So,what if I can only use array to implement, and 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?
Why won't a hash do if you don't want to spend the memory? Why do you think someone will have written a module that basically does what a hash does?

In reply to Re^3: Could we save the memory occupied by "undef" in an array? by JavaFan
in thread Could we save the memory occupied by "undef" in an array? by lightoverhead

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