Get unique 3 digit zips from a list C#

.net, c#, regex

Solution

A little Linq should work. If using a list of `int`s:

var zips = new[] { 10433, 30549, 10456, 54933, 60594, 30569, 30659 };
var results = zips.GroupBy(z => z / 100).Select(g => g.First());

Or if using a list of `string`s:

var zips = new[] { "10433", "30549", "10456", "54933", "60594", "30569", "30659" };
var results = zips.GroupBy(z => z.Remove(3)).Select(g => g.First());

Another solution would be to use a custom `IEqualityComparer<T>`. For `int`s:

class ZipComparer : IEqualityComparer<int> {
    public bool Equals(int x, int y) {
        return x / 100 == y / 100;
    }
    public int GetHashCode(int x) {
        return x / 100;
    }
}

For `string`s:

class ZipComparer : IEqualityComparer<string> {
    public bool Equals(string x, string y) {
        return x.Remove(3) == y.Remove(3);
    }
    public int GetHashCode(string x) {
        return x.Remove(3).GetHashCode();
    }
}

Then to use it, you can simply call `Distinct`:

var result = zips.Distinct(new ZipComparer());

Finally, you also use MoreLINQ's `DistinctBy` extension method (also available on NuGet):

var results = zips.DistinctBy(z => z / 100);
// or
var results = zips.DistinctBy(z => z.Remove(3));

Problem

I have a list of integers that represent US ZIP codes, and I want to get unique values based on the first three digits of the ZIP code. For example this is my list: ``` 10433 30549 10456 54933 60594 30569 30659 ``` My result should contain only: ``` 10433 30549 54933 60594 30659 ``` The US ZIP codes excluded from my list are: 10456 and 30659 because I already have the ZIPs that contain 104xx and 306xx. I really don't know how to get this done, I guess it's not that hard, but I have no idea. I've made a function, that saves me the unique first three digits, and I've added some random 2 digits at the end of each zip. But it didn't worked out because I got for example 10423 but 10423 is not in my list, and I don't have a specific pattern that all my numbers have the last 2 digits in a range.

Original source