In Perl how do I create a hash whose keys come from a given array

Question

Let s say I have an array  and I know I m going to be doing a lot of  Does the array contain X   checks  The efficient way to do this is to turn that array into a hash  where the keys are the array s elements  and then you can just say if  hash X            Is there an easy way to do this array-to-hash conversion  Ideally  it should be versatile enough to take an anonymous array and return an anonymous hash

User · Accepted Answer

hash   map        gt  1    array    It s not as short as the   hash  array         solutions  but those ones require the hash and array to already be defined somewhere else  whereas this one can take an anonymous array and return an anonymous hash   What this does is take each element in the array and pair it up with a  1   When this list of  key  1  key  1  key 1  pairs get assigned to a hash  the odd-numbered ones become the hash s keys  and the even-numbered ones become the respective values

User · Answer

There is a presupposition here  that the most efficient way to do a lot of  quot Does the array contain X  quot  checks is to convert the array to a hash  Efficiency depends on the scarce resource  often time but sometimes space and sometimes programmer effort  You are at least doubling the memory consumed by keeping a list and a hash of the list around simultaneously  Plus you re writing more original code that you ll need to test  document  etc  As an alternative  look at the List  MoreUtils module  specifically the functions any    none    true   and false    They all take a block as the conditional and a list as the argument  similar to map   and grep    print  quot At least one value undefined quot  if any    defined        list  I ran a quick test  loading in half of  usr share dict words to an array  25000 words   then looking for eleven words selected from across the whole dictionary  every 5000th word  in the array  using both the array-to-hash method and the any   function from List  MoreUtils  On Perl 5 8 8 built from source  the array-to-hash method runs almost 1100x faster than the any   method  1300x faster under Ubuntu 6 06 s packaged Perl 5 8 7   That s not the full story however - the array-to-hash conversion takes about 0 04 seconds which in this case kills the time efficiency of array-to-hash method to 1 5x-2x faster than the any   method  Still good  but not nearly as stellar  My gut feeling is that the array-to-hash method is going to beat any   in most cases  but I d feel a whole lot better if I had some more solid metrics  lots of test cases  decent statistical analyses  maybe some big-O algorithmic analysis of each method  etc   Depending on your needs  List  MoreUtils may be a better solution  it s certainly more flexible and requires less coding  Remember  premature optimization is a sin

User · Answer

usr bin perl -w  use strict  use Data  Dumper   my  a   qw 5 8 2 5 4 8 9   my  b   qw 7 6 5 4 3 2 1   my  h           h   a     b   print Dumper  h     gives  note repeated keys get the value at the greatest position in the array - ie 8- 2 and not 6    VAR1                8    gt   2              4    gt   3              9    gt   1              2    gt   5              5    gt   4

User · Answer

There is a presupposition here  that the most efficient way to do a lot of  quot Does the array contain X  quot  checks is to convert the array to a hash  Efficiency depends on the scarce resource  often time but sometimes space and sometimes programmer effort  You are at least doubling the memory consumed by keeping a list and a hash of the list around simultaneously  Plus you re writing more original code that you ll need to test  document  etc  As an alternative  look at the List  MoreUtils module  specifically the functions any    none    true   and false    They all take a block as the conditional and a list as the argument  similar to map   and grep    print  quot At least one value undefined quot  if any    defined        list  I ran a quick test  loading in half of  usr share dict words to an array  25000 words   then looking for eleven words selected from across the whole dictionary  every 5000th word  in the array  using both the array-to-hash method and the any   function from List  MoreUtils  On Perl 5 8 8 built from source  the array-to-hash method runs almost 1100x faster than the any   method  1300x faster under Ubuntu 6 06 s packaged Perl 5 8 7   That s not the full story however - the array-to-hash conversion takes about 0 04 seconds which in this case kills the time efficiency of array-to-hash method to 1 5x-2x faster than the any   method  Still good  but not nearly as stellar  My gut feeling is that the array-to-hash method is going to beat any   in most cases  but I d feel a whole lot better if I had some more solid metrics  lots of test cases  decent statistical analyses  maybe some big-O algorithmic analysis of each method  etc   Depending on your needs  List  MoreUtils may be a better solution  it s certainly more flexible and requires less coding  Remember  premature optimization is a sin

User · Answer

You can place the code into a subroutine  if you don t want pollute your namespace   my  hash ref     sub      my  hash       hash      qw one two three         undef      return   hash     - gt       Or even better   sub keylist       my  hash     hash       undef    return   hash     my  hash ref   keylist qw one two three      or  my  key list   qw one two three   my  hash ref   keylist  key list    If you really wanted to pass an array reference   sub keylist        my  hash     hash       0       undef if       return   hash     my  key list   qw one two three   my  hash ref   keylist  key list

User · Answer

Note that if typing if   exists  hash  key     isn   t too much work for you  which I prefer to use since the matter of interest is really the presence of a key rather than the truthiness of its value   then you can use the short and sweet   hash  key

User · Answer

You could also use Perl6  Junction   use Perl6  Junction qw any    my  arr     1  2  3     if  any  arr     1

User · Answer

There is a presupposition here  that the most efficient way to do a lot of  quot Does the array contain X  quot  checks is to convert the array to a hash  Efficiency depends on the scarce resource  often time but sometimes space and sometimes programmer effort  You are at least doubling the memory consumed by keeping a list and a hash of the list around simultaneously  Plus you re writing more original code that you ll need to test  document  etc  As an alternative  look at the List  MoreUtils module  specifically the functions any    none    true   and false    They all take a block as the conditional and a list as the argument  similar to map   and grep    print  quot At least one value undefined quot  if any    defined        list  I ran a quick test  loading in half of  usr share dict words to an array  25000 words   then looking for eleven words selected from across the whole dictionary  every 5000th word  in the array  using both the array-to-hash method and the any   function from List  MoreUtils  On Perl 5 8 8 built from source  the array-to-hash method runs almost 1100x faster than the any   method  1300x faster under Ubuntu 6 06 s packaged Perl 5 8 7   That s not the full story however - the array-to-hash conversion takes about 0 04 seconds which in this case kills the time efficiency of array-to-hash method to 1 5x-2x faster than the any   method  Still good  but not nearly as stellar  My gut feeling is that the array-to-hash method is going to beat any   in most cases  but I d feel a whole lot better if I had some more solid metrics  lots of test cases  decent statistical analyses  maybe some big-O algorithmic analysis of each method  etc   Depending on your needs  List  MoreUtils may be a better solution  it s certainly more flexible and requires less coding  Remember  premature optimization is a sin

User · Answer

You could also use Perl6  Junction   use Perl6  Junction qw any    my  arr     1  2  3     if  any  arr     1

User · Answer

Raldi s solution can be tightened up to this  the      from the original is not necessary    my  hash   map      1    array    This technique can also be used for turning text lists into hashes   my  hash   map      1   split      line    Additionally if you have a line of values like this   foo 1 bar 2 baz 3  you can do this   my  hash   map   split           split      line      EDIT to include     Another solution offered  which takes two lines  is   my  hash   The values in  hash can only be accessed by doing exists  hash  key    The assignment only works with    undef   and will not work properly with    1    if you do    1   only the hash key of  array 0  will be set to 1   hash  array    undef

User · Answer

In perl 5 10  there s the close-to-magic    operator   sub invite in       my  vampires     qw Angel Darla Spike Drusilla         return     0      vampires    0   1       See here  http   dev perl org perl5 news 2007 perl-5 10 0 html

User · Answer

In perl 5 10  there s the close-to-magic    operator   sub invite in       my  vampires     qw Angel Darla Spike Drusilla         return     0      vampires    0   1       See here  http   dev perl org perl5 news 2007 perl-5 10 0 html

User · Answer

Also worth noting for completeness  my usual method for doing this with 2 same-length arrays  keys and  vals which you would prefer were a hash     my  hash   map    keys       gt   vals        0   keys-1

User · Answer

You could also use Perl6  Junction   use Perl6  Junction qw any    my  arr     1  2  3     if  any  arr     1

User · Answer

I always thought that   foreach my  item   array     hash  item    1     was at least nice and readable   maintainable

User · Answer

Note that if typing if   exists  hash  key     isn   t too much work for you  which I prefer to use since the matter of interest is really the presence of a key rather than the truthiness of its value   then you can use the short and sweet   hash  key

User · Answer

usr bin perl -w  use strict  use Data  Dumper   my  a   qw 5 8 2 5 4 8 9   my  b   qw 7 6 5 4 3 2 1   my  h           h   a     b   print Dumper  h     gives  note repeated keys get the value at the greatest position in the array - ie 8- 2 and not 6    VAR1                8    gt   2              4    gt   3              9    gt   1              2    gt   5              5    gt   4

User · Answer

Raldi s solution can be tightened up to this  the      from the original is not necessary    my  hash   map      1    array    This technique can also be used for turning text lists into hashes   my  hash   map      1   split      line    Additionally if you have a line of values like this   foo 1 bar 2 baz 3  you can do this   my  hash   map   split           split      line      EDIT to include     Another solution offered  which takes two lines  is   my  hash   The values in  hash can only be accessed by doing exists  hash  key    The assignment only works with    undef   and will not work properly with    1    if you do    1   only the hash key of  array 0  will be set to 1   hash  array    undef

User · Answer

You can place the code into a subroutine  if you don t want pollute your namespace   my  hash ref     sub      my  hash       hash      qw one two three         undef      return   hash     - gt       Or even better   sub keylist       my  hash     hash       undef    return   hash     my  hash ref   keylist qw one two three      or  my  key list   qw one two three   my  hash ref   keylist  key list    If you really wanted to pass an array reference   sub keylist        my  hash     hash       0       undef if       return   hash     my  key list   qw one two three   my  hash ref   keylist  key list

User · Answer

You can place the code into a subroutine  if you don t want pollute your namespace   my  hash ref     sub      my  hash       hash      qw one two three         undef      return   hash     - gt       Or even better   sub keylist       my  hash     hash       undef    return   hash     my  hash ref   keylist qw one two three      or  my  key list   qw one two three   my  hash ref   keylist  key list    If you really wanted to pass an array reference   sub keylist        my  hash     hash       0       undef if       return   hash     my  key list   qw one two three   my  hash ref   keylist  key list

User · Answer

There is a presupposition here  that the most efficient way to do a lot of  quot Does the array contain X  quot  checks is to convert the array to a hash  Efficiency depends on the scarce resource  often time but sometimes space and sometimes programmer effort  You are at least doubling the memory consumed by keeping a list and a hash of the list around simultaneously  Plus you re writing more original code that you ll need to test  document  etc  As an alternative  look at the List  MoreUtils module  specifically the functions any    none    true   and false    They all take a block as the conditional and a list as the argument  similar to map   and grep    print  quot At least one value undefined quot  if any    defined        list  I ran a quick test  loading in half of  usr share dict words to an array  25000 words   then looking for eleven words selected from across the whole dictionary  every 5000th word  in the array  using both the array-to-hash method and the any   function from List  MoreUtils  On Perl 5 8 8 built from source  the array-to-hash method runs almost 1100x faster than the any   method  1300x faster under Ubuntu 6 06 s packaged Perl 5 8 7   That s not the full story however - the array-to-hash conversion takes about 0 04 seconds which in this case kills the time efficiency of array-to-hash method to 1 5x-2x faster than the any   method  Still good  but not nearly as stellar  My gut feeling is that the array-to-hash method is going to beat any   in most cases  but I d feel a whole lot better if I had some more solid metrics  lots of test cases  decent statistical analyses  maybe some big-O algorithmic analysis of each method  etc   Depending on your needs  List  MoreUtils may be a better solution  it s certainly more flexible and requires less coding  Remember  premature optimization is a sin

User · Answer

hash  array     1  x  array    It s a hash slice  a list of values from the hash  so it gets the list-y   in front   From the docs      If you re confused about why you use   an     there on a hash slice instead   of a      think of it like this  The   type of bracket  square or curly    governs whether it s an array or a   hash being looked at  On the other   hand  the leading symbol      or        on the array or hash indicates whether   you are getting back a singular value    a scalar  or a plural one  a list

User · Answer

hash  array     1  x  array    It s a hash slice  a list of values from the hash  so it gets the list-y   in front   From the docs      If you re confused about why you use   an     there on a hash slice instead   of a      think of it like this  The   type of bracket  square or curly    governs whether it s an array or a   hash being looked at  On the other   hand  the leading symbol      or        on the array or hash indicates whether   you are getting back a singular value    a scalar  or a plural one  a list

User · Answer

You can place the code into a subroutine  if you don t want pollute your namespace   my  hash ref     sub      my  hash       hash      qw one two three         undef      return   hash     - gt       Or even better   sub keylist       my  hash     hash       undef    return   hash     my  hash ref   keylist qw one two three      or  my  key list   qw one two three   my  hash ref   keylist  key list    If you really wanted to pass an array reference   sub keylist        my  hash     hash       0       undef if       return   hash     my  key list   qw one two three   my  hash ref   keylist  key list

User · Answer

If you do a lot of set theoretic operations - you can also use Set  Scalar or similar module   Then  s   Set  Scalar- gt new   array   will build the Set for you - and you can query it with   s- gt contains  m

User · Answer

hash  keys    undef    The syntax here where you are referring to the hash with an   is a hash slice   We re basically saying  hash  keys 0   AND  hash  keys 1   AND  hash  keys 2       is a list on the left hand side of the    an lvalue  and we re assigning to that list  which actually goes into the hash and sets the values for all the named keys   In this case  I only specified one value  so that value goes into  hash  keys 0    and the other hash entries all auto-vivify  come to life  with undefined values    My original suggestion here was set the expression   1  which would ve set that one key to 1 and the others to undef   I changed it for consistency  but as we ll see below  the exact values do not matter    When you realize that the lvalue  the expression on the left hand side of the    is a list built out of the hash  then it ll start to make some sense why we re using that      Except I think this will change in Perl 6    The idea here is that you are using the hash as a set   What matters is not the value I am assigning  it s just the existence of the keys   So what you want to do is not something like   if   hash  key     1    then key is in the hash   instead   if  exists  hash  key     then key is in the set   It s actually more efficient to just run an exists check than to bother with the value in the hash  although to me the important thing here is just the concept that you are representing a set just with the keys of the hash   Also  somebody pointed out that by using undef as the value here  we will consume less storage space than we would assigning a value    And also generate less confusion  as the value does not matter  and my solution would assign a value only to the first element in the hash and leave the others undef  and some other solutions are turning cartwheels to build an array of values to go into the hash  completely wasted effort

User · Answer

hash  array     1  x  array    It s a hash slice  a list of values from the hash  so it gets the list-y   in front   From the docs      If you re confused about why you use   an     there on a hash slice instead   of a      think of it like this  The   type of bracket  square or curly    governs whether it s an array or a   hash being looked at  On the other   hand  the leading symbol      or        on the array or hash indicates whether   you are getting back a singular value    a scalar  or a plural one  a list

User · Answer

hash  array     1  x  array    It s a hash slice  a list of values from the hash  so it gets the list-y   in front   From the docs      If you re confused about why you use   an     there on a hash slice instead   of a      think of it like this  The   type of bracket  square or curly    governs whether it s an array or a   hash being looked at  On the other   hand  the leading symbol      or        on the array or hash indicates whether   you are getting back a singular value    a scalar  or a plural one  a list

User · Answer

hash  keys    undef    The syntax here where you are referring to the hash with an   is a hash slice   We re basically saying  hash  keys 0   AND  hash  keys 1   AND  hash  keys 2       is a list on the left hand side of the    an lvalue  and we re assigning to that list  which actually goes into the hash and sets the values for all the named keys   In this case  I only specified one value  so that value goes into  hash  keys 0    and the other hash entries all auto-vivify  come to life  with undefined values    My original suggestion here was set the expression   1  which would ve set that one key to 1 and the others to undef   I changed it for consistency  but as we ll see below  the exact values do not matter    When you realize that the lvalue  the expression on the left hand side of the    is a list built out of the hash  then it ll start to make some sense why we re using that      Except I think this will change in Perl 6    The idea here is that you are using the hash as a set   What matters is not the value I am assigning  it s just the existence of the keys   So what you want to do is not something like   if   hash  key     1    then key is in the hash   instead   if  exists  hash  key     then key is in the set   It s actually more efficient to just run an exists check than to bother with the value in the hash  although to me the important thing here is just the concept that you are representing a set just with the keys of the hash   Also  somebody pointed out that by using undef as the value here  we will consume less storage space than we would assigning a value    And also generate less confusion  as the value does not matter  and my solution would assign a value only to the first element in the hash and leave the others undef  and some other solutions are turning cartwheels to build an array of values to go into the hash  completely wasted effort

User · Answer

You could also use Perl6  Junction   use Perl6  Junction qw any    my  arr     1  2  3     if  any  arr     1

User · Answer

hash  keys    undef    The syntax here where you are referring to the hash with an   is a hash slice   We re basically saying  hash  keys 0   AND  hash  keys 1   AND  hash  keys 2       is a list on the left hand side of the    an lvalue  and we re assigning to that list  which actually goes into the hash and sets the values for all the named keys   In this case  I only specified one value  so that value goes into  hash  keys 0    and the other hash entries all auto-vivify  come to life  with undefined values    My original suggestion here was set the expression   1  which would ve set that one key to 1 and the others to undef   I changed it for consistency  but as we ll see below  the exact values do not matter    When you realize that the lvalue  the expression on the left hand side of the    is a list built out of the hash  then it ll start to make some sense why we re using that      Except I think this will change in Perl 6    The idea here is that you are using the hash as a set   What matters is not the value I am assigning  it s just the existence of the keys   So what you want to do is not something like   if   hash  key     1    then key is in the hash   instead   if  exists  hash  key     then key is in the set   It s actually more efficient to just run an exists check than to bother with the value in the hash  although to me the important thing here is just the concept that you are representing a set just with the keys of the hash   Also  somebody pointed out that by using undef as the value here  we will consume less storage space than we would assigning a value    And also generate less confusion  as the value does not matter  and my solution would assign a value only to the first element in the hash and leave the others undef  and some other solutions are turning cartwheels to build an array of values to go into the hash  completely wasted effort

User · Answer

Raldi s solution can be tightened up to this  the      from the original is not necessary    my  hash   map      1    array    This technique can also be used for turning text lists into hashes   my  hash   map      1   split      line    Additionally if you have a line of values like this   foo 1 bar 2 baz 3  you can do this   my  hash   map   split           split      line      EDIT to include     Another solution offered  which takes two lines  is   my  hash   The values in  hash can only be accessed by doing exists  hash  key    The assignment only works with    undef   and will not work properly with    1    if you do    1   only the hash key of  array 0  will be set to 1   hash  array    undef

User · Answer

In perl 5 10  there s the close-to-magic    operator   sub invite in       my  vampires     qw Angel Darla Spike Drusilla         return     0      vampires    0   1       See here  http   dev perl org perl5 news 2007 perl-5 10 0 html

User · Answer

Note that if typing if   exists  hash  key     isn   t too much work for you  which I prefer to use since the matter of interest is really the presence of a key rather than the truthiness of its value   then you can use the short and sweet   hash  key

User · Answer

Note that if typing if   exists  hash  key     isn   t too much work for you  which I prefer to use since the matter of interest is really the presence of a key rather than the truthiness of its value   then you can use the short and sweet   hash  key

User · Answer

You might also want to check out Tie  IxHash  which implements ordered associative arrays  That would allow you to do both types of lookups  hash and index  on one copy of your data

User · Answer

Also worth noting for completeness  my usual method for doing this with 2 same-length arrays  keys and  vals which you would prefer were a hash     my  hash   map    keys       gt   vals        0   keys-1

User · Answer

hash  keys    undef    The syntax here where you are referring to the hash with an   is a hash slice   We re basically saying  hash  keys 0   AND  hash  keys 1   AND  hash  keys 2       is a list on the left hand side of the    an lvalue  and we re assigning to that list  which actually goes into the hash and sets the values for all the named keys   In this case  I only specified one value  so that value goes into  hash  keys 0    and the other hash entries all auto-vivify  come to life  with undefined values    My original suggestion here was set the expression   1  which would ve set that one key to 1 and the others to undef   I changed it for consistency  but as we ll see below  the exact values do not matter    When you realize that the lvalue  the expression on the left hand side of the    is a list built out of the hash  then it ll start to make some sense why we re using that      Except I think this will change in Perl 6    The idea here is that you are using the hash as a set   What matters is not the value I am assigning  it s just the existence of the keys   So what you want to do is not something like   if   hash  key     1    then key is in the hash   instead   if  exists  hash  key     then key is in the set   It s actually more efficient to just run an exists check than to bother with the value in the hash  although to me the important thing here is just the concept that you are representing a set just with the keys of the hash   Also  somebody pointed out that by using undef as the value here  we will consume less storage space than we would assigning a value    And also generate less confusion  as the value does not matter  and my solution would assign a value only to the first element in the hash and leave the others undef  and some other solutions are turning cartwheels to build an array of values to go into the hash  completely wasted effort

User · Answer

Raldi s solution can be tightened up to this  the      from the original is not necessary    my  hash   map      1    array    This technique can also be used for turning text lists into hashes   my  hash   map      1   split      line    Additionally if you have a line of values like this   foo 1 bar 2 baz 3  you can do this   my  hash   map   split           split      line      EDIT to include     Another solution offered  which takes two lines  is   my  hash   The values in  hash can only be accessed by doing exists  hash  key    The assignment only works with    undef   and will not work properly with    1    if you do    1   only the hash key of  array 0  will be set to 1   hash  array    undef

User · Answer

If you do a lot of set theoretic operations - you can also use Set  Scalar or similar module   Then  s   Set  Scalar- gt new   array   will build the Set for you - and you can query it with   s- gt contains  m

User · Answer

You might also want to check out Tie  IxHash  which implements ordered associative arrays  That would allow you to do both types of lookups  hash and index  on one copy of your data

User · Answer

I always thought that   foreach my  item   array     hash  item    1     was at least nice and readable   maintainable

User · Answer

In perl 5 10  there s the close-to-magic    operator   sub invite in       my  vampires     qw Angel Darla Spike Drusilla         return     0      vampires    0   1       See here  http   dev perl org perl5 news 2007 perl-5 10 0 html

[arrays] In Perl, how do I create a hash whose keys come from a given array?

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