I want to implement a custom string IComparer in C# and apply it to a ComboBox.
Actual Results
If I set the ComboBox's Sorted property to true, the output is :
A
AA
AAA
B
BB
BBB
Wanted Results
The wanted behavior of the sorting algorithm is the following (financial developers will understand why :) ) :
AAA
AA
A
BBB
BB
B
Question
Is it possible to do it ? Are sorting algorithms needed here ?
PS : I don't need a complete answer with code, i just need an idea of how it might be done ..
EDIT
This is about credit ratings. I've omitted something in my question. The ratings have to be sorted in this order :
XXX
XX+
XX
XX-
X+
X
X-
with X in ('A','B','C') and 'A' > 'B' > 'C'
Here's a mostly implemented version:
public class MyComparer : IComparer<string>
{
public int Compare(string x, string y)
{
//todo null checks on input
var pairs = x.Zip(y, (a, b) => new { x = a, y = b });
foreach (var pair in pairs)
{
int value = pair.x.CompareTo(pair.y);
if (value != 0)
return value;
}
//if we got here then either they are the same,
//or one starts with the other
return y.Length.CompareTo(x.Length); //note x and y are reversed here
}
}
So this uses Zip to get the pairs of chars from each corresponding string until one ends, returning the appropriate value if they aren't equal. If it makes it past that then one string start with the other. For a traditional string comparison we'd just compare the lengths in the same order as the input parameters. Since we're essentially reversing the order based on length, note that the x and y are swapped on the last line. That reverses the comparison logic.
Assuming this is for credit ratings, normally this is done by having a "sort order" column on the CreditRating class that you could use to sort the list before assigning it as the data source of the drop-down.
But, a quick workaround (based on the limited possible values) would be to sort by the first letter ascending, then by the length of the string descending:
if(left[0] != right[0])
return left[0].CompareTo(right[0]);
else
return right.Length - left.Length;
Another workaround if you want more control over the order is to create a list of possible values in the "right" order and then use that to sort the list:
public class MyComparer : IComparer<string>
{
private static readonly string[] Ratings = new [] {
"CC","C","CCC-","CCC","CCC+",
"B-","B","B+","BB-","BB","BB+","BBB-","BBB","BBB+",
"A-","A","A+","AA-","AA","AA+","AAA"};
// reverse the order so that any strings not found will be put at the end.
public int Compare(string left, string right)
{
return Array.IndexOf(Ratings, right).CompareTo(Array.IndexOf(Ratings, left));
}
}
Write the IComparer so that it takes strings but compares per character,
if A[0] == B[0] go to the next character.
if B[1] == null or A[1] < B[1], return A < B.
if A[1] == null or B[1] < A[1], return B < A.
if equal...continue as needed
Related
I want to write Comparer which will return sorted list by string and in alphabet order.
This is my List:
AA
AA MN
ADIDAS
ADIDAS MEN
2KC
ISANA MEN
Here is my comparer:
public class MyComparer : IComparer<string>
{
int IComparer<string>.Compare(string x, string y)
{
if (x == y)
{
return 0;
}
if (x.Contains(y))
{
return -1;
}
else
{
return 1;
}
}
}
And this comparer sort my list like that:
2KC
ADIDAS MEN
AA MEN
AA
ISANA MEN
ADIDAS
It's correct because I want first check "longest" brands (this removes error with finding brand in product), but with this sorting I want sort it alphabetically. So my list should look like
2KC
ADIDAS MEN
AA MEN
AA
ADIDAS
ISANA MEN
I'm trying with string.Sort(), not work correctly.
You have two problems. The first is that your comparer violates the rules for IComparer.Compare. The rules say that if A.CompareTo(B) returns 1, then B.CompareTo(A) must return -1. Your comparer doesn't do that. For example, this code:
string a = "abc";
string b = "def";
MyComparer comp = new MyComparer();
int rsltab = comp.Compare(a, b); // returns 1
int rsltba = comp.Compare(b, a); // returns 1
That's plainly wrong.
Whatever else you do in your comparer, you must ensure that:
If x.Compare(y) returns -1, then y.Compare(x) returns 1.
If y.Compare(x) returns -1, then x.Compare(y) returns 1.
If x.Compare(y) returns 0, then y.Compare(x) returns 0.
See the Notes to Implementers section of the documentation for ICompareble.CompareTo for more information about the rules for comparisons. I'm kind of surprised that this information isn't repeated in the documentation for IComparer<T>.
The other problem is that I don't think you really want to test to see if x contains y. Maybe you want to check to see if x starts with y. That is:
if (x.StartsWith(y)) return 1;
But that, too, could give you some strange results. For example, what if one company's name is just a prefix of the first word in another's? For example:
Art
Artificial Flavor Company
Do you really want Artificial Flavor Company to sort before Art, even though the companies aren't related?
I suspect that what you really want is to parse the string and see if the first word of the longer string is equal to the shorter string. So given "ADIDAS" and "ADIDAS MEN", you would parse the longer string to get the first word, and compare that against the shorter string. That's not a perfect solution, but it will be much more effective than what you currently have.
I am trying to find the differences in two lists. List, "y" should have 1 unique value when compared to list "x". However, Except, does not return the difference. The, "differences" list's count always equals 0.
List<EtaNotificationUser> etaNotifications = GetAllNotificationsByCompanyIDAndUserID(PrevSelectedCompany.cmp_ID);
IEnumerable<string> x = etaNotifications.OfType<string>();
IEnumerable<string> y = EmailList.OfType<string>();
IEnumerable<string> differences = x.Except(y, new StringLengthEqualityComparer()).ToList();
foreach(string diff in differences)
{
addDiffs.Add(diff);
}
After reading a few posts and articles on the post, I created a custom comparer. The comparer looks at string length (kept it simple for testing) and obtains the Hashcode, since these are two objects of a different type (even though I convert their types to string), I thought it may have been the issue.
class StringLengthEqualityComparer : IEqualityComparer<string>
{
public bool Equals(string x, string y)
{
return x.Length == y.Length;
}
public int GetHashCode(string obj)
{
return obj.Length;
}
}
This is my first time using Except. Sounds like a great, optimized way of comparing two lists, but I can't get it to work.
Update
X - Should hold Email Addresses from the database.
GetAllNotificationsByCompanyIDAndUserID - brings back email values from the DB.
Y - Should hold all Email Addresses in the UI Grid.
What I am trying to do is detect if a new e-mail has been added to the grid. So at this point X will have the saved values from past entries. Y will have any new e-mail addresses add by the user and have not been saved yet.
I have verified this is all working correctly.
The problem is here:
IEnumerable<string> x = etaNotifications.OfType<string>();
but etaNotifications is a List<EtaNotificationUser>, none of which can be a string since string is sealed. OfType returns all instances that are of the given type - it does not "convert" each member to that type.
So x will always be empty.
Maybe you want:
IEnumerable<string> x = etaNotifications.Select(e => e.ToString());
if EtaNotificationUser has overridden ToString to give you the value you want to compare. If the value you want to compare is in a property you can use:
IEnumerable<string> x = etaNotifications.Select(e => e.EmailAddress);
or some other property.
You'll likely have to do something similar for y (unless EmailList is already a List<string> which I doubt).
Assuming you have verified that your two enumerables x and y actually contain the strings you expect them to, I believe your problem is with your string comparer. According to the docs, Enumerable.Except "Produces the set difference of two sequences. The set difference is the members of the first sequence that don't appear in the second sequence." But your equality comparer equates all strings with the same length. Thus, if a string in the first sequence happens to have the same length as a string in the second, it will not be found as different using your comparer.
Update: yup, I just tested it:
public class StringLengthEqualityComparer : IEqualityComparer<string>
{
public bool Equals(string x, string y)
{
return x.Length == y.Length;
}
public int GetHashCode(string obj)
{
return obj.Length;
}
}
string [] array1 = new string [] { "foo", "bar", "yup" };
string[] array2 = new string[] { "dll" };
int diffCount;
diffCount = 0;
foreach (var diff in array1.Except(array2, new StringLengthEqualityComparer()))
{
diffCount++;
}
Debug.Assert(diffCount == 0); // No assert.
diffCount = 0;
foreach (var diff in array1.Except(array2))
{
diffCount++;
}
Debug.Assert(diffCount == 0); // Assert b/c diffCount == 3.
There is no assert with the custom comparer but there is with the standard.
I have several columns in a listview but I am just so dummb to think up a logical sorting method to both sort items out alphabetically and numerically. Because in case of numerical values
I'd like a column's content such as:
111
13
442
23
214
to be:
13
23
111
214
442
My current sorting class looks like this:
class itemsorter:IComparer
{
public int compare (object a, object b)
{
return string.compare(((lvitem)a).text,((lvitem)b).text));
}
}
Parse your Strings to numbers before doing the comparison, in which case you can simply return the difference of the 2 numbers as your result from the compare method.
As it sounds like you still want to sort both alphabetical and numerical values, this would have to be a combined, hybrid approach with the above - such that numbers are sorted against numbers, and alphabetical values with alphabetical. You'd just need to choose which takes precedence, such that either numerical or alphabetical values always come first - necessary to maintain a stable and reflexive sort. (For example, if a is a number, and b is a non-number, return 1. If a is a non-number, and b is a number, return -1. Else, they must be of equal types, and then you can defer to the type-specific sorting.)
As ziesemer said, you can take my sample code as below, hope this will give you a hand.
class itemsorter : IComparer
{
public int compare(object a, object b)
{
int resultA, resultB;
bool markA = int.TryParse(((lvitem)a).text, out resultA);
bool markB = int.TryParse(((lvitem)b).text, out resultB)
// They are number.
if (markA && markB)
{
if (resultA > resultB)
return 1;
else if (resultA < resultB)
return -1;
else
return 0;
}
// a can convert to number,
// b can't.
if (markA && !markB)
{
return 1;
}
// b can convert to number,
// a can't.
if(!markA && markB)
{
return -1;
}
}
}
I have a string that contains numbers separated by periods. When I sort it appears like this since it is a string: (ascii char order)
3.9.5.2.1.1
3.9.5.2.1.10
3.9.5.2.1.11
3.9.5.2.1.12
3.9.5.2.1.2
3.9.5.2.1.3
3.9.5.2.1.4
etc.
I want it to sort like this: (in numeric order)
3.9.5.2.1.1
3.9.5.2.1.2
3.9.5.2.1.3
...
3.9.5.2.1.9
3.9.5.2.1.10
3.9.5.2.1.11
3.9.5.2.1.12
I know that I can:
Use the Split function to get the individual numbers
Put the values into an object
Sort the object
I prefer to avoid all of that work if it is duplicating existing functionality. Is a method in the .net framework that does this already?
Here's my working solution that also takes care of strings that are not in the right format (e.g. contain text).
The idea is to get the first number within both strings and compare these numbers. If they match, continue with the next number. If they don't, we have a winner. If one if these numbers isn't a number at all, do a string comparison of the part, which wasn't already compared.
It would be easy to make the comparer fully compatible to natural sort order by changing the way to determine the next number.
Look at that.. just found this question.
The Comparer:
class StringNumberComparer : IComparer<string>
{
public int Compare(string x, string y)
{
int compareResult;
int xIndex = 0, yIndex = 0;
int xIndexLast = 0, yIndexLast = 0;
int xNumber, yNumber;
int xLength = x.Length;
int yLength = y.Length;
do
{
bool xHasNextNumber = TryGetNextNumber(x, ref xIndex, out xNumber);
bool yHasNextNumber = TryGetNextNumber(y, ref yIndex, out yNumber);
if (!(xHasNextNumber && yHasNextNumber))
{
// At least one the strings has either no more number or contains non-numeric chars
// In this case do a string comparison of that last part
return x.Substring(xIndexLast).CompareTo(y.Substring(yIndexLast));
}
xIndexLast = xIndex;
yIndexLast = yIndex;
compareResult = xNumber.CompareTo(yNumber);
}
while (compareResult == 0
&& xIndex < xLength
&& yIndex < yLength);
return compareResult;
}
private bool TryGetNextNumber(string text, ref int startIndex, out int number)
{
number = 0;
int pos = text.IndexOf('.', startIndex);
if (pos < 0) pos = text.Length;
if (!int.TryParse(text.Substring(startIndex, pos - startIndex), out number))
return false;
startIndex = pos + 1;
return true;
}
}
Usage:
public static void Main()
{
var comparer = new StringNumberComparer();
List<string> testStrings = new List<string>{
"3.9.5.2.1.1",
"3.9.5.2.1.10",
"3.9.5.2.1.11",
"3.9.test2",
"3.9.test",
"3.9.5.2.1.12",
"3.9.5.2.1.2",
"blabla",
"....",
"3.9.5.2.1.3",
"3.9.5.2.1.4"};
testStrings.Sort(comparer);
DumpArray(testStrings);
Console.Read();
}
private static void DumpArray(List<string> values)
{
foreach (string value in values)
{
Console.WriteLine(value);
}
}
Output:
....
3.9.5.2.1.1
3.9.5.2.1.2
3.9.5.2.1.3
3.9.5.2.1.4
3.9.5.2.1.10
3.9.5.2.1.11
3.9.5.2.1.12
3.9.test
3.9.test2
blabla
No, I don't believe there's anything in the framework which does this automatically. You could write your own IComparer<string> implementation which doesn't do any splitting, but instead iterates over both strings, only comparing as much as is required (i.e. parsing just the first number of each, then continuing if necessary etc) but it would be quite fiddly I suspect. It would also need to make assumptions about how "1.2.3.4.5" compared with "1.3" for example (i.e. where the values contain different numbers of numbers).
Since the comparison you want to do on the strings is different from how strings are normally compared in .Net, you will have to use a custom string string comparer
class MyStringComparer : IComparer<string>
{
public int Compare(string x, string y)
{
// your comparison logic
// split the string using '.' separator
// parse each string item in split array into an int
// compare parsed integers from left to right
}
}
Then you can use the comparer in methods like OrderBy and Sort
var sorted = lst.OrderBy(s => s, new MyStringComparer());
lst.Sort(new MyStringComparer());
This will give you the desired result. If not then just tweak the comparer.
What you are looking for is the natural sort order and Jeff Atwood bloged about it and has links to implementations in different languages. The .NET Framework does not contain an implementation.
Is it possible for you to pad your fields to the same length on the front with 0? If so, then you can just use straight lexicographic sorting on the strings. Otherwise, there is no such method built in to the framework that does this automatically. You'll have to implement your own IComparer<string> if padding is not an option.
Not really, though you may be able to use Regexes or Linq to avoid too much wheel-reinventing. Keep in mind it will cost you much the same computationally to use something built-in as to roll your own.
Try this:
List<string> myList = GetNumberStrings();
myList.Select(s=>s.Split('.')).ToArray().
.Sort((a,b)=>RecursiveCompare(a,b))
.Select(a=>a.Aggregate(new StringBuilder(),
(s,sb)=>sb.Append(s).Append(".")).Remove(sb.Length-1, 1).ToString())
.ToList();
...
public int RecursiveCompare(string[] a, string[] b)
{
return RecursiveCompare(a,b,0)
}
public int RecursiveCompare(string[] a, string[] b, int index)
{
return index == a.Length || index == b.Length
? 0
: a[index] < b[index]
? -1
: a[index] > b[index]
? 1
: RecursiveCompare(a,b, index++);
}
Not the most compact, but it should work and you could use a y-combinator to make the comparison a lambda.
Split each string by '.', iterate through the components and compare them numerically.
This code also assumes that the number of components is signficant (a string '1.1.1' will be greater than '2.1'. This can be adjusted by altering the first if statement in the Compare method below.
int Compare(string a, string b)
{
string[] aParts = a.Split('.');
string[] bParts = b.Split('.');
/// if A has more components than B, it must be larger.
if (aParts.Length != bParts.Length)
return (aParts.Length > bParts.Length) ? 1 : -1;
int result = 0;
/// iterate through each numerical component
for (int i = 0; i < aParts.Length; i++)
if ( (result = int.Parse(aParts[i]).CompareTo(int.Parse(bParts[i]))) !=0 )
return result;
/// all components are equal.
return 0;
}
public string[] sort()
{
/// initialize test data
string l = "3.9.5.2.1.1\n"
+ "3.9.5.2.1.10\n"
+ "3.9.5.2.1.11\n"
+ "3.9.5.2.1.12\n"
+ "3.9.5.2.1.2\n"
+ "3.9.5.2.1.3\n"
+ "3.9.5.2.1.4\n";
/// split the large string into lines
string[] arr = l.Split(new char[] { '\n' },StringSplitOptions.RemoveEmptyEntries);
/// create a list from the array
List<string> strings = new List<string>(arr);
/// sort using our custom sort routine
strings.Sort(Compare);
/// concatenate the list back to an array.
return strings.ToArray();
}
You can use the awesome AlphanumComparator Alphanum natural sort algorithm by David Koelle.
Code:
OrderBy(o => o.MyString, new AlphanumComparator())
If you're gonna use the C# version change it to:
AlphanumComparator : IComparer<string>
and
public int Compare(string x, string y)
In addition to implementing your own IComparer as Jon mentions, if you call ToList() on your array, you can call the .Sort() method and pass in a function parameter that compares two values, as shown here: http://msdn.microsoft.com/en-us/library/w56d4y5z.aspx
What is the best algorithm to take array like below:
A {0,1,2,3}
I expected to order it like array below:
B {3,1,0,2}
Any ideas?
So if you have two arrays and they hold the same data just in different order then just do this:
A = B
I suspect that is not your situation so I think we need more info.
What you need to do is determine the ordering of B and then apply that ordering to A. One way to accomplish this is to undo the ordering of B and keep track of what happens along the way. Then you can do the reverse to A.
Here's some sketchy C# (sorry, I haven't actually run this)...
Take a copy of B:
List<int> B2 = new List<int>(B);
Now sort it, using a sort function that records the swaps:
List<KeyValuePair<int,int>> swaps = new List<KeyValuePair<int,int>>();
B2.Sort( delegate( int x, int y ) {
if( x<y ) return -1;
if( x==y ) return 0;
// x and y must be transposed, so assume they will be:
swaps.Add( new KeyValuePair<int,int>(x,y) );
return 1;
});
Now apply the swaps, in reverse order, to A:
swaps.Reverse();
foreach( KeyValuePair<int,int> x in swaps )
{
int t = A[x.key];
A[x.key] = A[x.value];
A[x.value] = t;
}
Depending how the built-in sort algorithm works, you might need to roll your own. Something nondestructive like a merge sort should give you the correct results.
Here's my implementation of the comparer (uses LINQ, but can be easily adapted to older .net versions). You can use it for any sorting algorithms such as Array.Sort, Enumerable.OrderBy, List.Sort, etc.
var data = new[] { 1, 2, 3, 4, 5 };
var customOrder = new[] { 2, 1 };
Array.Sort(data, new CustomOrderComparer<int>(customOrder));
foreach (var v in data)
Console.Write("{0},", v);
The result is 2,1,3,4,5, - any items not listed in the customOrder are placed at the end in the default for the given type (unless a fallback comparator is given)
public class CustomOrderComparer<TValue> : IComparer<TValue>
{
private readonly IComparer<TValue> _fallbackComparer;
private const int UseDictionaryWhenBigger = 64; // todo - adjust
private readonly IList<TValue> _customOrder;
private readonly Dictionary<TValue, uint> _customOrderDict;
public CustomOrderComparer(IList<TValue> customOrder, IComparer<TValue> fallbackComparer = null)
{
if (customOrder == null) throw new ArgumentNullException("customOrder");
_fallbackComparer = fallbackComparer ?? Comparer<TValue>.Default;
if (UseDictionaryWhenBigger < customOrder.Count)
{
_customOrderDict = new Dictionary<TValue, uint>(customOrder.Count);
for (int i = 0; i < customOrder.Count; i++)
_customOrderDict.Add(customOrder[i], (uint) i);
}
else
_customOrder = customOrder;
}
#region IComparer<TValue> Members
public int Compare(TValue x, TValue y)
{
uint indX, indY;
if (_customOrderDict != null)
{
if (!_customOrderDict.TryGetValue(x, out indX)) indX = uint.MaxValue;
if (!_customOrderDict.TryGetValue(y, out indY)) indY = uint.MaxValue;
}
else
{
// (uint)-1 == uint.MaxValue
indX = (uint) _customOrder.IndexOf(x);
indY = (uint) _customOrder.IndexOf(y);
}
if (indX == uint.MaxValue && indY == uint.MaxValue)
return _fallbackComparer.Compare(x, y);
return indX.CompareTo(indY);
}
#endregion
}
In the example you gave (an array of numbers), there would be no point in re-ordering A, since you could just use B.
So, presumably these are arrays of objects which you want ordered by one of their properties.
Then, you will need a way to look up items in A based on the property in question (like a hashtable). Then you can iterate B (which is in the desired sequence), and operate on the corresponding element in A.
Both array's contain the same values (or nearly so) but I need to force them to be in the same order. For example, in array A the value "3045" is in index position 4 and in array B it is in index position 1. I want to reorder B so that the index positions of like values are the same as A.
If they are nearly the same then here is some pseudo code:
Make an ArrayList
Copy the contents of the smaller array to the arraylist
for each item I in the larger array
FInd I in the ArrayList
Append I to a new array
Remove I from the arraylist
Could the issue be resolved using a Dictionary so the elements have a relationship that isn't predicated on sort order at all?