Question

What I’d like to be able to do is construct a LINQ query that retrieved me a few values from some DataRows when one of the fields changes. Here’s a contrived example to illustrate:

Observation   Temp  Time
------------- ----  ------
Cloudy        15.0  3:00PM
Cloudy        16.5  4:00PM
Sunny         19.0  3:30PM
Sunny         19.5  3:15PM
Sunny         18.5  3:30PM
Partly Cloudy 16.5  3:20PM
Partly Cloudy 16.0  3:25PM
Cloudy        16.0  4:00PM
Sunny         17.5  3:45PM

I’d like to retrieve only the entries when the Observation changed from the previous one. So the results would include:

Cloudy        15.0  3:00PM
Sunny         19.0  3:30PM
Partly Cloudy 16.5  3:20PM
Cloudy        16.0  4:00PM
Sunny         17.5  3:45PM

Currently there is code that iterates through the DataRows and does the comparisons and construction of the results but was hoping to use LINQ to accomplish this.

What I’d like to do is something like this:

var weatherStuff = from row in ds.Tables[0].AsEnumerable()
                   where row.Field<string>("Observation") != weatherStuff.ElementAt(weatherStuff.Count() - 1) )
                   select row;

But that doesn’t work – and doesn’t compile since this tries to use the variable ‘weatherStuff’ before it is declared.

Can what I want to do be done with LINQ? I didn’t see another question like it here on SO, but could have missed it.

Answer 1

You could use the IEnumerable extension that takes an index.

var all = ds.Tables[0].AsEnumerable();
var weatherStuff = all.Where( (w,i) => i == 0 || w.Field<string>("Observation") != all.ElementAt(i-1).Field<string>("Observation") );