Multiple SUM using LINQ

Question

I have a loop like the following, can I do the same using multiple SUM?

foreach (var detail in ArticleLedgerEntries.Where(pd => pd.LedgerEntryType == LedgerEntryTypeTypes.Unload &&
                                                                pd.InventoryType == InventoryTypes.Finished))
{
     weight += detail.GrossWeight;
     length += detail.Length;
     items  += detail.NrDistaff;
}

Answer 1

You can call Sum three times, but it will be slower because it will make three loops.

For example:

var list = ArticleLedgerEntries.Where(pd => pd.LedgerEntryType == LedgerEntryTypeTypes.Unload
                                   && pd.InventoryType == InventoryTypes.Finished))

var totalWeight = list.Sum(pd => pd.GrossWeight);
var totalLength = list.Sum(pd => pd.Length);
var items = list.Sum(pd => pd.NrDistaff);

Because of delayed execution, it will also re-evaluate the Where call every time, although that's not such an issue in your case. This could be avoided by calling ToArray, but that will cause an array allocation. (And it would still run three loops)

However, unless you have a very large number of entries or are running this code in a tight loop, you don't need to worry about performance.

---

EDIT: If you really want to use LINQ, you could misuse Aggregate, like this:

int totalWeight, totalLength, items;

list.Aggregate((a, b) => { 
    weight += detail.GrossWeight;
    length += detail.Length;
    items  += detail.NrDistaff;
    return a;
});

This is phenomenally ugly code, but should perform almost as well as a straight loop.

You could also sum in the accumulator, (see example below), but this would allocate a temporary object for every item in your list, which is a dumb idea. (Anonymous types are immutable)

var totals = list.Aggregate(
    new { Weight = 0, Length = 0, Items = 0},
    (t, pd) => new { 
        Weight = t.Weight + pd.GrossWeight,
        Length = t.Length + pd.Length,
        Items = t.Items + pd.NrDistaff
    }
);

Answer 2

You'd be able to do this pivot-style, using the answer in this topic: http://stackoverflow.com/questions/167304/is-it-possible-to-pivot-data-using-linq

Answer 3

Technically speaking, what you have is probably the most efficient way to do what you are asking. However, you could create an extension method on IEnumerable<T> called Each that might make it simpler:

public static class EnumerableExtensions
{
    public static void Each<T>(this IEnumerable<T> col, Action<T> itemWorker)
    {
        foreach (var item in col)
        {
            itemWorker(item);
        }
    }
}

And call it like so:

// Declare variables in parent scope
double weight;
double length;
int items;

ArticleLedgerEntries
    .Where(
        pd => 
           pd.LedgerEntryType == LedgerEntryTypeTypes.Unload &&
           pd.InventoryType == InventoryTypes.Finished
    )
    .Each(
        pd => 
        {
            // Close around variables defined in parent scope
            weight += pd.GrossWeight; 
            lenght += pd.Length;
            items += pd.NrDistaff;
        }
    );

UPDATE: Just one additional note. The above example relies on a closure. The variables weight, length, and items should be declared in a parent scope, allowing them to persist beyond each call to the itemWorker action. I've updated the example to reflect this for clarity sake.

Answer 4

Ok. I realize that there isn't an easy way to do this using LINQ. I'll take may foreach loop because I understood that it isn't so bad. Thanks to all of you