I've a list that contains 4 sized arrays:
These arrays have 4 elements. I want to use another list that contains these arrays' first element's count. In addition, if their first elements are same, they should be summation. For example:
list[0] = {1,2,3,4}
list[1] = {1,1,5,3}
list[2] = {1,2,5,8}
list[3] = {2,2,3,3}
list[4] = {3,5,5,6}
list[5] = {4,4,4,4}
list[6] = {4,5,5,6}
So, anotherList should be:
anotherList = {3, 1, 1, 2}
How can I do this?
EDIT: Expected result is:
anotherList = list.Select(a => a[0]) // project each array to its first item
.GroupBy(x => x) // group first items by their value
.Select(g => g.Count()) // select count of same items
.ToList();
Output:
[ 3, 1, 1, 2 ]
NOTE: GroupBy internally uses Lookup which returns groups in same order as the are added, so it seems to be what you want.
UPDATE: Approach which does not depend on internal implementation of GroupBy
anotherList = list.Select((a,i) => new { Item = a[0], Index = i })
.GroupBy(x => x.Item)
.OrderBy(g => g.Min(x => x.Index))
.Select(g => g.Count())
.ToList();
Lookup, the output order is not "well defined" so at any later time they could choose to change how ordering is handled.
Enumerable.GroupBy is well-defined. From the Enumerable.GroupBy documentation: "The IGrouping<TKey, TElement> objects are yielded in an order based on the order of the elements in source that produced the first key of each IGrouping<TKey, TElement>. Elements in a grouping are yielded in the order they appear in source."
select count(column_1) from list group by column_1