c# - Does LINQ know how to optimize "queries"? -


suppose 

 var ordered = mylist.orderby(x => x.prop1).thenby(x => x.prop2);

does mylist.orderby(x => x.prop1) return filtered list, , further filter list thenby(x => x.prop2)? in other words, equivalent 

var orderedbyprop1 = mylist.orderby(x => x.prop1); var ordered = orderedbyprop1.orderby(x => x.prop2);

???

because it's possible optimize running sorting algorithm comparator: 

var ordered = mylist.sort( (x,y) => x.prop1 != y.prop1 ? x.prop1 < y.prop1 : ( x.prop2 < y.prop2 ) );

if sort of optimization , intermediate lists not returned in process, how know how that? how write class optimizes chains of methods on itself? makes no sense.

does mylist.orderby(x => x.prop1) return filtered list

no. linq methods (at least typically) return queries, not results of executing queries.

orderby returns object which, when ask item, return first item in collection given particular ordering. until ask result it's not doing anything.

note can decent idea what's going on looking @ orderby returns. returns iorderedenumerable<t>. interface has method createorderedenumerable which:

performs subsequent ordering on elements of iorderedenumerable according key.

that method thenby uses indicate there subsequent ordering.

this means you're building of comparers want used, orderby , thenby calls before ever need generate single item in result set.

for more specifics on exactly how can go creating behavior, see jon skeet's blog series on subject.


Comments

Post a Comment

Popular posts from this blog

authentication - Mongodb revoke acccess to connect test database -

i have ec2 instance mongodb 3.2.0 installed. have user admin access. can connect user , perform operation remotely. 1 can connect database test remote if knows ip mongo 11.11.11.11. although not able make operations, want restrict access user credentials can connect server. first need create users , grant roles users, example: use admin db.createuser( { user: "myuseradmin", pwd: "abc123", roles: [ { role: "useradminanydatabase", db: "admin" } ] } ) then need start server --auth parameter mongod --auth more information here
Read more

r - Update two sets of radiobuttons reactively - shiny -

i have read ( how make choices in radiobuttons reactive in shiny? ) shows me how update radiobutton s in reactive way. however, when try , update 2 sets of buttons same data, 1 set renders. example: server: # create example data wafer <- rep(c(1:3), each=3) length <- c(1,1,2,1,1,1,3,5,1) width <- c(3,1,6,1,1,1,1,1,6) dd <- data.frame(wafer, length, width) shinyserver(function(input, output, session){ # create reactive dataframe store data values <- reactivevalues() values$df <- data.frame() # lengths , widths of wafer user input <- eventreactive(input$do, { subset(dd, wafer %in% input$wafer, select = length:width) }) # update reactive data frame lengths , widths have been selected user input observe({ if(!is.null(a())) { values$df <- rbind(isolate(values$df), a()) } }) output$wl <- renderdatatable({ a() }) # update radio buttons unique length , widths stored in values$df # ever "observe" put first in co...
Read more

ios - Realm over CoreData should I use NSFetchedResultController or a Dictionary? -

i'm working on app using core data , nsfetchedresultcontroller many reasons switch use realm. saw there "realm version" of nsfetchedresultcontroller on github wouldn't compatible current code using core data. the view controller displaying list of people same school, address book. list sublist of people studied in same city. so thinking make 1 unique request database retrieve list of people , filter locally list within 1 dictionary per school [string: anyobject] string section name within tableview , anyobject array of people. first solution, make nsfetchedresultcontroller each school predicate filtering location. delete actions etc.. handled delegates -> not compatible realm create dictionaries , update them each actions... -> works realm it's annoying code. any better solution? edit: i need clarify request: i'd write class inherit uitableviewcontroller. this class has list of people sorted in alphabetical order the tableview has ...
Read more