C# Select only few member pairs from Object array and store it in Dictionary

Question

I have an Object array of my class (Item) like below,

class Item
{
    int id;
    int value;
    bool isSelected;
}

Item itemArr[] = new Item [MAXCOUNT];

To retrieve the selected Item ids and store into list, I am using below code

List<int> listOfItems = new List<int>();
listOfItems.AddRange(itemArr.Where(p => (p.IsSelected)
                            .Select(p => p.ID)).ToArray());

I have one more Dictionary which indicates item id as key and item value as value,

Dictionary<int,int> itemidValueMap = new Dictionary<int,int>();

How to get item id and value pair and store it into this dictionary?

This question is not duplicate. I need to select only the elements which satisfies the condition and store only them in dictionary. But previous question says about adding all the elements into dictionary.

Your code wonßt even compile, as `Item.IsSeleceted` has no member `Select`. You probably missed a bracket. I supoose you mean `istOfItems.AddRange(itemArr.Where(p => p.IsSelected).Select(p => p.ID).ToArray());`. — MakePeaceGreatAgain, Aug 10 '17 at 09:50
You can't add a range of items of the type if `ID` as they are int and not `Item`. Please add compiling code — Gilad Green, Aug 10 '17 at 09:51
You probably want to add those items into your list that are selected - not just their ids. Omit the `Select` from your code. — MakePeaceGreatAgain, Aug 10 '17 at 09:52
@HimBromBeere I have edited my code now to List from List. Hope this compiles. — srajeshnkl, Aug 10 '17 at 09:55
@MongZhu : How to convert array to dictionary explains about getting all the array elements into dictionary. I need solution to select only the values which satisfies the condition and store them into dictionary. — srajeshnkl, Aug 10 '17 at 10:05

Gilad Green · Answer 1 · 2017-08-10T10:02:03.057

2

To construct the dictionary use ToDictionary:

var dictionary = itemArr.Where(p => p.IsSelected)
                        .ToDictionary(key => key.id, value => value.value);

If you have the same id a few times it will cause an exception (keys are not unique) so use ToLookup or first GroupBy and then ToDictionary.

var lookup = itemArr.Where(p => p.IsSelected)
                    .ToLookup(key => key.id, value => value.value);

var dictionary = itemArr.Where(p => p.IsSelected)
                        .GroupBy(i => i.id)
                        .ToDictionary(key => key.Key, value => value.Select(i => i.value));

As a side note you can populate the ID list a bit nicer:

var listOfItems = itemArr.Where(p => p.IsSelected).Select(p => p.ID).ToList();

edited Aug 10 '17 at 10:02

answered Aug 10 '17 at 09:55

Gilad Green

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Your code selects all the itemid and value into dictionary. I need to select the items which has IsSelected value as true when adding it into dictionary. How to do ? – srajeshnkl Aug 10 '17 at 10:01
@RajeshSubramanian - Did this help you understand how to do so? – Gilad Green Aug 10 '17 at 10:16

score 2 · Answer 2 · answered Aug 10 '17 at 09:55

To get the selected IDs your query should look like this:

List<int> selectedIDs = itemArr.Where(item => item.IsSelected).Select(item => item.id).ToList();

Note that the result is a List<int>, not a List<Item>.

To get your dictionary try this:

var idToValue = itemArr.ToDictionary(item => item.id, item => item.value);

This only works if your IDs are unique. If they are not, you'll need to group them, but I don't know how you want to aggregate the values of items with the same ID.

If you want that dictionary only for selected items, you simply insert the Where clause again:

var idToValue = itemArr.Where(item => item.IsSelected).ToDictionary(item => item.id, item => item.value);

score 1 · Answer 3 · answered Aug 10 '17 at 09:57

You need to change this

listOfItems.AddRange(itemArr.Where
                  (p => (p.IsSelected).Select(p => p.ID)).ToArray());

to

listOfItems.AddRange(itemArr.Where
                  (p => p.IsSelected).ToArray());

then

Dictionary<int,int> itemidValueMap = listOfItems.ToDictionary(item => item.id,
                                   item => item.value)

C# Select only few member pairs from Object array and store it in Dictionary

3 Answers3