Use List<T>ForEach to add element to HashTable

Question

I have a list of string values that I want add to a hashtable or other array that can be accessed by key/index but cannot implement it. I have this working how I want but its ugly

        List<string> valueList = new List<string>();
        valueList.Add("1");
        valueList.Add("2");
        valueList.Add("3");

        Hashtable p = new Hashtable();


        valueList.ForEach(delegate(string f) { p.Add(valueList.FindIndex(v => v == f), f); });

EDIT: After James reminded me that a List will return values by index I went with a List and this is what I have ended up with

valueList.ForEach(f => sequenceList.Add(int.Parse(f)));

Answer 1

Assuming you want the strings to be the keys and the index to be the value:

Hashtable ht = new Hashtable();

for (var i = 0; i <= valueList.Count; i++)
{
    ht.Add(valueList[i], i);
}

Otherwise switch the ht.Add parameters around. However, if that is the case you would be best just to leave it as a List < string >.

Answer 2

List<string> list = new List<string>{"w", "y", "u", "i", "n"};

HashSet<string> hset = new HashSet<string>(list);

Answer 3

Even if it compiled, it wouldn't work - calling GetEnumerator().Current will always fail because it'll give you a new iterator positioned before the first item.

What do you want the key for each item to be? If it's just its position within the list, I don't see the benefit that gives you over a list (which is already indexed by int). However, you can do it like this (assuming that valueList is a List<string>:

var dictionary = valueList.Select((item, index) => new { item, index })
                          .ToDictionary(x => x.index, x => x.item);

Adding it to an existing dictionary, I'd just do:

for (int i=0; i < valueList.Count; i++)
{
    dictionary[i] = valueList[i];
}

Not everything has to be done with lambdas :)

Note that this won't have quite the same effect as using FindIndex if you have repeated values.

Answer 4

Try this:

valueList.ForEach(x => htable.Add(valueList.FindIndex(y => y == x), x));

Although, there's really no reason not to use a for here

for (var index = 0; index < valueList.Count; index++)
{
    htable.Add(index, valueList[index]);
}

It's more lines of code, but it's more straightforward and will perform much better (findIndex is far less efficient than using the index from the for statement).

Answer 5

Using ForEach for this is not efficient, as you have to use FindIndex to find out where you are. It works, but you will do a lot of extra work:

valueList.ForEach(x => htable.Add(valueList.FindIndex(y => y == x), x));

It's better to just use a regular loop, so that you get the index without having to seek through the table:

for (int i = 0; i < valueList.Count; i++) {
   htable.Add(i, valueList[i]);
}