Ruby Easiest Way to Filter Hash Keys

Question

I have a hash that looks something like this   params      irrelevant   gt   A String               choice1   gt   Oh look  another one               choice2   gt   Even more strings               choice3   gt   But wait               irrelevant2   gt   The last string      And I want a simple way to reject all the keys that aren t choice int  It could be choice1  or choice1 through choice10  It varies   How do I single out the keys with just the word choice and a digit or digits after them   Bonus   Turn the hash into a string with tab   t  as a delimiter  I did this  but it took several lines of code  Usually master Rubicians can do it in one or so lines

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Put this in an initializer  class Hash   def filter  args      return nil if args try  empty       if args size    1       args 0    args 0  to s if args 0  is a  Symbol        self select   key  key to s match args first        else       self select   key  args include  key       end   end end   Then you can do   a   1   b   b   c   c   d   d   filter  a   b      gt    a   1   b   b     or   a   1   b   b   c   c   d   d   filter   a        gt    a   1

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The easiest way is to include the gem  activesupport   or gem  active support     Then  in your class you only need to  require  active support core ext hash slice    and to call  params slice  choice1   choice2   choice3      gt    choice1  gt  Oh look  another one    choice2  gt  Even more strings    choice3  gt  But wait     I believe it s not worth it to be declaring other functions that may have bugs  and it s better to use a method that has been tweaked during last few years

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This is a one line to solve the complete original question   params select    k    k  choice    values join   t     But most the solutions above are solving a case where you need to know the keys ahead of time  using slice or simple regexp   Here is another approach that works for simple and more complex use cases  that is swappable at runtime  data      matcher   - gt  key value    COMPLEX LOGIC HERE   data select  amp matcher    Now not only this allows for more complex logic on matching the keys or the values  but it is also easier to test  and you can swap the matching logic at runtime   Ex to solve the original issue   def some method hash  matcher     hash select  amp matcher  values join   t   end  params      irrelevant   gt   A String               choice1   gt   Oh look  another one               choice2   gt   Even more strings               choice3   gt   But wait               irrelevant2   gt   The last string     some method params  - gt  k      k  choice         gt   Oh look  another one  tEven more strings  tBut wait  some method params  - gt    v    v  string         gt   Even more strings  tThe last string

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If you work with rails and you have the keys in a separate list  you can use the   notation   keys     foo   bar  hash1    foo  1  bar 2  baz  3  hash2   hash1 slice  keys    gt   foo  1  bar 2    As other answers stated  you can also use slice  to modify the hash in place  and return the erased key values

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params select   k v  k     choice d    map   k v  v  join   t

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I had a similar problem  in my case the solution was a one liner which works even if the keys aren t symbols  but you need to have the criteria keys in an array   criteria array     choice1   choice2   params select    k v  criteria array include  k       gt     choice1   gt   Oh look another one                                                             choice2   gt   Even more strings      Another example  criteria array    1  2  3   params     1   gt   A String              17   gt   Oh look  another one              25   gt   Even more strings              49   gt   But wait              105   gt   The last string     params select    k v  criteria array include  k       gt    1   gt   A String

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As for bonus question    If you have output from  select method like this  list of 2-element arrays       choice1   Oh look  another one      choice2   Even more strings      choice3   But wait      then simply take this result and execute   filtered params join   t     or if you want only values instead of pairs key-value filtered params map  amp  last  join   t    If you have output from  delete if method like this  hash      choice1  gt  Oh look  another one    choice2  gt  Even more strings    choice3  gt  But wait     then   filtered params to a join   t     or filtered params values join   t

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params      irrelevant   gt   A String               choice1   gt   Oh look  another one               choice2   gt   Even more strings               choice3   gt   But wait               irrelevant2   gt   The last string     choices   params select    key  value  key to s   choice d         gt    choice1  gt  Oh look  another one    choice2  gt  Even more strings    choice3  gt  But wait

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The easiest way is to include the gem  activesupport   or gem  active support     params slice  choice1   choice2   choice3

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In Ruby  the Hash select is a right option  If you work with Rails  you can use Hash slice and Hash slice   e g   rails 3 2 13   h1     a   gt  1   b   gt  2   c   gt  3   d   gt  4   h1 slice  a   b            return   a  gt 1   b  gt 2   but h1 is not changed  h2   h1 slice   a   b      h1     a  gt 1   b  gt 2   h2     c   gt  3   d   gt  4

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If you want the remaining hash   params delete if   k  v    k match  choice 0-9        or if you just want the keys   params keys delete if   k    k match  choice 0-9

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Edit to original answer  Even though this is answer  as of the time of this comment  is the selected answer  the original version of this answer is outdated   I m adding an update here to help others avoid getting sidetracked by this answer like I did   As the other answer mentions  Ruby    2 5 added the Hash slice method which was previously only available in Rails   Example    gt    one  1  two  2  three  3   slice  one   two    gt    one  gt 1   two  gt 2    End of edit  What follows is the original answer which I guess will be useful if you re on Ruby  lt  2 5 without Rails  although I imagine that case is pretty uncommon at this point     If you re using Ruby  you can use the select method  You ll need to convert the key from a Symbol to a String to do the regexp match  This will give you a new Hash with just the choices in it   choices   params select    key  value  key to s match   choice d        or you can use delete if and modify the existing Hash e g   params delete if    key  value   key to s match  choice d        or if it is just the keys and not the values you want then you can do   params keys select    key  key to s match   choice d        and this will give the just an Array of the keys e g    choice1   choice2   choice3

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With Hash  select   params   params select    key  value    choice d    match key to s

[ruby] Ruby: Easiest Way to Filter Hash Keys?

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