Most efficient way to remove a value from a series of lists within a dictionary?

Question

I have a Dictionary set up like so:

private Dictionary<string, List<string>> Directory = new Dictionary<string, List<string>>();

I am needing to iterate over every Key in the Dictionary and remove all occurrences of a string within each of the lists.

I believe I can do so with a series of for/for-each loops, possibly by creating a new list for each without the string that I am trying to remove but I feel this may become inefficient when the Dictionary grows in size.

Are there any methods that can make this more efficient?

`foreach (var list in Directory.Values) list.RemoveAll(item => item == "SomeValue");` — Dmitry Bychenko, Jan 15 '20 at 13:37
What about using `Directory.Values.ForEach(list => list.RemoveAll(...)));`? — fredrik, Jan 15 '20 at 13:37
Does this answer your question? [get dictionary key by value](https://stackoverflow.com/questions/2444033/get-dictionary-key-by-value) — Jawad, Jan 15 '20 at 13:38
@cillsley If you are not storing duplicate string values in the Lists, you could use `HashSet` instead of `List` as your values in the dictionary and use the `.Remove(T)` method of `HashSet` — Ryan Wilson, Jan 15 '20 at 13:39
@fredrik This looks like it would be the simplest solution, thanks! — cillsley, Jan 15 '20 at 13:45
@cillsley You can use a linq expression to remove the item from all elements. — Jawad, Jan 15 '20 at 13:47

ilkerkaran · Answer 1 · 2020-01-15T13:45:28.013

2

I would use a traditional foreach loop

var strToRemove == "remove me!";
foreach (var listObj in dic.Values)
{
    listObj.RemoveAll(x => x.Equals(strToRemove ,StringComparison.OrdinalIgnoreCase));
}

edited Jan 15 '20 at 13:45

answered Jan 15 '20 at 13:37

ilkerkaran

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score 1 · Answer 2 · answered Jan 15 '20 at 13:42

1

You can use RemoveAll() in a foreach loop to remove all occurence in all lists of a specified string :

foreach (var val in Directory.Values)
{
    var numberOfElementsRemoved = val.RemoveAll(x => x.ToLower() == StringToRemove.ToLower());
}

answered Jan 15 '20 at 13:42

Cid

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Jawad · Answer 3 · 2020-01-15T14:35:52.323

0

You can use the Where statement to look up the pair that has the value you need to remove... then update the value of that pair.

    Dictionary<string, List<string>> Directory = new Dictionary<string, List<string>>();
    Directory.Add("1", new List<string>() { "alpha", "bravo", "charlie" });
    Directory.Add("2", new List<string>() { "delta", "echo", "foxtrot" });
    Directory.Add("3", new List<string>() { "golf", "charlie", "India" });

    string stringToRemove = "charlie";
    Directory.Where(x => x.Value.Contains(stringToRemove))
                .ToList()
                .ForEach(x => x.Value.Remove(stringToRemove));

After the execution, "charlie" is gone from all dictionary's pairs.

As per Cid's suggestion, you can also use RemoveAll(lambda) if you dont have a distinct list.

Directory.Where(x => x.Value.Contains(stringToRemove))
           .ToList()
           .ForEach(x => x.Value.RemoveAll(x => x.Equals(strToRemove ,StringComparison.OrdinalIgnoreCase)));

edited Jan 15 '20 at 14:35

answered Jan 15 '20 at 13:43

Jawad

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3
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This looks nice and clean! Thanks for the reply! – cillsley Jan 15 '20 at 13:48
1

`Remove` removes only the first occurence in the list. – Cid Jan 15 '20 at 13:49
The lists will only contain one occurrence anyways so this should work fine! – cillsley Jan 15 '20 at 14:15
Added the note with addition of how you can use RemoveAll to remove all instances of the string from the Dictionary's list. – Jawad Jan 15 '20 at 14:36

score 0 · Accepted Answer · answered Jan 15 '20 at 13:47

Solution:

class Program
{
    static void Main(string[] args)
    {
        Dictionary<string, List<string>> dictionary = new Dictionary<string, List<string>>();
        dictionary.Add("Test1", new List<string>() { "Test1", "Test2", "Test3" });
        dictionary.Add("Test2", new List<string>() { "Test2", "Test3", "Test1" });
        dictionary.Add("Test3", new List<string>() { "Test3", "Test2", "Test1" });
        dictionary.RemoveStringFromEveryList("Test1");
    }
}

public static class Extensions
{
    public static void RemoveStringFromEveryList(this Dictionary<string, List<string>> dictionary, string wordToRemove)
    {
        foreach(var keyValue in dictionary)
        {
            keyValue.Value.RemoveAll(x => x == wordToRemove);
        }
    }
}

I would use an extension method for a better usage and readability of your code.

Most efficient way to remove a value from a series of lists within a dictionary?

4 Answers4