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POHow to calculate changes of nullable numbers with Linq
 

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  1. POHow to calculate changes of nullable numbers with Linq
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    <p>I need to calculate changes off of time series of nullable numbers. The following code gets the job done:</p> <pre><code>public static double?[] GetChanges(double?[] x) { if(x.Length == 1) throw new Exception("Time Series Too Short"); var ret = new double?[x.Length - 1]; for (int i = 1; i &lt; x.Length; i++) { ret[i-1] = (x[i - 1].HasValue &amp;&amp; x[i].HasValue) ? x[i] - x[i - 1] : null; } return ret; } </code></pre> <p>Is there a better way to accomplish that with Linq? The library is using .Net 3.5. Right now I cannot use Zip, because that comes with .Net 4.</p> <p>Edit: following the advice by mquander and Eric Lippert, I have come up with the following code which runs on 3.5:</p> <pre><code>public class Tuple&lt;T&gt; { public Tuple(T first) { First = first; } public T First { get; set; } } public class Tuple&lt;T, T2&gt; : Tuple&lt;T&gt; { public Tuple(T first, T2 second) : base(first) { Second = second; } public T2 Second { get; set; } public static Tuple&lt;T1, T2&gt; New&lt;T1, T2&gt;(T1 t1, T2 t2) { return new Tuple&lt;T1, T2&gt;(t1, t2); } } public static class EnumerableExtensions { public static IEnumerable&lt;Tuple&lt;T, T&gt;&gt; Pairs&lt;T&gt;(this IEnumerable&lt;T&gt; seq) { using (var enumerator = seq.GetEnumerator()) { enumerator.MoveNext(); var prior = enumerator.Current; while (enumerator.MoveNext()) { yield return Tuple&lt;T, T&gt;.New(prior, enumerator.Current); prior = enumerator.Current; } } } } </code></pre> <p>I am using this code as follows:</p> <pre><code> public static IEnumerable&lt;double?&gt; GetChanges2(double?[] x) { if (x.Length == 1) throw new Exception("Time Series Too Short"); return x.Pairs().Select(p =&gt; p.Second - p.First); } </code></pre> <p>Any suggestions on further improvement are welcome. I will be back when I have VS2010 and .Net 4, so that I can try out the approaches suggested in both answers.</p> <p>Thanks!</p>
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    1. COI am confused by this code, specifically by the body of the loop. What purpose does the conditional operator achieve? Why is the body of the loop not simply "ret[i-1] = x[i]-x[i-1];" ???
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    2. COOh, I didn't personally know that you could subtract nullable numeric types and have it handle nulls correctly. Awesome.
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    3. CO@mquander: yep, that's the whole point of nullable arithmetic. We automatically "lift" the operations to nullable. Where it breaks down, unfortunately, is in comparisons. In C#, when you compare a null double equal to another null double the result should be *null*. "null" means "I don't know", and the answer to "does one thing I don't know equal another thing I don't know" is not "true", is not "false", is "I don't know!" But comparisons always produce bools, not nullable bools. VB gets it right; in VB equality is three-valued.
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