C#•3y ago

❔ How do you merging Dictionaries into one Dictionary?

Has anyone merged two dictionaries of same type into one?

I read something like this


var result = dictionaries.SelectMany(dict => dict)
                         .ToLookup(pair => pair.Key, pair => pair.Value)
                         .ToDictionary(group => group.Key, group => group.First());

I read something like this


var result = dictionaries.SelectMany(dict => dict)
                         .ToLookup(pair => pair.Key, pair => pair.Value)
                         .ToDictionary(group => group.Key, group => group.First());

Lookup(https://learn.microsoft.com/en-us/dotnet/api/system.linq.lookup-2?view=net-7.0). But will this code make the dictionary distinct (that values and keys don't repeat). I rather use same key and add the value from the dictionary i'm merging/joining. Maybe this option makes it ignore

new Dictionary<string, long>(StringComparer.OrdinalIgnoreCase);

new Dictionary<string, long>(StringComparer.OrdinalIgnoreCase);

Lookup Class (System.Linq)

Represents a collection of keys each mapped to one or more values.

40 Replies

Pokey•3y ago

This one got my curiosity going Short answer is yes, it does appear to, and I think I understand why Long answer is... Yea sure but theres gotta be a simpler way to achieve what you want to achieve? Because you're just taking the first instance of each key and discarding any others

Pokey•3y ago

See https://dotnetfiddle.net/rynDoT for my example demo of your code

Hercules @ C# | C# Online Compiler | .NET Fiddle

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HerculesOP•3y ago

@Pokey I'll check it out. @Pokey It works fine. But since you had two dictionaries with A,a that means that your result should be.

A:aa 
B:bb
C:cc
D:dd
E:ee
F:ff

<--- two a's because you had that value on two dictionaries

A:aa 
B:bb
C:cc
D:dd
E:ee
F:ff

<--- two a's because you had that value on two dictionaries

Pokey•3y ago

Well no, that thing literally runs the code. The result you see was produced on the fly by running the code You do not have two of each, because you have chosen to take the .First() Nowhere have you concatenated anything

HerculesOP•3y ago

No, the result is correct! for the code. But what im looking for is 2 a's

Pokey•3y ago

I see Then you will want to not select the .First()

HerculesOP•3y ago

Let's remove it then. In you example.

Pokey•3y ago

https://dotnetfiddle.net/mtEx2o

Hercules @ C# | C# Online Compiler | .NET Fiddle

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Pokey•3y ago

It works, but the important thing is, do you understand why it works?

HerculesOP•3y ago

Thanks i saw you append fix for each group item. Yeah i do, Tell me if im wrong. 1. I like to think select as a mapper and SelectMany same for Collections/lists/etc. Basiclly looping through all dictionaries 2. To lookup every values on keys (a collection of keys each mapped to one or more values. MSLEARN) 3. Make it to Dictionary, where group (kvp) goes through key and value but 4. instead of picking group.first we append each char in value (string) with a loop. So in short group is all keys and values. left side and right side.

group => group.Key () (left side) , group => right side

group => group.Key () (left side) , group => right side

I tried changing group.values but that dosen't go because of the lookup gets the values from key.

Pokey•3y ago

I think you pretty much have it It ultimately comes down to understanding what each function in the linq produces You may find it even easier to put each linq call into a variable and use your IDE's debugger to view the result after each step

HerculesOP•3y ago

Yes. A nice trick for that is to read it backwards. 1. create dict 2.Get value by lookup (with key) 3. loop through all dicts in listofDicts (selectMany)

Pokey•3y ago

I think there is probably an easier way to achieve what you want to ultimately but without more context it would be hard to know

HerculesOP•3y ago

I think this is the simplest way to do it. Your example was perfect. i .net fiddle

Pokey•3y ago

well I mean the "simplest" way would be to foreach each dictionary, foreach each item in that dictionary and then have an output dictionary which you use to check key existence and assemble with .Add

HerculesOP•3y ago

Something like this?

foreach(var dict in listOfDicts)
             foreach (var kvp in dict)
             {
                 if (newDict.ContainsKey(kvp.Key))
                 {
                     newDict[kvp.Key] += kvp.Value;
                 }
                 else
                 {
                     newDict.Add(kvp.Key, kvp.Value);
                 }
             }

foreach(var dict in listOfDicts)
             foreach (var kvp in dict)
             {
                 if (newDict.ContainsKey(kvp.Key))
                 {
                     newDict[kvp.Key] += kvp.Value;
                 }
                 else
                 {
                     newDict.Add(kvp.Key, kvp.Value);
                 }
             }

Pokey•3y ago

If newDict is Dictionary<string, string> yea I think that should work

HerculesOP•3y ago

That dosen't work, it basically copies the first Dictionary on the listOfDicts

Pokey•3y ago

let me see one second

Pokey•3y ago

https://dotnetfiddle.net/Lj0aK3

Hercules @ C# | C# Online Compiler | .NET Fiddle

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Pokey•3y ago

works fine

HerculesOP•3y ago

Huhh.. https://dotnetfiddle.net/cVXJef It does, i must've missed something. when i tried that. you see me newDict is of <string, long>

Hercules @ C# | C# Online Compiler | .NET Fiddle

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Pokey•3y ago

Would have been useful if you said this at the start xD

HerculesOP•3y ago

That just means that the lowercase letters are numbers instead. The way we handle the types are the same, really. xD Maybeee not with the stringbuilder because i have to cast it to int64

Pokey•3y ago

Well no, actually, it changes my first answer a lot

HerculesOP•3y ago

How? Now im curious

Pokey•3y ago

instead of .First I would use .Sum This is one of those situations where I ask, what do you REALLY want to achieve here? Where are you coming from and where are you going to?

HerculesOP•3y ago

You know i have to try it out now. .Sum() @Pokey Works perfectly

Pokey•3y ago

Your example works? So

HerculesOP•3y ago

https://dotnetfiddle.net/gRV4lb this is your work really. 👏🏼

Hercules @ C# | C# Online Compiler | .NET Fiddle

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HerculesOP•3y ago

I have two options to choose now. xD

dictionaries.SelectMany(dict => dict).ToLookup(pair => pair.Key, pair => pair.Value).ToDictionary(group => group.Key, group => group.Sum());

or 

foreach(var dict in dictionaries){
             foreach (var kvp in dict)
             {
                 if (newDict.ContainsKey(kvp.Key))
                 {
                     newDict[kvp.Key] += kvp.Value;
                 }
                 else
                 {
                     newDict.Add(kvp.Key, kvp.Value);
                 }
             }
        }

dictionaries.SelectMany(dict => dict).ToLookup(pair => pair.Key, pair => pair.Value).ToDictionary(group => group.Key, group => group.Sum());

or 

foreach(var dict in dictionaries){
             foreach (var kvp in dict)
             {
                 if (newDict.ContainsKey(kvp.Key))
                 {
                     newDict[kvp.Key] += kvp.Value;
                 }
                 else
                 {
                     newDict.Add(kvp.Key, kvp.Value);
                 }
             }
        }

Pokey•3y ago

https://dotnetfiddle.net/ltw7bf Here's my take

Hercules @ C# | C# Online Compiler | .NET Fiddle

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Pokey•3y ago

https://dotnetfiddle.net/EmciP6 Simplified a little further. Unnecessary .Select()

Hercules @ C# | C# Online Compiler | .NET Fiddle

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HerculesOP•3y ago

Why not just add .Sum that gives same result.

Pokey•3y ago

where?

HerculesOP•3y ago

You changed lookup to groupby and added itm => itm.value inside sum; You should just leave it and have group.sum() that would result on same. Question here is whats faster groupby or lookupfor() -- and just thinking about it i think groupby key is faster xD nvm i like your solution

Pokey•3y ago

Pokey•3y ago

You meant this? Because this doesn't work because its a KVP not a long https://benchmarkdotnet.org/

Pokey•3y ago

https://tryitands.ee

Accord•3y ago

Was this issue resolved? If so, run /close - otherwise I will mark this as stale and this post will be archived until there is new activity.

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❔ How do you merging Dictionaries into one Dictionary?