Ordering Linq list by array elements

Question

I have below code -

var refNosToOrder = new int[9] {1,2,3,4,5,6,7,8,9}

var orderedList = lst.OrderBy(x=>x.RefNo==7)
                     .ThenBy(y=> refNosToOrder.Contains(y.RefNo)).ToList();

lst is list of class object containing int property - RefNo : i.e. List<SampleClass>

class SampleClass
{
  public int RefNo {get;set;}
}

lst contains all the unsorted data of RefNo:

lst = 2,4,6,9,7,5,8,1,3

What I want to do -

First I want to order lst by keeping first element as - 7; then for the rest of the list, it should be ordered as the array refNosToOrder

i.e. Final output I am expecting to be -

7,1,2,3,4,5,6,8,9

With the above code -

var orderedList = lst.OrderBy(x=>x.RefNo==7)
                         .ThenBy(y=> refNosToOrder.Contains(y.RefNo)).ToList();

It is giving - 2,4,6,9,7,5,8,1,3 i.e. this code is not at all ordering the list.

Shouldn't your ThenBy() contain somethin like ThenBy(y => y.RefNo) ? Look at this post where I saw that — Error 404 Brain not found
– Error 404 Brain not found, Commented Dec 14, 2021 at 13:34
No .. Rest of the RefNo should get ordered by refNosToOrder array @Error404Brainnotfound — C Sharper
– C Sharper, Commented Dec 14, 2021 at 13:36
So what if refNosToOrder is 6, 6, 6, 6? Would you want four copies of the original? Is refNosToOrder always going to be (effectively) a number followed by a linear sequence that doesn't have that number? — Jeroen Mostert
– Jeroen Mostert, Commented Dec 14, 2021 at 13:38
refNosToOrder will always have unique elements in it. I dont want to have copies. @JeroenMostert — C Sharper
– C Sharper, Commented Dec 14, 2021 at 13:39

Mureinik · Accepted Answer · 2021-12-14 13:56:03Z

5

Contains returns a boolean of whether an element is in a list or not, which won't be very helpful here. Instead, you could sort by the index of that element:

var orderedList = 
    lst.OrderBy(x => x.RefNo != 7)
       .ThenBy(y => Array.IndexOf(refNosToOrder, y.RefNo))
       .ToList();

EDIT:
Following up on Jeroen Mostert's comment, this sorting has quadratic complexity. For large refNosToOrder it may be more efficient to first convert the array to a dictionary of orders and then use it for the sorting:

var orderDict = 
    Enumerable.Range(0, refNosToOrder.Length).ToDictionary(i => refNosToOrder[i]);
var orderedList = 
    lst.OrderBy(x => x.RefNo != 7).ThenBy(y => orderDict[y.RefNo]).ToList();

edited Dec 14, 2021 at 13:56

answered Dec 14, 2021 at 13:42

Mureinik

316k54 gold badges400 silver badges405 bronze badges

Sign up to request clarification or add additional context in comments.

3 Comments

Jeroen Mostert Over a year ago

Do note that this solution has quadratic performance due to the repeated linear search in refNosToOrder -- OK if your arrays are 10 elements, no longer OK if they're 10 000 elements. At that point creating lookups first starts to make sense.

Mureinik Over a year ago

@JeroenMostert good point, thanks! Edited that in to the answer (with the appropriate credit, of course)

C Sharper Over a year ago

@Mureinik - If lst contains elements outside the refNosToOrder - then it is giving preference to those numbers after 7. Eg. if lst contains - 2,4,6,9,7,5,8,1,3 and refNosToOrder = 3,4,5 then it is outputing as - 7,1,2,3,4,5,6,8,9 ; where it should have been - 7,3,4,5,1,2,6,8,9

Collectives™ on Stack Overflow

Ordering Linq list by array elements

1 Answer 1

3 Comments

Your Answer

Linked

Hot Network Questions

Collectives™ on Stack Overflow

1 Answer 1

3 Comments

Your Answer

Sign up or log in

Post as a guest

Linked

Related