如何在Linq查询中计算一系列整数的运行总和？

Question

I am trying to come up with a linq query to convert an IEnumerable<int> to another IEnumerable<int> , where each int in the result is the sum of all the ints up to that position from the initial 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] and so on

Alternatively, the sums above can be written as b[1] = b[0] + a[1], b[2] = b[1] + a[2] and so on, but I don't see how that would help.

I can, of course, do this with a for loop, but I obtain the a[] sequence from a query and I thought it would look nicer if I continue that query instead of suddenly adding a for there :)

Answer 1

Well, you can do it with side effects easily enough, although it's pretty icky...

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

It would be nice if the framework provided a sort of "running aggregate" to encapsulate this, but it doesn't as far as I'm aware.

Answer 2

I wrote a function to do this a while ago. It's similar to Haskell's 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 drop the requirement for a linq query and more literally address "convert an 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 don't want to create the whole array at once.

Answer 4

The above answer doesn't quite work....and doesnt share the same signature as Haskell's scanl.... basically its an extension to the idea of linq's "aggregate"...I think this matches the Haskell implementation better

    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;
            }
        }
    }

usage

    [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);
    }