Is there a more concise Linq expression for inserting a constant between each element in a list?

Question

Here is what I have at the moment:

public List<double> GetStrokeDashArray(List<double> dashLengths, double gap) { return dashLengths .SelectMany(dl => new[] { dl, gap }) .Take(dashLengths.Count * 2 - 1) .ToList(); } 

Results for GetStrokeDashArray(new List<double> { 2, 4, 7, 11, 16 }, 2);

 2, 2, 4, 2, 7, 2, 11, 2, 16 

Answer 1

I think the best way is to create a special extension method for it:

 public static IEnumerable<T> Intersperse<T>(this IEnumerable<T> source, T value) { bool first = true; foreach(T item in source) { if (!first) yield return value; yield return item; first = false; } } 

Then you can write your method as follows:

 public List<double> GetStrokeDashArray(List<double> dashLengths, double gap) { return dashLengths .Intersperse(gap) .ToList(); } 

Answer 2

Instead, you can use an iterator block:

 IEnumerable<Double> GetStrokeDashArray(IEnumerable<Double> dashLengths, Double gap) { using (var enumerator = dashLengths.GetEnumerator()) { if (enumerator.MoveNext()) yield return enumerator.Current; while (enumerator.MoveNext()) { yield return gap; yield return enumerator.Current; } } } 

Answer 3

This is the best I could offer for the Zip method:

 var gaps = Enumerable.Repeat(gap, dashLengths.Count - 1); return dashLengths.Take(1) .Concat(gaps.Zip(dashLengths.Skip(1), (x, y) => new[] {x, y}) .SelectMany(x => x)) .ToList(); 

Less compressed ... but uses Zip .