Develop Reference

C# sort object list with start position and loop

I have a strange question :)
I have a object list looking like this:
var list = new []
{
new { Id = 1, Name = "Marcus" },
new { Id = 2, Name = "Mattias" },
new { Id = 3, Name = "Patric" },
new { Id = 4, Name = "Theodor" },
};
I would like to sort the list providing a "start id"
For example, if I provide "start id" 3, the result should look like this:
Id
Name
3
Patric
4
Theodor
1
Marcus
2
Mattias
I have no idea where to start, so I really need some help from you coding gods
The list is from a sql table, but it does not matter for me where the sort take place (in sql query or in c# code)

Try this:
var list = new []
{
new { Id = 1, Name = "Marcus" },
new { Id = 2, Name = "Mattias" },
new { Id = 3, Name = "Patric" },
new { Id = 4, Name = "Theodor" },
};
var start_id = 3;
var max_id = list.Max(y => y.Id);
var result =
from x in list
orderby (x.Id + max_id - start_id) % max_id
select x;
I get:

With LINQ to objects you can do something like that:
var list = new []
{
new { Id = 1, Name = "Marcus" },
new { Id = 2, Name = "Mattias" },
new { Id = 3, Name = "Patric" },
new { Id = 4, Name = "Theodor" },
};
var startId = 3;
var result = list
.GroupBy(i => i.Id >= startId ? 1 : 0) // split in two groups
.OrderByDescending(g => g.Key) // sort to have the group with startId first
.Select(g => g.OrderBy(i => i.Id)) // sort each group
.SelectMany(i => i) // combine result
.ToList();
Console.WriteLine(string.Join(", ", result.Select(i => i.Id))); // prints "3, 4, 1, 2"

You require 2 criteria to apply:
Order ascending by Id.
Return the Ids greater than threshold before the Ids less than threshold.
You can try:
var offset = 3;
var sorted1 = list
.OrderBy(item => item.Id < offset)
.ThenBy(item => item.Id);
The OrderBy condition yields true if Id is less than offset and false otherwise.
true is greater than false and therefore is returned later
A dirty way could also be:
var offset = 3;
var sorted2 = list
.OrderBy(item => unchecked((uint)(item.Id - offset)));
Here the offset is subtracted from Id and the result converted to unsigned int to make the negative values become very large positive ones. A little hacky. Might not work with queries against SQL providers.

Here's a toy Non-Linq Version
object[] ShiftList(int id)
{
var list = new dynamic[]
{
new { Id = 1, Name = "Marcus" },
new { Id = 2, Name = "Mattias" },
new { Id = 3, Name = "Patric" },
new { Id = 4, Name = "Theodor" },
};
Span<dynamic> listSpan = list;
int indexFound = -1;
for (int i = 0; i < list.Length; i++)
{
if (listSpan[i].Id == id)
{
indexFound = i;
}
}
if (indexFound is -1)
{
return list;
}
var left = listSpan.Slice(0, indexFound);
var right = listSpan[indexFound..];
object[] objs = new object[list.Length];
Span<object> objSpan = objs;
right.CopyTo(objSpan);
left.CopyTo(objSpan[right.Length..]);
return objs;
}

Try using foreach and iterate over each object in your list:
foreach (var item in list)
{
}
from here you should be able to use some of the collection methods for a list to reorder your list.

Finding differences within 2 Lists of string arrays

I am looking to find the differences between two Lists of string arrays using the index 0 of the array as the primary key.
List<string[]> original = new List<string[]>();
List<string[]> web = new List<string[]>();
//define arrays for List 'original'
string[] original_a1 = new string[3]{"a","2","3"};
string[] original_a2 = new string[3]{"x","2","3"};
string[] original_a3 = new string[3]{"c","2","3"};
//define arrays for List 'web'
string[] web_a1 = new string[3]{"a","2","3"};
string[] web_a2 = new string[3]{"b","2","3"};
string[] web_a3 = new string[3]{"c","2","3"};
//populate Lists
original.Add(original_a1);
original.Add(original_a2);
original.Add(original_a3);
web.Add(web_a1);
web.Add(web_a2);
web.Add(web_a3);
My goal is to find what is in List 'original' but NOT in 'web' by using index 0 as the primary key
This is what I tried.
List<string> differences = new List<string>(); //differences go in here
string tempDiff = ""; // I use this to try and avoid duplicate entries but its not working
for(int i = 0; i < original.Count; i++){
for(int j = 0; j< web.Count; j++){
if(!(original[i][0].Equals(web[j][0]))){
tempDiff = original[i][0];
}
}
differences.Add(tempDiff);
}
OUTPUT:
foreach(string x in differences){
Console.WriteLine("SIZE " + differences.Count);
Console.WriteLine(x);
ConSole.ReadLine();
}
SIZE 3
SIZE 3
x
SIZE 3
x
Why is it reporting the mismatch 3 times instead of once?

Using linq you can just go:
var differences = orignal.Except(web).ToList();
Reference here
This will give you the values that are in original, that don't exist in web
Sorry didn't read your question properly, to answer your question:
You have a nested for-loop. So for each value of original (3) it will loop through all values of web (3), which is 9 loops total.
In 3 cases it doesn't match and therefore outputs 3 times.

I think this is what you want. I use Linq to grab the primary keys, and then I use Except to do original - web. By the way, you can use == instead of Equals with strings in C# because C# does a value comparison as opposed to a reference comparison.
List<string[]> original = new List<string[]>
{
new string[3] { "a", "2", "3" },
new string[3] { "x", "2", "3" },
new string[3] { "c", "2", "3" }
};
List<string[]> web = new List<string[]>
{
new string[3] { "a", "2", "3" },
new string[3] { "b", "2", "3" },
new string[3] { "c", "2", "3" }
};
var originalPrimaryKeys = original.Select(o => o[0]);
var webPrimaryKeys = web.Select(o => o[0]);
List<string> differences = originalPrimaryKeys.Except(webPrimaryKeys).ToList();
Console.WriteLine("The number of differences is {0}", differences.Count);
foreach (string diff in differences)
{
Console.WriteLine(diff);
}
And here it is without Linq:
var differences = new List<string>();
for (int i = 0; i < original.Count; i++)
{
bool found = false;
for (int j = 0; j < web.Count; j++)
{
if (original[i][0] == web[j][0])
{
found = true;
}
}
if (!found)
{
differences.Add(original[i][0]);
}
}

To answer your question: It is a nested for loop as stated in JanR's answer. This approach will make you reiterate to your web count 9 times, thus listing your mismatched key three times.
What could be a better way to do is this:
//Check for originals not introduced in web.
if(original.Count > web.Count)
{
for(int y = web.Count; y < original.Count; y++)
{
differences.Add(original[y][0]);
}
}
//Check if Web has value, if not, everything else is done on the first for loop
if(web.Count > 0)
{
for(int i = 0; i < original.Count; i++)
{
if(!original[i][0].Equals(web[i][0]))
differences.Add(original[i][0]);
}
}
Also, the output is in a for loop, when you just need one result, the length of the mismatch. You can do that without a loop.
Console.WriteLine("SIZE " + differences.Count);
This is, of course to make it kinda simpler if you're not used to using LINQ statements, but if you can do so with LINQ, then by all means, use LINQ as it's more efficient.

You can get the difference by using Except extension method like this:
var originalDic = original.ToDictionary(arr => arr.First());
var webDic = web.ToDictionary(arr => arr.First());
var differences =
originalDic
.Except(webDic, kvp => kvp.Key)
.Select(kvp => kvp.Value)
.ToList();
The trick here is to first convert your original and web lists into a Dictionary using the first element of each array as key and then perform Except.

Compare two list of objects C#

I want to compare two list of objects. These lists contains the same type of objects. I create a new List in my programme and i want to compare it at the old list which is in the database. I get it with a stored procedure, then i put it into an object.
The old list : the new list :
*Category 1* Category 5
*Category 2* Category 6
*Category 3* *Category 4*
Category 4
Here the aim is to delete the first three Category in the old list, beacause they don't exist in the new list. And to delete the Category 4 in the new list because category 4 already exists in the old list.
It is possible to use à method like Equals() or use two foreach loop to browse the lists ?
Thanks for you answers and advises

You can use the linq, except and where
var a = new List<string> { "a", "b", "c" };
var b = new List<string> { "c", "d", "e" };
var temp = a.Intersect(b).ToList();
b = b.Except(a).ToList();
a = temp;
Output:
a: "c"
b: "d", "e"
Note: It is probably more efficient to do this without linq
var a = new List<string> { "a", "b", "c" };
var b = new List<string> { "c", "d", "e" };
for(int i = 0; i < a.Count; i++)
if(b.Contains(a[i]))
b.Remove(a[i]);
else
a.Remove(a[i--]);
If you need to compare based on a particular value
for(int i = 0; i < a.Count; i++)
{
var obj = b.Where(item => item.Category == a[i].Category);
if(obj.Any())
b.Remove(obj.First());
else
a.Remove(a[i--]);
}

It's not the most pretty of implementations but the fastest way you can do this is:
var tempA = new HashSet<int>(inputA.Select(item => item.Id));
var tempB = new HashSet<int>(inputB.Select(item => item.Id));
var resultA = new List<Category>(inputA.Count);
var resultB = new List<Category>(inputB.Count);
foreach (var value in inputA)
if (tempB.Contains(value.Id))
resultA.Add(value);
foreach (var value in inputB)
if (!tempA.Contains(value.Id))
resultB.Add(value);
resultA.TrimExcess();
resultB.TrimExcess();
// and if needed:
inputA = resultA;
inputB = resultB;
If you need more than item.id as unique then use a new Tuple such as:
inputA.Select(item => new Tuple<int, string>(item.Id, item.Title));
Another option is to override .GetHashCode in your category class such as:
public override int GetHashCode()
{
return Id.GetHashCode();
}
public override bool Equals(object obj)
{
var typedObj = obj as Category;
if (typedObj == null)
return false;
return Title == typedObj.Title && Id == typedObj.Id && Rank == typedObj.Rank;
}

I would solve this by sorting the two list and iterating over the first and second list. I would compare the current item of the first list to the current item from the second. If a match is found I remove the match from the second list and I move to the next item in both lists, otherwise the current item of the first list is removed from it and the iteration continues in the first list.

unequal size lists to merge

I have searched without success to a similar situation as follows.
I have two lists, list A and list B.
List A is composed of 10 objects created from ClassA which contains only strings.
List B is composed of 100 objects created from ClassB which only contains decimals.
List A is the header information.
List B is the data information.
The relationship between the two lists is:
Row 1 of list A corresponds to rows 1-10 of list B.
Row 2 of list A corresponds to rows 11-20 of list B.
Row 3 of list A corresponds to rows 21-30 of list B.
etc.........
How can I combine these two lists so that when I display them on the console the user will see a header row followed immediately by the corresponding 10 data rows.
I apologize if this has been answered before.

Ok, that should work. Let me know in case I got anything wrong.
List<ClassA> listA = GetListA()// ...
List<ClassB> listB = GetListA()// ...
if(listB.Count % listA.Count != 0)
throw new Exception("Unable to match listA to listB");
var datasPerHeader = listB.Count / listA.Count;
for(int i = 0; i < listA.Count;i++)
{
ClassA header = listA[i];
IEnumerable<ListB> datas = listB.Skip(datasPerHeader*i).Take(datasPerHeader);
Console.WriteLine(header.ToString());
foreach(var data in datas)
{
Console.WriteLine("\t{0}", data.ToString());
}
}

Here is some code that should fulfill your request - I am going to find a link for the partition extension as I can't find it in my code anymore:
void Main()
{
List<string> strings = Enumerable.Range(1,10).Select(x=>x.ToString()).ToList();
List<decimal> decimals = Enumerable.Range(1,100).Select(x=>(Decimal)x).ToList();
var detailsRows = decimals.Partition(10)
.Select((details, row) => new {HeaderRow = row, DetailsRows = details});
var headerRows = strings.Select((header, row) => new {HeaderRow = row, Header = header});
var final = headerRows.Join(detailsRows, x=>x.HeaderRow, x=>x.HeaderRow, (header, details) => new {Header = header.Header, Details = details.DetailsRows});
}
public static class Extensions
{
public static IEnumerable<List<T>> Partition<T>(this IEnumerable<T> source, Int32 size)
{
for (int i = 0; i < Math.Ceiling(source.Count() / (Double)size); i++)
yield return new List<T>(source.Skip(size * i).Take(size));
}
}
That Partition method is the one that does the grunt work...
And here is the link to the article - LINK
EDIT 2
Here is better code for the Main() method... Rushed to answer and forgot brain:
void Main()
{
List<string> strings = Enumerable.Range(1,10).Select(x=>x.ToString()).ToList();
List<decimal> decimals = Enumerable.Range(1,100).Select(x=>(Decimal)x).ToList();
var detailsRows = decimals.Partition(10);
var headerRows = strings; //just renamed for clarity from other code
var final = headerRows.Zip(detailsRows, (header, details) => new {Header = header, Details = details});
}

This should be pretty straight forward unless I'm missing something.
var grouped = ListA.Select((value, index) =>
new {
ListAItem = value,
ListBItems = ListB.Skip(index * 10).Take(10)
})
.ToList();
Returns back an anonymous type you can loop through.
foreach (var group in grouped)
{
Console.WriteLine("List A: {0}", group.Name);
foreach (var listBItem in group.ListBItems)
{
Console.WriteLine("List B: {0}", listBItem.Name);
{
}

The easiest way may be something like this:
var listA = new List<string>() { "A", "B", "C", ... }
var listB = new List<decimal>() { 1m, 2m, 3m, ... }
double ratio = ((double)listA.Count) / listB.Count;
var results =
from i in Enumerable.Range(0, listB.Count)
select new { A = listA[(int)Math.Truncate(i * ratio)], B = listB[i] };
Or in fluent syntax:
double ratio = ((double)listA.Count) / listB.Count;
var results = Enumerable.Range(0, listB.Count)
.Select(i => new { A = listA[(int)Math.Truncate(i * ratio)], B = listB[i] });
Of course if you know you will always have 10 items in listB for each item in listA, you can simplify this to:
var results =
from i in Enumerable.Range(0, listB.Count)
select new { A = listA[i / 10], B = listB[i] };
Or in fluent syntax:
var results = Enumerable.Range(0, listB.Count)
.Select(i => new { A = listA[i / 10], B = listB[i] });
This will return a result set like
{ { "A", 1 },
{ "A", 2 },
{ "A", 3 }
..,
{ "A", 10 },
{ "B", 11 },
{ "B", 12 },
{ "B", 13 },
...
{ "B", 20 },
{ "C", 21 },
...
{ "J", 100 }
}

Tricky algorithm... finding multiple combinations of subsets within nested HashSets?

I have a problem where I have to find multiple combinations of subsets within nested hashsets. Basically I have a "master" nested HashSet, and from a collection of "possible" nested HashSets I have to programmatically find the "possibles" that could be simultaneous subsets of the "master".
Lets say I have the following:
var master = new HashSet<HashSet<string>>(new HashSet<string>[] {
new HashSet<string>( new string[] { "A", "B", "C"}),
new HashSet<string>( new string[] { "D", "E"}),
new HashSet<string>( new string[] { "F"})
}
);
var possible1 = new HashSet<HashSet<string>>(new HashSet<string>[] {
new HashSet<string>( new string[] { "A", "B", "C"}),
new HashSet<string>( new string[] { "F"})
}
);
var possible2 = new HashSet<HashSet<string>>(new HashSet<string>[] {
new HashSet<string>( new string[] { "D", "E"})
}
);
var possible3 = new HashSet<HashSet<string>>(new HashSet<string>[] {
new HashSet<string>( new string[] { "F"})
}
);
var possible4 = new HashSet<HashSet<string>>(new HashSet<string>[] {
new HashSet<string>( new string[] { "X", "Y", "Z"})
}
);
var possible5 = new HashSet<HashSet<string>>(new HashSet<string>[] {
new HashSet<string>( new string[] { "A", "B" }),
new HashSet<string>( new string[] { "D", "E"})
}
);
The output I should get from my algorithm should be as follows:
All possible combination subsets:
possible1 and possible2
possible3 and possible5
possible2 and possible3
possible1
possible2
possible3
possible5
I'm trying to figure out the best way to approach this. There is, of course, the brute force option, but I'm trying to avoid that if I can.
I just hope my question was clear enough.
EDIT
To further elaborate on what constitutes a subset, here are some examples, given the master {{"A","B","C"},{"C","D","E",F"},{"X","Y","Z"}} :
{{"A","B"}{"C","D"}} would be a subset of
{{"A","B","C"},{"X","Y"}} would be a subset
{{"A","B"},{"A","B"}} would NOT be a subset
{{"A","B","C","D"}} would NOT be a subset
{{"A","B","C"},{"C","D","X"}} would NOT be a subset
Basically each child set needs to be a subset of a corresponding child in the master.

Use bruteforce:
public static int IsCsInMaster(HashSet<string> childSubset, List<HashSet<string>> master, int startIndex)
{
for (int i = startIndex; i < master.Count; i++)
if (childSubset.IsSubsetOf(master[i])) return i;
return -1;
}
public static bool IsChildInMaster(List<HashSet<string>> child, List<HashSet<string>> master)
{
foreach (var childSubset in child) if (IsCsInMaster(childSubset, master, 0) == -1) return false;
return true;
}
public static bool IsChildInMasterMulti(List<HashSet<string>> child, List<HashSet<string>> master)
{
Dictionary<int, int> subsetChecker = new Dictionary<int, int>();
List<IEnumerable<int>> multiMatches = new List<IEnumerable<int>>();
int subsetIndex;
// Check for matching subsets.
for (int i = 0; i < child.Count; i++)
{
subsetIndex = 0;
List<int> indexes = new List<int>();
while ((subsetIndex = IsCsInMaster(child[i], master, subsetIndex)) != -1)
{
indexes.Add(subsetIndex++);
}
if (indexes.Count == 1)
{
subsetIndex = indexes[0];
if (subsetChecker.ContainsKey(subsetIndex)) return false;
else subsetChecker[subsetIndex] = subsetIndex;
}
else
{
multiMatches.Add(indexes);
}
}
/*** Check for multi-matching subsets. ***/ //got lazy ;)
var union = multiMatches.Aggregate((aggr, indexes) => aggr.Union(indexes));
// Filter the union so only unmatched subset indexes remain.
List<int> filteredUion = new List<int>();
foreach (int index in union)
{
if (!subsetChecker.ContainsKey(index)) filteredUion.Add(index);
}
return (filteredUion.Count >= multiMatches.Count);
}
And in code:
IsChildInMasterMulti(possible2, master)
The code does not handle the {{"A","B"},{"A","B"}} case, though. That is a LOT more difficult (flagging used subsets in master, maybe even individual elements - recursively).
Edit2: The third method handles the {{"A","B"},{"A","B"}} case as well (and more).

Use the simplest solution possible.
Keep in mind that if someone else has to look at your code they should be able to understand what it's doing with as little effort as possible. I already found it hard to understand from your description what you want to do and I haven't had to read code yet.
If you find that it's too slow after it's working optimize it then.
If possible write unit tests. Unit tests will ensure that your optimized solution is also working correctly and will help others ensure their changes don't break anything.