the value of x by passing it some url and determine which of the xs really get processed into valid non-nil results - then  may be Enumerabble group by is a better option than Enumerable map   h   items group by   x   process x  nil     Bad     Good      gt    Bad   gt  1  2    Good   gt  3  4  5    h  Good      gt   3 4 5

User · Answer

If you wanted a looser criterion for rejection  for example  to reject empty strings as well as nil  you could use    1  nil  3  0      reject  amp  blank      gt   1  3  0     If you wanted to go further and reject zero values  or apply more complex logic to the process   you could pass a block to reject    1  nil  3  0      reject do  value  value blank     value  0 end    gt   1  3    1  nil  3  0      1000  reject do  value  value blank     value  0    value gt 10 end    gt   1  3

User · Answer

each with object is probably the cleanest way to go here   new items   items each with object     do  x  memo      ret   process x x      memo  lt  lt  ret unless ret nil  end   In my opinion  each with object is better than inject reduce in conditional cases because you don t have to worry about the return value of the block

User · Answer

In your example   items map     x  process x url      1  2  3  4  5    gt   1  nil  3  nil  nil    it does not look like the values have changed other than being replaced with nil  If that is the case  then   items select  x  process x url    will suffice

[ruby] How to map and remove nil values in Ruby

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