I'm reading a book on data structures and there is a comparison between linked list, array and dynamic array. The parameter name is wasted space. Here are the values:

\begin{array}{cc} \text{Linked list} & O(n)\\ \text{array} & 0 \\ \text{dynamic array}& O(n) \end{array}

What is the wasted space parameter and why is it $O(n)$ for a linked list?

  • $\begingroup$ While I've actually never seen "wasted space" defined as a term, it may be useful to contrast to the concepts of implicit and succinct data structures. $\endgroup$ – Derek Elkins left SE Apr 25 '17 at 6:32

Different people seem to define "wasted space" in different ways. Some authors define it as any space in a data structure that's not used to store actual data; others define it as space that could be used to store data but isn't being so used at the moment.

So, in the first definition, a linked list has $O(n)$ wasted space because every entry of the list contains some piece of data but also a pointer, so a constant fraction of the space taken up by the data structure is "wasted". (I think this is a silly definition of "waste": the space taken up by the pointers isn't wasted; it's an overhead.) In the second definition, a half-full hash-table has 50% wasted space (again, $O(n)$).

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  • $\begingroup$ thanks, so it's a space complexity, correct? $\endgroup$ – Max Koretskyi Apr 24 '17 at 8:42
  • $\begingroup$ It's a measure of space usage. I hesitate to use the word "complexity" as that normally refers to the resource requirements to decide a particular language, which isn't quite what we mean, here. $\endgroup$ – David Richerby Apr 24 '17 at 8:53
  • $\begingroup$ Got it, thank you $\endgroup$ – Max Koretskyi Apr 24 '17 at 9:09
  • $\begingroup$ I don't understand then why it's also O(n) for a dynamic array, can you please clarify? Dynamic array usually holds only a pointer to the beginning for an array and some boolean indicator whether an array is full or not. $\endgroup$ – Max Koretskyi May 6 '17 at 5:31
  • $\begingroup$ @Maximus Because, when a dynamic array is full, its size is increased by some factor $\alpha$. This means that, immediately after growing, a dynamic array holding $n$ items has size about $n\alpha$, so the amount of wasted space is $n(\alpha-1) = O(n)$. $\endgroup$ – David Richerby May 6 '17 at 9:57

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