How to calculate the current sum of an int series in a Linq query?

Question

I am trying to come up with a linq query to convert an IEnumerable<int> to another IEnumerable<int> , where each int as a result is the sum of all ints to this position from the original list:

Given int[] a
I need int[] b
Where b[0] = a[0], b[1] = a[0] + a[1], b[2] = a[0] + a[1] + a[2] , etc.

Alternatively, the amounts above can be written as b[1] = b[0] + a[1], b[2] = b[1] + a[2] etc., but I don’t see how that might help.

I can, of course, do this with a for loop, but I get the sequence [] from the request, and I thought it would look better if I continued this request instead of suddenly adding for there :)

Answer 1

Well, you can do it with side effects quite easily, although it's pretty bad ...

 int sum = 0; int[] b = a.Select(x => (sum += x)).ToArray(); 

It would be nice if the infrastructure provided a kind of “working aggregate” to encapsulate this, but this is not as far as I know.

Answer 2

I wrote a function to do this a while ago. It is similar to the Haskell scanl function.

 public static IEnumerable<TResult> Scan<T, TResult>( this IEnumerable<T> source, Func<T, T, TResult> combine) { using (IEnumerator<T> data = source.GetEnumerator()) if (data.MoveNext()) { T first = data.Current; yield return first; while (data.MoveNext()) { first = combine(first, data.Current); yield return first; } } } int[] b = a .Scan((running, current) => running + current) .ToArray(); 

Answer 3

An alternative to Mr. Skeet's solution: if we refuse the linq query and more literally refer to "convert IEnumerable<int> to another IEnumerable<int> ", we can use this:

  static IEnumerable<int> Sum(IEnumerable<int> a) { int sum = 0; foreach (int i in a) { sum += i; yield return sum; } } 

which we can apply to an infinite series:

  foreach (int i in Sum(MyMath.NaturalNumbers)) Console.WriteLine(i); 

This is also useful if you do not want to instantly create the entire array.

Answer 4

The above answer does not quite work .... and does not have the same signature as Haskell scanl .... basically it is an extension of the linq "aggregate" idea ... I think this is better suited to the Haskell implementation

  public static IEnumerable<TResult> Scanl<T, TResult>( this IEnumerable<T> source, TResult first, Func<TResult, T, TResult> combine) { using (IEnumerator<T> data = source.GetEnumerator()) { yield return first; while (data.MoveNext()) { first = combine(first, data.Current); yield return first; } } } 

Using

  [TestMethod] public void Scanl_Test() { var xs = new int[] { 1, 2, 3, 4, 5, 6, 7 }; var lazyYs = xs.Scanl(0, (y, x) => y + x); var ys = lazyYs.ToArray(); Assert.AreEqual(ys[0], 0); Assert.AreEqual(ys[1], 1); Assert.AreEqual(ys[2], 3); Assert.AreEqual(ys[3], 6); Assert.AreEqual(ys[4], 10); Assert.AreEqual(ys[5], 15); Assert.AreEqual(ys[6], 21); Assert.AreEqual(ys[7], 28); }