in reply to Re^2: Could we save the memory occupied by "undef" in an array?
in thread Could we save the memory occupied by "undef" in an array?

Before you complain about the speed of a hash, benchmark it. The difference between a hash and an array shouldn't be much. I doubt a sparse array would be any faster.

Your program is already heavily invested in the performance of hashes. Most objects use hashes as their underlying data structure. Every reference to a package variable results in multiple hash lookups (I think). And every call to a subroutine results in multiple hash lookups (I think).

Is there a method or package that can squash the memory occupied by "undef" in an array and return the the positions back when I try to fill something into the position that was occupied by a undef, sort of like autovivification?

The benefit of arrays come from knowing exactly where to look into memory for a given element knowing only a base address and the element index. That becomes impossible if some of the elements are missing.

Now, there could be something more efficient data structure than a hash for your needs, but accessing it would require calling a subroutine/method. I believe that results in a hash lookup to find the code pointer associated with the subroutine name, so you're worse off than you started.

On the plus side, arrays use the most efficient of two kinds of undef by default if I read what follows correctly. Instead of creating a new undef SV, uninitialized array elements refer to a global undef constant. That means only sizeof(void*) bytes are wasted per unused slot instead of sizeof(void*) + sizeof(SV).

>perl -MDevel::Peek -e"$a[2]=undef; $a[3]=3; Dump \@a" SV = RV(0x183c518) at 0x22609c REFCNT = 1 FLAGS = (TEMP,ROK) RV = 0x226db0 SV = PVAV(0x22bfcc) at 0x226db0 REFCNT = 2 FLAGS = () IV = 0 NV = 0 ARRAY = 0x1833f9c FILL = 3 MAX = 3 ARYLEN = 0x0 FLAGS = (REAL) Elt No. 0 # Not allocated Elt No. 1 # Not allocated Elt No. 2 # Allocated but undef SV = NULL(0x0) at 0x225f7c REFCNT = 1 FLAGS = () Elt No. 3 # Allocated and defined SV = IV(0x18287b4) at 0x22606c REFCNT = 1 FLAGS = (IOK,pIOK) IV = 3