Real average filesize in a set of files

I have an apparently "simple" problem but I can't find the solution for some reason...

I have n millions files of different sizes and I want to find the average filesize.

To simplify it, I grouped them in multiples of 16KB.

< 16 KB = 18689546 files

< 32 KB = 1365713 files

< 48 KB = 1168186 files

...

Of course, the simple (total_size / number of files) does not work. It gives a开发者_StackOverflow社区n average of 291KB...

What would be the algorithm to calculate the real average...?

Thx, JD

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You might be running into a problem with overruns when summing the file sizes (the total size probably doesn't fit into a 32-bit value). The easiest fix might be to try using a 64-bit int for the variable that's holding the sum.