在不使用foreach的情况下在List中查找对象

Question

I have list of following class Student

class student
{
   Guid id;
   string name;
}

The list contains multiple students. To search a student with specific id, I need to use foreach loop and compare id of each student.

I am looking for a better alternative instead of foreach loop. Is there any alternative available?

[EDIT]: What I meant by better alternative is optimized solution in terms of execution time and performance

[EDIT2] One more twist, what if id is Guid.

Thanks,

Ram

Answer 1

If each student can appear in the list only once, you might want to use a Dictionary<int, stutent> instead. Then you will have a very efficient way of looking up students by id.

Dictionary<int, student> students = GetSomeStudents();

// locate student with id = 42
if (students.ContainsKey(42))
{
    var student = students[42];
    // do something useful
}

Answer 2

Nothing will really change the fact that you have to iterate over the list. But you can use LINQ:

List<Student> studentsList = ReadStudentsList();
var student = studentsList.Where(s => s.id == ID_IM_LOOKING_FOR).Single();

Based on an answer by @Fredrik Mörk, this could be shortened to:

var student = studentsList.Single(s => s.id == ID_IM_LOOKING_FOR);

Also note, that Single() will throw an exception, if no student is found. If you'd prefer to just return null , use SingleOrDefault() .

But what you actually want to be doing is storing your students in a map:

Dictionary<int, Student> students = ReadStudentsMap();
var student = students[ID_IM_LOOKING_FOR];

This has a way better performance (O(1) for hashtables, O(log(n)) for trees) than looking through the list (O(n))!

Answer 3

You can use LINQ:

var student = students.SingleOrDefault(s => s.id == 12);

Enumerable.SingleOrDefault Method

Answer 4

如果您不想使用foreach lop，LINQ是可选项，因此您的代码将如下所示

var student = studentsList.FirstOrDefault(s => s.id == ID_IM_LOOKING_FOR);

Answer 5

Here's an idea, to avoid having to iterate over the list :

• Sort the list by Student.Id
• Implement a more sophisticated search algorithm (eg Binary Search binary search )

Answer 6

You can use any collection that uses hash lookup because their member look up is very quick. eg Dictionary <K,V> , OrderedDictionary . There is also the nongeneric Hashtable which is mostly rendered redundant by the generic Dictionary<K,V> . In Big-O notation, element retrieval time by key for these collections is O(1) , which is as good as it gets.

For accessing elements, to not incur the cost of two lookups, instead of using ContainsKey , you can use the TryGetValue method, eg:

Dictionary<Guid, Student> students = GetStudents();

Student student;
if (students.TryGetValue(guid, out student))
{
    // found
}

BTW, OrderedDictionary also allows you to access elements by index.