c# - Lambda: efficiently find, modify, then group elements -

let's have list<pagehit>, pagehit having these properties: datetime requesttime , int pageresponsetime.

ultimately i'm trying round each requesttime value nearest 30 minute increment (using how can round time nearest x minutes?), group them together, , overall average of pageresponsetime increment block (real world user story: track average page response time per 30 minute block).

this far i've gotten, brain won't show me how efficiently average each increment without gross loop. there way in step 1?

// step 1: round request times pagehitlist.foreach(x => x.requesttime = x.requesttime.roundup(timespan.fromminutes(30));  // step 2: average of each increment block ? 

haven't tested it, do:

var avg = pagehitlist.groupby(x => x.requesttime.roundup(timespan.fromminutes(30)));                      .select(hit => new {                                   hit.key,                                   average = hit.average(x => x.pageresponsetime)                               });