Replace sequence of duplicates

Question

Is it possible to replace every sequential duplicate using linq? I tried using groups with no success. Basically, I need to get the following results:

string[] arr = new [] { "a", "a", "a", "b", "b", "b", "b", "c", "c", "c", "a", "a" }; // input
string[] res = new [] { "a", "R", "R", "b", "R", "R", "R", "c", "R", "R", "a", "R" }; // output

Selman Genç · Accepted Answer · 2019-05-02T11:25:14.390

7

Select method has an overload that takes the index, you can make use of it to check previous item:

res = arr.Select((x, idx) => idx != 0 && arr[idx - 1] == x ? "R" : x).ToArray();

edited May 02 '19 at 11:25

answered May 02 '19 at 10:12

Selman Genç

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This code won't compile because `res` is `string[]`, not `IEnumerable` – tukaef May 02 '19 at 10:18
3

@tukaef Don't be that picky a simple call to `.ToArray` will convert it back to `string[]`. This is a fine answer – Fabjan May 02 '19 at 10:19
Agreed, `.ToArray()` will convert `IEnumerable` to `string[]` – Prasad Telkikar May 02 '19 at 10:20
This code doesn't compile. `CS0173 Type of conditional expression cannot be determined because there is no implicit conversion between 'char' and 'string'` – Enigmativity May 02 '19 at 11:11
@Enigmativity You are likely trying to access an index of a string instead of the array (so `someString[4]`) which gives a char. – EpicKip May 02 '19 at 12:15
@EpicKip - No, the OP had `'R'` instead of `"R"`. He's edited the question. Check the change log. – Enigmativity May 02 '19 at 12:29
@Enigmativity As far as I can tell he had neither `"` nor `'` in the original code – EpicKip May 02 '19 at 12:32

score -1 · Answer 2 · answered May 02 '19 at 10:16

You could use John Skeets Extension which he stated here.

  public static IEnumerable<TResult> SelectWithPrevious<TSource, TResult>
    (this IEnumerable<TSource> source,
     Func<TSource, TSource, TResult> projection)
{
    using (var iterator = source.GetEnumerator())
    {
        if (!iterator.MoveNext())
        {
             yield break;
        }
        TSource previous = iterator.Current;
        while (iterator.MoveNext())
        {
            yield return projection(previous, iterator.Current);
            previous = iterator.Current;
        }
    }
}

And then use it like this:

var out  = arr.SelectWithPrevious((prev, curr) => prev==curr? R:curr));

Did you test this before posting? – Enigmativity May 02 '19 at 11:16 — Enigmativity, May 02 '19 at 11:16

score -2 · Answer 3 · answered May 02 '19 at 11:08

-2

This works fine:

string[] arr = new [] { "a", "a", "a", "b", "b", "b", "b", "c", "c", "c", "a", "a" }; // input

string [] res = arr.StartWith("_").Zip(arr, (a0, a1) => a0 == a1 ? "R" : a1).ToArray();

answered May 02 '19 at 11:08

Enigmativity

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I would love to know why this got a down-vote. It's better in some ways to the accepted answer - this works with any fixed enumerable, not just arrays. – Enigmativity May 02 '19 at 12:31
@tukaef - Ah, `StartWith` is from Microsoft's Interactive Extensions. – Enigmativity May 02 '19 at 22:39

Replace sequence of duplicates

3 Answers3