check if value already exists

Question

I have dictionary which holds my books:

Dictionary<string, book> books

Book definiton:

class book
{
    string author { get; set; }

    string title { get; set; }
}

I have added some books to the dictionary.

How can I check if there is a book in the Dictionary that matches the title provided by the user?

What value are you providing for the 'string' part of ''? – zimdanen May 03 '12 at 17:57 — zimdanen, May 03 '12 at 17:57

score 41 · Answer 1 · answered May 03 '12 at 17:57

41

books.ContainsKey("book name");

answered May 03 '12 at 17:57

Brendan

3,415
24
26

8

He want to know if it contains a book (value of dictionnary) with a known title property I think, not the dictionnary key. – Amaranth May 03 '12 at 17:59

score 39 · Accepted Answer · answered May 03 '12 at 18:00

39

If you're not using the book title as the key, then you will have to enumerate over the values and see if any books contain that title.

foreach(KeyValuePair<string, book> b in books) // or foreach(book b in books.Values)
{
    if(b.Value.title.Equals("some title", StringComparison.CurrentCultureIgnoreCase))
        return true
}

Or you can use LINQ:

books.Any(tr => tr.Value.title.Equals("some title", StringComparison.CurrentCultureIgnoreCase))

If, on the other hand, you are using the books title as the key, then you can simply do:

books.ContainsKey("some title");

answered May 03 '12 at 18:00

SPFiredrake

3,852
18
26

THANK YOU, your first code helped, that comes to LINQ, I have never tried it before – Bublik May 03 '12 at 18:11
There is .ContainsValue currently in newer version of .net framework. See @ https://msdn.microsoft.com/en-us/library/a63811ah(v=vs.110).aspx – nkalfov Oct 31 '17 at 15:20

score 6 · Answer 3 · answered May 03 '12 at 18:06

6

If you are allowed to use LINQ, try using the below code:

bool exists = books.Any(b => (b.Value != null && b.Value.title == "current title"));

answered May 03 '12 at 18:06

Channs

2,091
1
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20

score 2 · Answer 4 · answered May 03 '12 at 18:01

In your dictionary, does the key hold the title? If yes, use ContainsKey as the other answers. If the key is something else altogether, and you want to check the value's (Book object's) title attribute, you'd have to do it manually like this:

foreach(KeyValuePair<string,book> kvp in books) {
    if (kvp.Value.title == "some title")
        return kvp.Key;
}

return String.Empty; //not found

check if value already exists

4 Answers4

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