Question

I want to take a List, and generate a Dictionary which maps each element to its index in the List. I can do this like so, for a List<string>:

var myList = new List<string>{ /* populate list */ };
var orderMap = new Dictionary<string, int>();

foreach (var element in myList)
{
    orderMap[element] = myList.IndexOf(element);
}

Basically, I want to take a list like:

Apple
Banana
Orange

And return a map showing indices:

Apple -> 0
Banana -> 1
Orange -> 2

How can I do this with Linq? I think something like this should work:

orderMap = myList.Select( x => /* return a key value pair mapping x to myList.IndexOf(x) */ );

But I can’t figure out the right syntax for it. Besides, can you refer to the list itself in the delegate used for Select?

Answer 1

While you can refer to the list within the delegate, it’s not generally a good idea. You really want to use the overload of Select which provides the index as well as the value:

var dictionary = list.Select((value, index) => new { value, index })
                     .ToDictionary(p => p.value, p => p.index);

Note that this will throw an exception if you have any duplicate elements.