Fast sum of values in a multidimensional array (C#)

Question

With a 1D array, I can use the sum method to get the sum of all the values.

int[] array = {6,3,1};
Console.WriteLine(array.Sum());

With a multidimensional array (3D in my case), this can't be done. Obviously I could go all foreach on it, but this seems verbose and I suspect it will perform badly.

Is there a way to flatten the array? Or a nice way just to get the sum that I've not seen?

Answer 1

Sum does exactly foreach. There is no magic behind them. If you are so performance hungry use for instead of foreach. You can do this in parallel also, this operation can be easily parallelized.

Answer 2

Why should a foreach perform badly? You have to read every value at least once to calculate the sum. There is no way around this (assuming "random" values, of course). So maybe there is a more beautyful way, but not a more performant one (speaking in terms of Big O).

Answer 3

If you have a jagged array and would like clean code you could use

int[][] array = { new []{ 6, 3, 1 }, new []{ 6, 3, 1 } };
Console.WriteLine(array.Sum(i => i.Sum()));

Answer 4

this'll do the trick.

var i = array.SelectMany(j => j).Sum()

you could parallelize this in .net 4 like this

var i = array.AsParallel().SelectMany(k => k).Sum();

Answer 5

How many entries will the lists contain?

A sum-function of a 3-dimensional array using nested for-loops will run in O(n^3). Assuming you don't have millions of entries it shouldn't really be a performance issue.

Doing it in parallel would give you theoretical running time of O(3*n) = O(n), but really it isn't that big of an improvement in actual seconds unless you, like mentioned above, have a huge load of entries.