Calculate difference from previous item with LINQ

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One option (for LINQ to Objects) would be to create your own LINQ operator:

// I don't like this name :(
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;
        }
    }
}

This enables you to perform your projection using only a single pass of the source sequence, which is always a bonus (imagine running it over a large log file).

Note that it will project a sequence of length n into a sequence of length n-1 – you may want to prepend a “dummy” first element, for example. (Or change the method to include one.)

Here’s an example of how you’d use it:

var query = list.SelectWithPrevious((prev, cur) =>
     new { ID = cur.ID, Date = cur.Date, DateDiff = (cur.Date - prev.Date).Days) });

Note that this will include the final result of one ID with the first result of the next ID… you may wish to group your sequence by ID first.

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