Question

I want to use LINQ to convert this

IEnumerable<int>[] value1ByType = new IEnumerable<int>[3];
value1ByType[0]= new [] { 0};
value1ByType[1]= new [] {10,11};
value1ByType[2]= new [] {20};

var value2ToType = new Dictionary<int,int> {
{100,0},
{101,1},
{102,2},
{103,1}};

to this

var value2ToValue1 = new Dictionary<int,int> { 
{100, 0},
{101,10},
{102,20},
{103,11}};

Is there a way to do this with LINQ? Without LINQ I would use multiple IEnumerators, one for each IEnumerable of value1ByType. like this:

// create enumerators
var value1TypeEnumerators = new List<IEnumerator<int>>();
for (int i = 0; i < value1ByType.Length; i++)
{
    value1TypeEnumerators.Add(value1ByType[i].GetEnumerator());
    value1TypeEnumerators[i].MoveNext();
}

// create wanted dictionary
var value2ToValue1 = new Dictionary<int, int>();
foreach (var item in Value2ToType)
{
    int value1=value1TypeEnumerators[item.Value].Current;
    value2ToValue1.Add(item.Key, value1);
    value1TypeEnumerators[item.Value].MoveNext();
}

Any Idea how to do this in LINQ?

Answer 1

Not pure but you can at least do …

var enumerators = value1ByType.Select(v => v.GetEnumerator()).ToArray();
var value2ToValue1 = value2ToType
                        .ToDictionary(x => x.Key, x => { enumerators[x.Value].MoveNext(); return enumerators[x.Value].Current; });

But there are so many ways this could go wrong it begs the question – why was the data in those data-structures anyway? and can you fix that instead? How did you end up with exactly the right number of references in the 2nd data structure to elements in the first?