Space complexity of C# sort on string lists

Question

I'm implementing a program to sort large files that maybe don't fit in memory. All files will be sorted by lines so I'm thinking in use a List to do it.

I already calculated how many lines I can have in memory to split the files in smaller files, but I don't know how many space I need in memory to sort a List of N elements.

The question is, knowing the maximum number of elements (strings of known size) and the available memory, how much space in memory will the List.Sort method will need?

Side note: for future question skip putting thank you notes and upvote/accept useful answers. I.e. in this one in addition to commenting "thank you" on bubble sort suggestion you could upvoted it. — Alexei Levenkov, Jul 13 '12 at 15:57

score 3 · Accepted Answer · answered Jul 13 '12 at 15:37

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List<T>.Sort uses a quicksort algorithm, which is O(log n) space.

http://en.wikipedia.org/wiki/Quicksort#Space_complexity

answered Jul 13 '12 at 15:37

SLaks

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I'd imagine that you'd need O(n) memory to simply store the string. – Sam I am says Reinstate Monica Jul 13 '12 at 15:38
Yeah, it depends on your point of reference. You might choose to point out that it takes an *additional* O(log n) space, but otherwise this is totally correct. +1 – BlackVegetable Jul 13 '12 at 15:40

score 2 · Answer 2 · answered Jul 13 '12 at 15:39

Quick sort is O(logn) space complexity, because it stores constant information (beginning and end of the partition) at every recursive level, and there's at most logn levels.

The merge itself is in-place, as is the bubble sort fallback, so they're O(1) and as such don't count.

score 2 · Answer 3 · answered Jul 13 '12 at 15:39

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Do some research: List.Sort() uses the QuickSort algorithm by default, which has O(logn) space complexity.

If you are really worried about space complexity and don't mind a slower algorithm you could use Bubble sort which has O(1) space complexity.

answered Jul 13 '12 at 15:39

NominSim

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I'm not that worried about space complexity, I just needed to know how much space I needed. I will take that in consideration though. Thanks. – Sheol Jul 13 '12 at 15:52

Space complexity of C# sort on string lists

3 Answers3