C# List of objects, how do I get the sum of a property

Question

I have a list of objects. One property of the individual object entry is amount. How do I get the sum of amount?

If my list was of type double I may be able to do something like this:

double total = myList.Sum();

However I want to something similar to this, yet this syntax is incorrect.

double total = myList.amount.Sum();

How should I go about accomplishing this? I would love to use the Sum function if possible instead of looping through and calculating the value.

score 395 · Accepted Answer · edited May 18 '17 at 10:29

395

using System.Linq;

...

double total = myList.Sum(item => item.Amount);

edited May 18 '17 at 10:29

Paul C

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answered Dec 04 '10 at 04:11

Alex LE

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14

Is this quicker than foreach out of interest? – Paul C Feb 04 '13 at 14:08
4

I am also interested in @CodeBlend's question. Will this computation be faster than a for loop? – rex Feb 25 '14 at 11:29
33

@Coops - To answer your question...using a list containing 100,000 objects, each object having a single property of datatype double, summing up the property 1,000 times using solution above (myList.Sum) takes 2.44 seconds compared to 0.98 seconds using foreach. The elapsed time is measured using the Stopwatch class for accuracy. Therefore foreach is over 2x faster than using myList.Sum. – Joe Gayetty Nov 11 '16 at 15:23
4

Linq is never quicker than pure foreach. Use it only in convenient, non-time-critical places. – Vitalii Vasylenko Oct 25 '20 at 14:26

score 53 · Answer 2 · answered Oct 18 '14 at 22:05

53

And if you need to do it on items that match a specific condition...

double total = myList.Where(item => item.Name == "Eggs").Sum(item => item.Amount);

answered Oct 18 '14 at 22:05

Greg Quinn

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score 15 · Answer 3 · answered Mar 22 '16 at 21:45

15

Another alternative:

myPlanetsList.Select(i => i.Moons).Sum();

answered Mar 22 '16 at 21:45

usefulBee

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Instead of ```.Select(i => i.Moons).Sum()``` you can use ```.Sum(i => i.Moons)``` – Mason Jan 05 '18 at 07:33
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@Mason, right and that is how Alex approached the problem in his earlier answer, so I just provided a different way of doing the same thing. – usefulBee Jan 05 '18 at 15:52
1

Ah yes, sorry for my misconception. – Mason Jan 08 '18 at 14:28
2

I appreciate that this is a unique answer, but wouldn't performance suffer? – Peter Mghendi Feb 23 '20 at 08:30

Puchacz · Answer 4 · 2016-02-19T10:16:59.853

5

Here is example code you could run to make such test:

var f = 10000000;
var p = new int[f];

for(int i = 0; i < f; ++i)
{
    p[i] = i % 2;
}

var time = DateTime.Now;
p.Sum();
Console.WriteLine(DateTime.Now - time);

int x = 0;
time = DateTime.Now;
foreach(var item in p){
   x += item;
}
Console.WriteLine(DateTime.Now - time);

x = 0;
time = DateTime.Now;
for(int i = 0, j = f; i < j; ++i){
   x += p[i];
}
Console.WriteLine(DateTime.Now - time);

The same example for complex object is:

void Main()
{
    var f = 10000000;
    var p = new Test[f];

    for(int i = 0; i < f; ++i)
    {
        p[i] = new Test();
        p[i].Property = i % 2;
    }

    var time = DateTime.Now;
    p.Sum(k => k.Property);
    Console.WriteLine(DateTime.Now - time);

    int x = 0;
    time = DateTime.Now;
    foreach(var item in p){
        x += item.Property;
    }
    Console.WriteLine(DateTime.Now - time);

    x = 0;
    time = DateTime.Now;
    for(int i = 0, j = f; i < j; ++i){
        x += p[i].Property;
    }
    Console.WriteLine(DateTime.Now - time);
}

class Test
{
    public int Property { get; set; }
}

My results with compiler optimizations off are:

00:00:00.0570370 : Sum()
00:00:00.0250180 : Foreach()
00:00:00.0430272 : For(...)

and for second test are:

00:00:00.1450955 : Sum()
00:00:00.0650430 : Foreach()
00:00:00.0690510 : For()

it looks like LINQ is generally slower than foreach(...) but what is weird for me is that foreach(...) appears to be faster than for loop.

edited Feb 19 '16 at 10:16

answered Feb 19 '16 at 09:45

Puchacz

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for future reference, have a look at `Stopwatch` in `System.Diagnostics` as it is a high-performance time recorder. (I didn't downvote you btw) – Meirion Hughes Feb 19 '16 at 09:50
1

Do not use `DateTime.Now` for measuring. It has terrible performance as it returns always local time. `DateTime.UtcNow` is faster; however, it still does not use as high resolution as the `Stopwatch` class. – György Kőszeg Feb 19 '16 at 09:54
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This does not answer the question. – Mark Pattison Feb 19 '16 at 09:57
Ok, thanks for tip. Score's are very repeatable so I assumed that such resolution is enough – Puchacz Feb 19 '16 at 09:58
2

While your intention is good -Mark is right- you're not explicitly answering the question. I'd recommend you change it to: "Here is how you could do it" and "Here is the cpu performance of each option". Also in principle if you describe your testing methodology you need-not show us the code. – Meirion Hughes Feb 19 '16 at 10:08
"but what is weird for me is that foreach(...) appears to be faster than for loop." Its not indexing an array, its enumerating along a list, lists use pointers to the next item. – Andy Morris Feb 19 '16 at 10:22
@AndyMorris Firstly, he's using arrays. Secondly, [lists use arrays internally](http://referencesource.microsoft.com/#mscorlib/system/collections/generic/list.cs). Thirdly, the times are not accurate because of the use of DateTime.Now. As the answer currently is, it both does not answer the question and has meaningless tests and results. – kjbartel Feb 21 '16 at 05:20

C# List of objects, how do I get the sum of a property

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