如何使用LINQ获得与条件总和匹配的属性？

Question

Ok, I get the title sounds weird, but here's my scenario. I think ts a simple query, but its hard to explain on paper. And I'm having trouble with the LINQ myself. Best to explain by example:

I have a list of objects (in psuedocode) that looks like:

var data = [
  {
    quantity: 123,
    rate: 0.12
  },
  {
    quantity: 456,
    rate: 0.13
  },
  {
    quantity: 3,
    rate: 0.14
  },
  {
    quantity: 92,
    rate: 0.15
  }
]

I am trying to generate a LINQ query that returns rate equaling the total sum of an input quantity.

Explaination by example:

Using the example data above, I want to find the rate that matches a total quantity of 581 (a variable input from elsewhere). The first three objects in my list have quantities that total 582, so I want my LINQ query to return 0.14 (the rate matching the condition that the total quantity fields is greater than my input quantity).

I should be able to use LINQ to do this, right? Its sort of a .Sum and a .Where , and that's where I'm getting confused.

Are there any references out that to address this type of scenario? Does anyone have any experience writing a "conditional sum"?

Answer 1

In functional programming, this would be a variation of a reduce . In LINQ, the Aggregate method can be used for this purpose. Psuedo-LINQ:

var limit = 581;
var rate = data.Aggregate((acc, cur) =>
{
    if (acc.quantity < limit)
    {
        return new
        {
            quantity = acc.quantity + cur.quantity,
            rate = cur.rate
        };
    }
    else
    {
        return acc;
    }
}).rate;

EDIT: A more compact, one-line version:

var rate = data.Aggregate((acc,cur) => acc.quantity < limit ? new { quantity = acc.quantity + cur.quantity, rate = cur.rate } : acc).rate;

Answer 2

Was going for Aggregate but failed to get in one line :(

For the desired of one-liner , this is a funny usage of LINQ :)

var target = 581;
var sum = 0;
var rate = data.First(i =>(sum += i.Quantity) > target).Rate; // 0.14