Overlaying one list on another list in C#

Question

I know the code to efficiently overlay one list on another in python, using dictionaries. Here is what i mean:

input:

 dictA = {'c':2, 'e':1}; 
 print(dictA);

output:

{'c': 2, 'e': 1}

input:

dictB = {'a':0,'b':0,'c':0,'d':0,'e':0,'f':0}; 
print(dictB);

output:

{'a': 0, 'b': 0, 'c': 0, 'd': 0, 'e': 0, 'f': 0}

input:

dictB.update(dict(dictA)); 
print(dictB);

output:

{'a': 0, 'b': 0, 'c': 2, 'd': 0, 'e': 1, 'f': 0}

dictA is overlayed on dictB with the 'c' and 'e' now having the values they were assigned in dictA.

Question: What is the equivalent syntax in C# to overlay one list another as above?

Grrr, just about to post an answer and this gets closed a minute before. Unless I'm misunderstanding the code in the question, `listA` and `listB`, as their names imply, are both _lists_, **not** _dictionaries_, before and after this operation. That `Dictionary<>` would be the most logical choice in C# and that both answers happen to use `Dictionary<>` I don't think necessarily makes this a dictionary question, and, therefore, not a duplicate of that dictionary question. I'm voting to reopen because this is about lists which, as the sample data shows, have a defined ordering. — Lance U. Matthews, Apr 01 '20 at 04:06
@BACON I'm pretty sure those are python dictionaries in the question. If thats true, then their is no ordering, and they are the same as Dictionaries in C#. I think the OP just forgot to change their variables from `listA` to `dictA`. — RoadRunner, Apr 01 '20 at 04:07
@RoadRunner The title and body say "list" three times. `listA` seems to be converted to a dictionary ("using dictionaries") with `dict(listA)`. Even if that's not the case, how do we know that the title and "Question:" at the end aren't to be taken literally? It does say that the code presented is the way to "efficiently overlay" lists in Python; if those are, in fact, Python dictionaries that shouldn't presuppose that they have to be dictionaries in C#, too. Also, that's curious that you were able to answer after the question was closed. — Lance U. Matthews, Apr 01 '20 at 04:24
@BACON Looks like the question has been reopened. Also having `Console.WriteLine` printing python code is incorrect. I would have just left it as `print`. Those are not lists, but dictionaries, and this question needs to address this. It will only cause confusion to future readers. — RoadRunner, Apr 01 '20 at 12:02

score 3 · Accepted Answer · answered Apr 01 '20 at 02:58

3

You can just use foreach.

foreach (var i in listB)
    listA[i.Key] = i.Value;

Note that listA and listB is actually Dictionary type.

answered Apr 01 '20 at 02:58

MyBug18

2,135
2
11
25

While I think this is a viable solution you've reversed his notation. I'm going to vote yours up over the more complete accepted solution because this is simpler and doesn't require hidden magic with extension methods. – Peter Wone Apr 01 '20 at 03:08

score 0 · Answer 2 · answered Apr 01 '20 at 03:03

I'd suggest doing something like this:

void Main()
{
    var listA = new Dictionary<char, int>() { { 'c', 2 }, { 'e', 1 } };
    var listB = new Dictionary<char, int>() { { 'a', 0 }, { 'b', 0 }, { 'c', 0 }, { 'd', 0 }, { 'e', 0 }, { 'f', 0 },  };

    listA.ForEach(x => listB[x.Key] = x.Value);
}

public static class Ex
{
    public static void ForEach<T>(this IEnumerable<T> source, Action<T> action)
    {
        foreach (var t in source)
            action(t);
    }
}

score 0 · Answer 3 · answered Apr 01 '20 at 04:02

If you don't mind using LINQ to create a new Dictionary<char, int>, you could use ToDictionary():

var merged = listB
    .ToDictionary(kvp => kvp.Key, 
                  kvp => listA.TryGetValue(kvp.Key, out int value) ? value : kvp.Value);

Which basically attempts to access the key in listA with TryGetValue(), and if it succeeds it sets the value in listA, otherwise keep the listB value.

Overlaying one list on another list in C#

3 Answers3