I would like to take a list of objects and convert it to a dictionary where the key is a field in the object, and the value is a list of a different field in the objects that match on the key. I can do this now with a loop but I feel this should be able to be accomplished with linq and not having to write the loop. I was thinking a combination of GroupBy and ToDictionary but have been unsuccessful so far.
Here's how I'm doing it right now:
var samplesWithSpecificResult = new Dictionary<string, List<int>>();
foreach(var sample in sampleList)
{
List<int> sampleIDs = null;
if (samplesWithSpecificResult.TryGetValue(sample.ResultString, out sampleIDs))
{
sampleIDs.Add(sample.ID);
continue;
}
sampleIDs = new List<int>();
sampleIDs.Add(sample.ID);
samplesWithSpecificResult.Add(sample.ResultString, sampleIDs);
}
The farthest I can get with .GroupBy().ToDictionay() is Dictionary<sample.ResultString, List<sample>>.
Any help would be appreciated.
Try the following
var dictionary = sampleList
.GroupBy(x => x.ResultString, x => x.ID)
.ToDictionary(x => x.Key, x => x.ToList());
The GroupBy clause will group every Sample instance in the list by its ResultString member, but it will keep only the Id part of each sample. This means every element will be an IGrouping<string, int>.
The ToDictionary portion uses the Key of the IGrouping<string, int> as the dictionary Key. IGrouping<string, int> implements IEnumerable<int> and hence we can convert that collection of samples' Id to a List<int> with a call to ToList, which becomes the Value of the dictionary for that given Key.
Yeah, super simple. The key is that when you do a GroupBy on IEnumerable<T>, each "group" is an object that implements IEnumerable<T> as well (that's why I can say g.Select below, and I'm projecting the elements of the original sequence with a common key):
var dictionary =
sampleList.GroupBy(x => x.ResultString)
.ToDictionary(
g => g.Key,
g => g.Select(x => x.ID).ToList()
);
See, the result of sampleList.GroupBy(x => x.ResultString) is an IEnumerable<IGrouping<string, Sample>> and IGrouping<T, U> implements IEnumerable<U> so that every group is a sequence of Sample with the common key!
Dictionary<string, List<int>> resultDictionary =
(
from sample in sampleList
group sample.ID by sample.ResultString
).ToDictionary(g => g.Key, g => g.ToList());
You might want to consider using a Lookup instead of the Dictionary of Lists
ILookup<string, int> idLookup = sampleList.ToLookup(
sample => sample.ResultString,
sample => sample.ID
);
used thusly
foreach(IGrouping<string, int> group in idLookup)
{
string resultString = group.Key;
List<int> ids = group.ToList();
//do something with them.
}
//and
List<int> ids = idLookup[resultString].ToList();
var samplesWithSpecificResult =
sampleList.GroupBy(s => s.ResultString)
.ToDictionary(g => g.Key, g => g.Select(s => s.ID).ToList());
What we 're doing here is group the samples based on their ResultString -- this puts them into an IGrouping<string, Sample>. Then we project the collection of IGroupings to a dictionary, using the Key of each as the dictionary key and enumerating over each grouping (IGrouping<string, Sample> is also an IEnumerable<Sample>) to select the ID of each sample to make a list for the dictionary value.
Related
I have List<Dictionary<DateTime, Points[]>> taskResult generated from tasks
var taskResult = tasks.Select(t => t.Result).ToList();
var data = new Dictionary<DateTime, Points[]>();
in my function I want to return Dictionary<DateTime, Points[]> data but I cant figure out how to do that. I tried using foreach but had no luck
Enumerable.SelectMany extension method is right tool for the job, which combines many collections into one. Dictionary is a collection of key-value pairs.
var combined = dictionaries
.SelectMany(dictionary => dictionary.Select(pair => pair))
.GroupBy(pair => pair.Key)
.ToDictionary(
group => group.Key,
group => group.SelectMany(pair => pair.Value).ToArray());
Approach above will merge points of same date if original dictionaries contain duplicated dates
Because Dictionary implements IEnumerable you can remove .Select in first call of SelectMany.
Alternative for .GroupBy is .ToLookup method, which can have multiple values per one key.
var combined = dictionaries
.SelectMany(dictionary => dictionary)
.ToLookup(pair => pair.Key, pair.Value)
.ToDictionary(
lookup => lookup.Key,
lookup => lookup.SelectMany(points => points).ToArray());
I often find myself doing the below to extract properties from a list of objects just to create an aggregated list. How would this be expressed with LINQ?
var totalErrors =
new Dictionary<string, string>();
foreach (var res in results)
{
foreach (var err in res.Errors)
{
totalErrors
.Add(err.Key, err.Value);
}
}
return totalErrors;
You can use SelectMany and ToDictionary methods:
var result = results
.SelectMany(x => x.Errors) // get all Errors in one sequence
.ToDictionary(x => x.Key, x => x.Value); // create new dictionary based on this Enumerable
SelectMany() projects each element of a sequence to an IEnumerable<T> and flattens the resulting sequences into one sequence. And ToDictionary() creates a Dictionary<TKey, TValue> from an IEnumerable<T> according to a specified key selector function.
You can do an aggregation on two levels with SelectMany, like this:
var totalErrors = results
.SelectMany(r => r.Errors)
.ToDictionary(e => e.Key, e => e.Value);
SelectMany "flattens" a collection of collections into a single level, at which point you can apply ToDictionary to a flattened list.
You can use SelectMany :
var allErrors = Results.SelectMany( res=>res.Errors );
//foreach( var error in allErrors )...
var dictionary = allErrors.ToDictionary( x=>x.Key, x=> x.Value );
I have a Dictionary<string, string> and another List<string>. What I am trying to achieve is a linq query to get all items out of the dictionary where any values from said dictionary are in the List<string>.
I found this post to be helpful, LINQ querying a Dictionary against a List . And was able to write the following linq expression, however my results never actually return anything.
What I have so far.
Data is the dictionary and PersonList is the list of strings.
var Persons = PersonList.Where(x => Data.ContainsKey(x))
.Select(z => new { key = z, value = Data[z] })
.ToList();
Are you looking for keys or values? If you're looking for values use
var Persons = Data.Where(kvp => PersonList.Contains(kvp.Value))
.ToDictionary(kvp => kvp.Key, kvp => kvp.Value);
If instead you really want keys then your code should work but another option would be:
var Persons = Data.Where(kvp => PersonList.Contains(kvp.Key))
.ToDictionary(kvp => kvp.Key, kvp => kvp.Value);
Try this one:
var Persons = Data.Where(x=>PersonList.Contains(x.Value))
.Select(x=>new { key=x.Key, value=x.Value})
.ToList();
I converted the result to a list, because I noticed that you used it in your code. If you want it to a dictionary, just take a look to the answer provided by D Stanley.
I think you don't have to convert it ToDictionary, because your source is a dictionary:
var Persons = Data.Where(kvp => personList.Contains(kvp.Key))
.Select(x => x);
I quickly tested it in LinqPad, but if this is a bad idea or I'm wrong, please leave a comment.
Can somebody suggest me what Linq operator should I use to write an elegant code to convert List to Dictionary of list?
Eg. I have a list of person (List<Person>) and I wanted to convert that to dictionary of list (like Dictionary<string, List<Person>> using the person's last name as a key. I needed that for quickly lookup the list of persons by the last name
To get a Dictionary<string, List<Person>> from List<Person>:
var dictionary = list
.GroupBy(p => p.LastName)
.ToDictionary(g => g.Key, g => g.ToList());
This isn't what you asked for, but you can use Philip response for that :-)
var lookup = myList.ToLookup(p => p.Surname);
This will create an ILookup<string, Person> that is something very similar to what you wanted (you won't have a Dictionary<string, List<Person>> but something more similar to a Dictionary<string, IEnumerable<Person>> and and the ILookup is readonly)
you can also use as follows.
foreach(var item in MyList)
{
if(!myDictionary.Keys.Contains(item))
myDictionary.Add(item,item.value);
}
edit: Thanks Jason, the fact that it was a dictionary isn't that important. I just wanted the runtime to have a low runtime. Is that LINQ method fast? Also, I know this is off topic but what does the n => n mean?
I have a list of numbers and I want to make another list with the numbers that appear most at the beginning and the least at the end.
So what I did was when through the list and checked if the number x was in the dictionary. If it wasn't then I made the key x and the value one. If it was then I changed the value to be the value plus one.
Now I want to order the dictionary so that I can make a list with the ones that appear the most at the beginning and the least at the end.
How can I do that in C#?
ps. runtime is very important.
So it sounds like you have a Dictionary<int, int> where the key represents some integer that you have in a list and corresponding value represents the count of the number of times that integer appeared. You are saying that you want to order the keys by counts sorted in descending order by frequency. Then you can say
// dict is Dictionary<int, int>
var ordered = dict.Keys.OrderByDescending(k => dict[k]).ToList();
Now, it sounds like you started with a List<int> which are the values that you want to count and order by count. You can do this very quickly in LINQ like so:
// list is IEnumerable<int> (e.g., List<int>)
var ordered = list.GroupBy(n => n)
.OrderByDescending(g => g.Count())
.Select(g => g.Key)
.ToList();
Or in query syntax
var ordered = (from n in list
group n by n into g
orderby g.Count() descending
select g.Key).ToList();
Now, if you need to have the intermediate dictionary you can say
var dict = list.GroupBy(n => n)
.ToDictionary(g => g.Key, g => g.Count());
var ordered = dict.Keys.OrderByDescending(k => dict[k]).ToList();
Use the GroupBy extension on IEnumerable() to group the numbers and extract the count of each. This creates the dictionary from the list and orders it in one statement.
var ordered = list.GroupBy( l => l )
.OrderByDescending( g => g.Count() )
.ToDictionary( g => g.Key, g.Count() );
You may also consider using SortedDictionary.
It sorts the items on the basis of key, while insertion. more..
List<KeyValuePair<type, type>> listEquivalent =
new List<KeyValuePair<type, type>>(dictionary);
listEquivalent.Sort((first,second) =>
{
return first.Value.CompareTo(second.Value);
});
Something like that maybe?
edit: Thanks Jason for the notice on my omission