从列表中选择项目与保存在其他列表中的索引有关？

Question

i have:
a list of list x;
a list of list A that contains indexces of some elements in list d as A[i]= [1,3]
a list of list d that contains values as d[i]=[500,200,1000, 40,60,..]
i want to select from d elements which has the indexes saved in A and add them to x
then x[0] will = [200,40]

i use this code:

List<int> sublist = new List<int>();
for (int b = 0; b < A[i1].Count(); b++)
{
    sublist .Add(d[i1][A[i1][b]]);
 }
x.Add(sublist );

can you help me to do it with an efficient methode?

Answer 1

List<int> sublist = new List<int>();
foreach(int i in A)
{
    sublist.Add(d[i]);
}
x.Add(sublist);

Answer 2

It really depends what you mean by "efficient".

If you want the fastest execution time, then nested for loops will probably be your best bet. You might be able to save some time by specifying the initial size for sublist to match the number of elements in A[i1] , and by dropping the .Count() extension method in favour of the .Count or .Length property, depending on the precise types you're using.

If you just want simple code, then LINQ can help:

List<List<int>> x = A
    .Select((value, index) => value.Select(i => d[index][i]).ToList())
    .ToList();

Demo

Or:

List<List<int>> x = A
    .Zip(d, (ai, di) => ai.Select(i => di[i]).ToList())
    .ToList();

Demo