C# Intersect Two Dictionaries and Store The Key and The Two Values in a List

Question

I want to intersect the 2 dictionaries, but I am not sure how to store historicalHashTags[x] too, I managed to store only the key and value of 1 dictionary.

var intersectedHashTags = currentHashTags.Keys.Intersect(historicalHashTags.Keys).
ToDictionary(x => x, x => currentHashTags[x]);

But, I want the result to be stored in a list that includes Key, currentHashTags[x], and historicalHashTags[x]

Example:

Dictionary<string, int> currentHashTags = 
{ ["#hashtag1", 100], ["#hashtag2", 77], ["#hashtag3", 150], ...} 
Dictionary<string, int> historicalHashTags = 
{ ["#hashtag1", 144], ["#hashtag4", 66], ["#hashtag5", 150], ...} 

List<(string,int,int)> result = { ["#hashtag1", 100, 144] }

Can you present a working minimalist sample that creates the actual data for the Dictionary? — radian, Jan 18 '23 at 08:09
I suspect you want to Intersect to use the same `IEqualityComparer` as the dictionaries but, what if they differ ... — Jodrell, Jan 18 '23 at 08:50

AoooR · Accepted Answer · 2023-01-18T08:47:31.350

To keep only the elements common to both dictionaries, you can use the Intersect method of type Dictionary on their keys. Then with Select method you transform this result into an IEnumerable<(string, int, int)>. Finally convert it to a list.

Dictionary<string, int> currentHashTags = new()
{
    {"#hashtag1", 11},
    {"#hashtag2", 12},
    {"#hashtag3", 13},
    {"#hashtag4", 14}
};
Dictionary<string, int> historicalHashTags = new()
{
    {"#hashtag2", 22},
    {"#hashtag3", 23},
    {"#hashtag4", 24},
    {"#hashtag5", 25}
};

List<(string, int, int)> intersectedHashTags = currentHashTags.Keys
    .Intersect(historicalHashTags.Keys)
    .Select(key => (key, currentHashTags[key], historicalHashTags[key]))
    .ToList();
// Content of intersectedHashTags:
// ("#hashtag2", 12, 22)
// ("#hashtag3", 13, 23)
// ("#hashtag4", 14, 24)

Enigmativity · Answer 2 · 2023-01-18T10:01:21.043

1

Assuming your dictionaries are of type Dictionary<string, string> and you want Dictionary<string, List<string>> as output, then this works:

Dictionary<string, List<string>> combinedHashTags =
    currentHashTags
        .Concat(historicalHashTags)
        .GroupBy(x => x.Key, x => x.Value)
        .ToDictionary(x => x.Key, x => x.ToList());

To get the intersection a simple join would work:

List<(string Key, int current, int historical)> combined =
(
    from c in currentHashTags
    join h in historicalHashTags on c.Key equals h.Key
    select (c.Key, c.Value, h.Value)
).ToList();

edited Jan 18 '23 at 10:01

answered Jan 18 '23 at 07:50

Enigmativity

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Thank you, No, my Dictionary is Dictionary, i want the result to be List, where string is the intersected key that is a unique value, int is currentHashTags values and int is historicalHashTags values. – bob mason Jan 18 '23 at 07:58
@bobmason The type `List` doesn't exist, did you mean `List<(string, int, int)>`, so a list of tuples? If so, you lost properties of dictionaries (i.g. access elements in O(1) time) – AoooR Jan 18 '23 at 08:12
yes sorry, List<(string, int, int)> , there are no lost properties, as the string is the intersected key between the 2 dictionaries, and the other 2 ints are the 2 dictionaries values, and that's all that I need. – bob mason Jan 18 '23 at 08:25

C# Intersect Two Dictionaries and Store The Key and The Two Values in a List

2 Answers2