LINQ - is SkipWhile broken?

c# linq linq-to-objects
Judah Gabriel Himango picture Judah Gabriel Himango · Mar 26, 2010 · Viewed 12.7k times · Source

I'm a bit surprised to find the results of the following code, where I simply want to remove all 3s from a sequence of ints:

var sequence = new [] { 1, 1, 2, 3 };
var result = sequence.SkipWhile(i => i == 3); // Oh noes! Returns { 1, 1, 2, 3 }

Why isn't 3 skipped?

My next thought was, OK, the Except operator will do the trick:

var sequence = new [] { 1, 1, 2, 3 };
var result = sequence.Except(i => i == 3); // Oh noes! Returns { 1, 2 }

In summary,

  • Except removes the 3, but also removes non-distinct elements. Grr.
  • SkipWhile doesn't skip the last element, even if it matches the condition. Grr.

Can someone explain why SkipWhile doesn't skip the last element? And can anyone suggest what LINQ operator I can use to remove the '3' from the sequence above?

Answer

Ahmad Mageed picture Ahmad Mageed · Mar 26, 2010

It's not broken. SkipWhile will only skip items in the beginning of the IEnumerable<T>. Once that condition isn't met it will happily take the rest of the elements. Other elements that later match it down the road won't be skipped.

int[] sequence = { 3, 3, 1, 1, 2, 3 };
var result = sequence.SkipWhile(i => i == 3); 
// Result: 1, 1, 2, 3

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