What s the safest way to iterate through the keys of a Perl hash

Question

If I have a Perl hash with a bunch of  key  value  pairs  what is the preferred method of iterating through all the keys   I have heard that using each may in some way have unintended side effects   So  is that true  and is one of the two following methods best  or is there a better way     Method 1 while  my   key   value    each  hash           Something      Method 2 foreach my  key  keys  hash           Something

User · Answer

Using the each syntax will prevent the entire set of keys from being generated at once. This can be important if you're using a tie-ed hash to a database with millions of rows. You don't want to generate the entire list of keys all at once and exhaust your physical memory. In this case each serves as an iterator whereas keys actually generates the entire array before the loop starts.

So, the only place "each" is of real use is when the hash is very large (compared to the memory available). That is only likely to happen when the hash itself doesn't live in memory itself unless you're programming a handheld data collection device or something with small memory.

If memory is not an issue, usually the map or keys paradigm is the more prevelant and easier to read paradigm.

User · Answer

One thing you should be aware of when using each is that it has the side effect of adding  state  to your hash  the hash has to remember what the  next  key is   When using code like the snippets posted above  which iterate over the whole hash in one go  this is usually not a problem  However  you will run into hard to track down problems  I speak from experience     when using each together with statements like last or return to exit from the while     each loop before you have processed all keys   In this case  the hash will remember which keys it has already returned  and when you use each on it the next time  maybe in a totaly unrelated piece of code   it will continue at this position    Example   my  hash     foo   gt  1  bar   gt  2  baz   gt  3  quux   gt  4       find key  baz  while   my   k   v    each  hash         print  found key  k n       last if  k eq  baz     found it       later      print  the hash contains  n      iterate over all keys  while   my   k   v    each  hash         print   k   gt   v n       This prints   found key bar found key baz the hash contains  quux   gt  4 foo   gt  1   What happened to keys  bar  and baz   They re still there  but the second each starts where the first one left off  and stops when it reaches the end of the hash  so we never see them in the second loop

User · Answer

I may get bitten by this one but I think that it s personal preference   I can t find any reference in the docs to each   being different than keys   or values    other than the obvious  they return different things  answer   In fact the docs state the use the same iterator and they all return actual list values instead of copies of them  and that modifying the hash while iterating over it using any call is bad   All that said  I almost always use keys   because to me it is usually more self documenting to access the key s value via the hash itself   I occasionally use values   when the value is a reference to a large structure and the key to the hash was already stored in the structure  at which point the key is redundant and I don t need it   I think I ve used each   2 times in 10 years of Perl programming and it was probably the wrong choice both times

User · Answer

A few miscellaneous thoughts on this topic    There is nothing unsafe about any of the hash iterators themselves  What is unsafe is modifying the keys of a hash while you re iterating over it   It s perfectly safe to modify the values   The only potential side-effect I can think of is that values returns aliases which means that modifying them will modify the contents of the hash  This is by design but may not be what you want in some circumstances  John s accepted answer is good with one exception  the documentation is clear that it is not safe to add keys while iterating over a hash  It may work for some data sets but will fail for others depending on the hash order  As already noted  it is safe to delete the last key returned by each  This is not true for keys as each is an iterator while keys returns a list

User · Answer

The rule of thumb is to use the function most suited to your needs   If you just want the keys and do not plan to ever read any of the values  use keys     foreach my  key  keys  hash            If you just want the values  use values     foreach my  val  values  hash            If you need the keys and the values  use each     keys  hash    reset the internal iterator so a prior each   doesn t affect the loop while my  k   v    each  hash            If you plan to change the keys of the hash in any way except for deleting the current key during the iteration  then you must not use each     For example  this code to create a new set of uppercase keys with doubled values works fine using keys      h    a   gt  1  b   gt  2    foreach my  k  keys  h       h uc  k     h  k    2      producing the expected resulting hash    a   gt  1  A   gt  2  b   gt  2  B   gt  4    But using each   to do the same thing    h    a   gt  1  b   gt  2    keys  h  while my  k   v    each  h       h uc  k     h  k    2    BAD IDEA      produces incorrect results in hard-to-predict ways   For example    a   gt  1  A   gt  2  b   gt  2  B   gt  8    This  however  is safe   keys  h  while my  k   v    each  h      if              delete  h  k     This is safe         All of this is described in the perl documentation     perldoc -f keys   perldoc -f each

User · Answer

I always use method 2 as well   The only benefit of using each is if you re just reading  rather than re-assigning  the value of the hash entry  you re not constantly de-referencing the hash

User · Answer

The place where each can cause you problems is that it s a true  non-scoped iterator   By way of example   while   my   key  val    each  a hash         print   key   gt   val n       last if  val   exits loop when  val is true      but  each  hasn t reset   while   my   key  val    each  a hash           continues where the last loop left off     print   key   gt   val n       If you need to be sure that each gets all the keys and values  you need to make sure you use keys or values first  as that resets the iterator    See the documentation for each

User · Answer

I woudl say    Use whatever s easiest to read understand for most people  so keys  usually  I d argue  Use whatever you decide consistently throught the whole code base     This give 2 major advantages    It s easier to spot  common  code so you can re-factor into functions methiods  It s easier for future developers to maintain    I don t think it s more expensive to use keys over each  so no need for two different constructs for the same thing in your code

User · Answer

I usually use keys and I can t think of the last time I used or read a use of each   Don t forget about map  depending on what you re doing in the loop   map   print       gt   hash     n    keys  hash

[perl] What's the safest way to iterate through the keys of a Perl hash?

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