How can I check if a Perl array contains a particular value

Question

I am trying to figure out a way of checking for the existence of a value in an array without iterating through the array   I am reading a file for a parameter   I have a long list of parameters I do not want to deal with  I placed these unwanted parameters in an array  badparams   I want to read a new parameter and if it does not exist in  badparams  process it  If it does exist in  badparams  go to the next read

User · Accepted Answer

Simply turn the array into a hash   my  params   map        gt  1    badparams   if exists  params  someparam              You can also add more  unique  params to the list    params  newparam    1    And later get a list of  unique  params back    badparams   keys  params

User · Answer

Even though it s convenient to use  it seems like the convert-to-hash solution costs quite a lot of performance  which was an issue for me      usr bin perl use Benchmark  my  list  for  1  10 000        push  list         timethese 10000       grep       gt  sub               if   grep   5000  o   list                        code                             hash       gt  sub               my  params   map        gt  1    list              if   exists  params 5000                         code                                Output of benchmark test   Benchmark  timing 10000 iterations of grep  hash              grep   8 wallclock secs   7 95 usr    0 00 sys    7 95 CPU    1257 86 s  n 10000            hash  50 wallclock secs  49 68 usr    0 01 sys   49 69 CPU    201 25 s  n 10000

User · Answer

This blog post discusses the best answers to this question   As a short summary  if you can install CPAN modules then the most readable solutions are   any  ingredients  eq  flour     or   ingredients- gt contains  flour      However  a more common idiom is    any      eq  flour     ingredients   But please don t use the first   function  It doesn t express the intent of your code at all  Don t use the     Smart match  operator  it is broken  And don t use grep   nor the solution with a hash  they iterate through the whole list   any   will stop as soon as it finds your value   Check out the blog post for more details

User · Answer

my  badparams    1 2 5 7  a   zzz     my  badparams   join      badparams           or any other character not present in params  foreach my  par  4 5 6 7  a   z   zzz         if   badparams      b par b                 print   par is present n             else               print   par is not present n             You may want to check for numerical leading spaces consistancy

User · Answer

eakssjo s benchmark is broken - measures creating hashes in loop vs creating regexes in loop  Fixed version  plus I ve added List  Util  first and List  MoreUtils  any    use List  Util qw first   use List  MoreUtils qw any   use Benchmark   my  list     1  10 000    my  hit   5 000  my  hit regex   qr   hit      precompute regex my  params   params       1 for  list     precompute hash timethese      100 000             any    gt  sub               die unless   any    hit regex    list                        first    gt  sub               die unless   first    hit regex    list                        grep    gt  sub               die unless   grep    hit regex    list                        hash    gt  sub               die unless    params  hit                          And result  it s for 100 000 iterations  ten times more than in  eakssjo s answer    Benchmark  timing 100000 iterations of any  first  grep  hash           any   0 wallclock secs   0 67 usr    0 00 sys    0 67 CPU    149253 73 s  n 100000       first   1 wallclock secs   0 63 usr    0 01 sys    0 64 CPU    156250 00 s  n 100000        grep  42 wallclock secs  41 95 usr    0 08 sys   42 03 CPU    2379 25 s  n 100000        hash   0 wallclock secs   0 01 usr    0 00 sys    0 01 CPU    10000000 00 s  n 100000               warning  too few iterations for a reliable count

User · Answer

If you need to know the amount of every element in array besides existing of that element you may use my  bad param lookup   bad param lookup   bad params       1   x  bad params   bad param lookup   map        gt   bad param lookup         bad params   and then for every  i that is in  bad params   bad param lookup  i  contains amount of  i in  bad params

User · Answer

You can use smartmatch feature in Perl 5 10 as follows   For literal value lookup doing below will do the trick   if    value      array      For scalar lookup  doing below will work as above   if   val     array    For inline array doing below  will work as above   if    var      bar    value    foo        In Perl 5 18 smartmatch is flagged as experimental therefore you need to turn off the warnings by turning on experimental pragma by adding below to your script module   use experimental  smartmatch     Alternatively if you want to avoid the use of smartmatch - then as Aaron said use   if   grep     value     array          TODO

User · Answer

You certainly want a hash here   Place the bad parameters as keys in the hash  then decide whether a particular parameter exists in the hash   our  bad params   map        gt  1   qw badparam1 badparam2 badparam3   if   bad params  new param       print  That is a bad parameter n       If you are really interested in doing it with an array  look at List  Util or List  MoreUtils

User · Answer

There are two ways you can do this  You can use the throw the values into a hash for a lookup table  as suggested by the other posts    I ll add just another idiom     my  bad param lookup   bad param lookup   bad params       1   x  bad params    But if it s data of mostly word characters and not too many meta  you can dump it into a regex alternation    use English qw lt  LIST SEPARATOR gt    my  regex str   do        local  LIST SEPARATOR                 bad params             front delim and  back delim being any characters that come before and after    my  regex   qr  front delim regex str back delim     This solution would have to be tuned for the types of  bad values  you re looking for  And again  it might be totally inappropriate for certain types of strings  so caveat emptor

User · Answer

files is an existing array  my  new values    grep   2  d    d  A-za-z     files    print join   n    new values    print   n       2  d    d  A-za-z      vaues starting from 2 here you can put any regular expression

User · Answer

Method 1  grep  may careful while value is expected to be a regex   Try to avoid using grep  if looking at resources  if   grep     value     badparams         print  quot found quot      Method 2  Linear Search for   badparams        if     eq  value           print  quot found quot          last           Method 3  Use a hash my  hash   map       gt  1   badparams  print  quot found quot  if  exists  hash  value     Method 4  smartmatch  added in Perl 5 10  marked is experimental in Perl 5 18   use experimental  smartmatch      for perl 5 18 print  quot found quot  if   value     badparams    Method 5  Use the module List  MoreUtils use List  MoreUtils qw any    badparams    1 2 3    value   1  print  quot found quot  if any         value   badparams

User · Answer

Best general purpose - Especially short arrays  1000 items or less  and coders that are unsure of what optimizations best suit their needs      value can be any regex  be safe if   grep     value     array         print  found it       It has been mentioned that grep passes through all values even if the first value in the array matches   This is true  however grep is still extremely fast for most cases   If you re talking about short arrays  less than 1000 items  then most algorithms are going to be pretty fast anyway   If you re talking about very long arrays  1 000 000 items  grep is acceptably quick regardless of whether the item is the first or the middle or last in the array   Optimization Cases for longer arrays   If your array is sorted  use a  binary search    If the same array is repeatedly searched many times  copy it into a hash first and then check the hash   If memory is a concern  then move each item from the array into the hash  More memory efficient but destroys the original array   If same values are searched repeatedly within the array  lazily build a cache   as each item is searched  first check if the search result was stored in a persisted hash  if the search result is not found in the hash  then search the array and put the result in the persisted hash so that next time we ll find it in the hash and skip the search    Note  these optimizations will only be faster when dealing with long arrays   Don t over optimize

[perl] How can I check if a Perl array contains a particular value?

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