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Care este cea mai rapida metoda pentru a obtine index-ul unui element din IEnumerable

De cate ori nu v-ati lovit de acest caz? Sa aveti un obiect de tip enumerable si sa fim nevoiti sa obtineti indexul unui element.
Exista mai multe metode pentru a rezolva aceasta problema, cea mai simpla din ele este:
obj.ToList().FindIndex( x => x =="Blog" )
Pentru cazul in care am cautat elementul care are valoarea "Blog".
Daca ne punem problema vitezei putem sa scriem noi o metoda popie:
public static int FindIndex<T>(this IEnumerable<T> elemente, Predicate<T> conditie)
int index = 0;
foreach (var item in elemente)
if (conditie(item))
return index;
return -1;
Prima optiune ne sperie putin din cauta la ToList(). Ne punem problema performantei, am prefera parca sa nu folosim ToList() si sa mergem pe metoda scrisa de noi.
elemente.Select((item, index) => new { item, index })
.Where(p => p.item == "Blog")
.Select(p => p.index + 1)
.FirstOrDefault() - 1;
Ce am facut mai sus? Nimic complicat, am obtinut toate elementele din enumeratie sub forma (element, index). Daca nu stiati Select-ul ne permite acest lucru, ne poate fi de ajutor aceasta metoda in multe alte situatii. Apoi am selectat doar elementele care respecta conditia, iar apoi am selectat index-ul la acest element. FistOrDefault ne garanteaza ca acest algoritm se va oprii dupa ce a gasit primul element care indeplineste conditia data. M-am folosit de +1 si -1 la Select si la FirstOrDefault pentru a putea avea rezultatul -1 cand nu se gaseste nici un element in enumeratie ce sa indeplineasca conditia data.
Mai exista si alte mecanisme, folosind TakeWhile.
Problema nu este neaparat cum, ci care este mai optim mecanism.
Desii e greu de crezut cel mai rapid mecanism este ToList().FistIndex(). Se pare ca nu intoadeauna un cod scris de mana este mai rapid ca si un API default oferit de catre .NET, chiar daca vorbim de LINQ.


  1. Interesant - ai facut niste masuratori ca sa vezi cu cat e mai rapida metoda ce foloseste LINQ/Select ?
    (la prima metoda, nu mi-e prea clar de ce era nevoie sa se foloseasca ToList())

  2. Problema e ca query-ul scris cu LINQ e gresit. Punand clauza Select((item, index) => new { item, index }) prima, am invalidat tot lazy-loading-ul care il ofera LINQ.
    Corect ar fi:
    elemente.Where(p => p.item == "Blog")
    .Select((item, index) => new { item, index })


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