How do you add an array to another array in Ruby and not end up with a multi-dimensional result

Question

somearray     some    thing    anotherarray     another    thing    somearray push anotherarray flatten     I expected    some   thing   another   thing

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somearray     some    thing    anotherarray     another    thing    somearray   anotherarray

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quot some quot    quot thing quot       quot another quot    quot thing quot

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The question  essentially  is  how to concatenate arrays in Ruby   Naturally the answer is to use concat or   as mentioned in nearly every answer   A natural extension to the question would be  how to perform row-wise concatenation of 2D arrays in Ruby   When I googled  ruby concatenate matrices   this SO question was the top result so I thought I would leave my answer to that  unasked but related  question here for posterity     In some applications you might want to  concatenate  two 2D arrays row-wise  Something like     a  b       x        a  b  x     c  d       y     gt    c  d  y     This is something like  augmenting  a matrix  For example  I used this technique to create a single adjacency matrix to represent a graph out of a bunch of smaller matrices  Without this technique I would have had to iterate over the components in a way that could have been error prone or frustrating to think about  I might have had to do an each with index  for example  Instead I combined zip and flatten as follows     given two multi-dimensional arrays that you want to concatenate row-wise m1      a   b     c   d   m2      x     y    m1m2   m1 zip m2  map  amp  flatten      gt     a   b   x     c   d   y

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Try this  it will combine your arrays removing duplicates  array1     foo    bar   array2     foo1    bar1    array3   array1 array2   http   www ruby-doc org core classes Array html  Further documentation look at  Set Union

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Just another way of doing it    somearray  anotherarray  flatten   gt    some    thing    another    thing

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a     a    b   b     c    d   arr    a  b  flatten   This won t remove dups  but   a b   removes dups

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Elaborating on  Pilcrow s answer the only suitable answer for huge arrays is concat     since is fast and does not allocate a new object to be garbage-collected when operating inside a loop   Here s the benchmark   require  benchmark   huge ary 1   Array new 1 000 000    rand 5 000 000  30 000 00     huge ary 2   Array new 1 000 000    rand 35 000 000  55 000 00     Benchmark bm do  bm    p  -------------------CONCAT ----------------    bm report   huge ary 1 concat huge ary 2       p  ------------------- PUSH ----------------    bm report   huge ary 1 push  huge ary 2     end   Results          user     system      total        real  -------------------CONCAT ----------------    0 000000   0 000000   0 000000    0 009388   ------------------- PUSH ----------------    example array concat vs push rb 13 in  block  2 levels  in  lt main gt    stack level too deep  SystemStackError    As you can see using push throws an ERROR  stack level too deep  SystemStackError  when the arrays are big enough

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You ve got a workable idea  but the  flatten  is in the wrong place -- it flattens its receiver  so you could use it to turn  1  2    foo    bar    into  1 2  foo   bar     I m doubtless forgetting some approaches  but you can concatenate   a1 concat a2 a1   a2                creates a new array  as does a1    a2   or prepend append   a1 push  a2            note the asterisk a2 unshift  a1         note the asterisk  and that a2 is the receiver   or splice   a1 a1 length  0    a2 a1 a1 length  0    a2 a1 insert a1 length   a2    or append and flatten    a1  lt  lt  a2  flatten     a call to  flatten instead would return a new array

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array1   array2  uniq  This way you get array1 elements first  You will get no duplicates

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a     some    thing   b     another    thing     To append b to a and store the result in a   a push  b    or  a    b   In either case  a becomes     some    thing    another    thing     but in the former case  the elements of b are appended to the existing a array  and in the latter case the two arrays are concatenated together and the result is stored in a

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You can just use the   operator   irb main  001 0 gt  a    1 2    gt   1  2  irb main  002 0 gt  b    3 4    gt   3  4  irb main  003 0 gt  a   b   gt   1  2  3  4    You can read all about the array class here  http   ruby-doc org core classes Array html

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If the new data could be an array or a scalar  and you want to prevent the new data to be nested if it was an array  the splat operator is awesome  It returns a scalar for a scalar  and an unpacked list of arguments for an array   1 9 3-p551  020  gt  a    1  2     gt   1  2   1 9 3-p551  021  gt  b    3  4     gt   3  4   1 9 3-p551  022  gt  c   5    gt  5  1 9 3-p551  023  gt  a object id    gt  6617020  1 9 3-p551  024  gt  a push  b    gt   1  2  3  4   1 9 3-p551  025  gt  a object id    gt  6617020  1 9 3-p551  026  gt  a push  c    gt   1  2  3  4  5   1 9 3-p551  027  gt  a object id    gt  6617020

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I m surprised nobody has mentioned reduce  which works well when you have an array of arrays   lists      a    b      c    d    flatlist   lists reduce          a    b    c    d

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Easy method that works with Ruby version    2 0 but not with older versions    irb main  001 0 gt  a  1 2    gt   1  2  irb main  003 0 gt  b  3 4    gt   3  4  irb main  002 0 gt  c  5 6    gt   5  6  irb main  004 0 gt    a  b  c    gt   1  2  3  4  5  6

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I find it easier to push or append arrays and then flatten them in place  like so   somearray     some    thing   anotherarray     another    thing   somearray push anotherarray     gt    some    thing     another    thing     or somearray  lt  lt  anotherarray     gt    some    thing     another    thing    somearray flatten       gt    some    thing    another    thing   somearray     gt    some    thing    another    thing

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Here are two ways  notice in this case that the first way assigns a new array   translates to somearray   somearray   anotherarray    somearray     some    thing    anotherarray     another    thing    somearray    anotherarray     gt    some    thing    another    thing    somearray     some    thing   somearray concat anotherarray     gt    some    thing    another    thing

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The cleanest approach is to use the Array concat method  it will not create a new array  unlike Array   which will do the same thing but create a new array   Straight from the docs  http   www ruby-doc org core-1 9 3 Array html method-i-concat    concat other ary   Appends the elements of other ary to self    So  1 2  concat  3 4       gt   1 2 3 4     Array concat will not flatten a multidimensional array if it is passed in as an argument  You ll need to handle that separately  arr   3  4 5   arr  arr flatten      gt   3 4 5   1 2  concat arr      gt   1 2 3 4 5   Lastly  you can use our corelib gem  https   github com corlewsolutions corelib  which adds useful helpers to the Ruby core classes   In particular we have an Array add all method which will automatically flatten multidimensional arrays before executing the concat

[ruby] How do you add an array to another array in Ruby and not end up with a multi-dimensional result?

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