List
, equalsTwo C# Lists may be equal when element order is ignored. We develop an algorithm to test for this condition. Order, and element counts, matter here.
There are many possible approaches, but we want the simplest. We look at some research and develop a C# solution.
We use Dictionary
and compare frequencies. We see a generic method—it receives parameters of a caller-specified type. The syntax uses the angle brackets.
TryGetValue
is used instead of ContainsKey
. The TryGetValue
method eliminates extra hash key computations.UnorderedEqual
receives ICollections
and checks length. If the 2 parameters have different counts, they cannot be equal.using System; using System.Collections.Generic; class Program { static void Main() { var la = new List<int>() { 1, 0, 4, 200, -40 }; var lb = new List<int>() { -40, 200, 4, 1, 0 }; var lc = new List<int>() { 3, 5, 4, 9, 11 }; var ld = new List<int>() { 6, 6, 100 }; var le = new List<int>() { 6, 100, 100 }; Console.WriteLine(UnorderedEqual(la, lb)); Console.WriteLine(UnorderedEqual(la, lc)); Console.WriteLine(UnorderedEqual(lc, ld)); Console.WriteLine(UnorderedEqual(ld, le)); var a1 = new int[] { 1, 2, 5 }; var a2 = new int[] { 2, 5, 1 }; var a3 = new int[] { 1, 1, 3 }; var a4 = new int[] { 3, 3, 1 }; Console.WriteLine(UnorderedEqual(a1, a2)); Console.WriteLine(UnorderedEqual(a1, a3)); Console.WriteLine(UnorderedEqual(a3, a4)); } static bool UnorderedEqual<T>(ICollection<T> a, ICollection<T> b) { // Require that the counts are equal. if (a.Count != b.Count) { return false; } // Initialize new Dictionary of the type. var d = new Dictionary<T, int>(); // Add each key's frequency from collection A to the Dictionary. foreach (T item in a) { int c; if (d.TryGetValue(item, out c)) { d[item] = c + 1; } else { d.Add(item, 1); } } // Add each key's frequency from collection B to the Dictionary. // ... Return early if we detect a mismatch. foreach (T item in b) { int c; if (d.TryGetValue(item, out c)) { if (c == 0) { return false; } else { d[item] = c - 1; } } else { // Not in dictionary return false; } } // Verify that all frequencies are zero. foreach (int v in d.Values) { if (v != 0) { return false; } } // We know the collections are equal. return true; } }True False False False True False False
The Dictionary
used in this method is for storing each key of type T and its frequency. If a specific key is found once, its frequency will be set to 1, for example.
Dictionary
as a key. If an element occurs twice, its frequency is incremented.TryGetValue
helps avoid hash computations. Each element in the second collection is decremented in the Dictionary
.List 1 contents: 1, 2, 4 List 2 contents: 2, 1, 4 Equal?: True List 1 contents: 5, 4, 6 List 2 contents: 6, 5, 4 Equal?: True List 1 contents: 1, 2, 4 List 2 contents: 1, 4 Equal?: False List 1 contents: 1, 5 List 2 contents: 2, 5 Equal?: False List 1 contents: 1, 2 List 2 contents: 1, 2 Equal?: True
We use a custom generic. You can rewrite the solution simply by changing the arguments to int[]
, and the Dictionary
and loops to use int
instead of T.
We saw a solution to checking unordered elements that is a generic method, which works for both string
and int
. It uses Dictionary
and avoids excessive lookups.