如何使用lambda表达式从字典中删除项目

Question

I am not into LINQ solutions,

I am using simple predicat to determine if the key should be removed, For example if the dictionary is construct like Dictionary<int, int> , so how should I remove all the entries with negative data

I am prefer to use the same dictionary, not to create new one, I don't have preformance issues

Is there a way to do it, without using LINQ, but using Lambda expressions?

I didn't want solutions in LINQ because no one is using them in my project, didn't want to be the first.., but because I saw the LINQ solutions look better, I will use them them..

Answer 1

The simplest way is probably to create a new dictionary, if that's okay for you:

var newDictionary = oldDictionary.Where(pair => pair.Value >= 0)
                                 .ToDictionary(pair => pair.Key,
                                               pair => pair.Value);

If you have to mutate the existing dictionary (eg because several other objects have reference to the same dictionary) you'd need to build a list of keys to remove, then remove them afterwards:

var toRemove = dictionary.Where(pair => pair.Value < 0)
                         .Select(pair => pair.Key)
                         .ToList();

foreach (var key in toRemove)
{
    dictionary.Remove(key);
}

EDIT: I've just noticed the first sentence: "I am not into LINQ solutions". If that means you don't want to use a LINQ solution, here's the by-hand version:

List<int> toRemove = new List<int>();
foreach (KeyValuePair<int, int> pair in dictionary)
{
    if (pair.Value < 0)
    {
        toRemove.Add(pair.Key);
    }
}

foreach (var key in toRemove)
{
    dictionary.Remove(key);
}

... but if you can use LINQ, I'd encourage you do. My second solution is equivalent to the "by-hand" version, but more readable IMO.

Answer 2

By merely using lambda expression:

foreach (var i in myDict.Where(d => (d.Value  < 0 || d.key <0)).ToList() ) 
{
  myDict.Remove(i.Key);
}

Answer 3

var toRemove = dict.Keys.Where(predicate).ToArray();
foreach (var key in toRemove) {
    dict.Remove(key);
}

Answer 4

Well if you add

namespace MMExtensions
{
    public static class DictionaryExtensions
    {
        public delegate bool Predicate<TKey, TValue>(KeyValuePair<TKey, TValue> d);

        [MethodImpl(MethodImplOptions.Synchronized)]
        public static void Filter<TKey, TValue>(
            this Dictionary<TKey, TValue> hashtable, Predicate<TKey, TValue> p)
        {
            foreach (KeyValuePair<TKey, TValue> value in hashtable.ToList().Where(value => !p(value)))
                hashtable.Remove(value.Key);
        }
    }
}

And you had some dataset as dictionary:

    Dictionary<string, int> d =
            new Dictionary<string, int> {{"v", -3}, {"val1", 1}, {"val2", 2}};

Then you could use:

    d.Filter(delegate(KeyValuePair<string, int> kv) { return kv.Value >= 0; });
    d.Filter(kv => kv.Value >= 0);// or as lambda

Answer 5

Do you want to remove the items from that dictionary, or are you happy to use a new dictionary without those items included?

var d = new Dictionary<int,int>();
var newDict = d.Where(entry => entry.Value >= 0).ToDictionary(entry => entry.Key, entry => entry.Value);

Answer 6

Easiest one:

Dictionary<long, long> dict...
Dictionary<long, long> result = dict.Were(x => x.Value >= 0).ToDictionary(x => x.Key, x => x.Value);

Or just loop over all in 'for' in reverse order and remove invalid ones.

Answer 7

I know you said you are not into Linq, but I could not contain myself with the following solution, plus it is still useful if you read the title of your question. This is probably the most elegant solution to your problem:

dictionary.Where(pair => pair.Value < 0)
          .Select(pair => { 
              dictionary.Remove(pair.Key);
              return pair.Key;
          });