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Split List of int and keep reference to the original elements c# [duplicate]

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C# List of references to another list elements
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Closed 2 years ago.
I have a list of integer:
List<int> foo = new List<int>() {1, 2, 3, 4, 5, 6, 7, 8};
what i need to slice it based on the parity of the index so:
List<int> bar1 = foo.Where((i, v) => v % 2 == 0).ToList();
List<int> bar2 = foo.Where((i, v) => v % 2 == 1).ToList();
The issue is that if now i set
bar1[0] = -1
the original foo list remains unchanged.
Is there any work-around in order to change the value of bar1 and bar2 while also changing the values of the original foo list?

To keep the format you want and i do not recommend it you have to fill the collection with reference type object for example this would work
public class ReferenceInt
{
public int Value { get; set; } = 0;
}
now this would work :
List<ReferenceInt> foo = new List<ReferenceInt>()
{
new ReferenceInt(){ Value = 1 },
new ReferenceInt(){ Value = 2 },
new ReferenceInt(){ Value = 3 },
new ReferenceInt(){ Value = 4 },
new ReferenceInt(){ Value = 5 },
new ReferenceInt(){ Value = 6 },
new ReferenceInt(){ Value = 7 },
new ReferenceInt(){ Value = 8 }
};
now you have to filter per value inside the reference object
List<ReferenceInt> bar1 = foo.Where(v => v.Value % 2 == 0).ToList();
List<ReferenceInt> bar2 = foo.Where(v => v.Value % 2 == 1).ToList();
and this changed the value in all collections that has the ReferenceInt in it.
So both foo and bar1 will show the same thing
bar1[1].Value = 17;

You can create a proxy to work in-between of source list and filtered view.
public class Indexer<T> : IEnumerable<T>
{
public T this[int index]
{
get => All().ElementAt(index);
set
{
var i = 0;
foreach (var item in _source)
if (_filter(item))
{
if (i++ == index)
_source[i] = value;
break;
}
}
}
public IEnumerable<T> All() => _source.Where(o => _filter(o));
readonly IList<T> _source;
readonly Func<T, bool> _filter;
public Indexer(IList<T> source, Func<T, bool> filter)
{
_source = source;
_filter = filter;
}
IEnumerator<T> IEnumerable<T>.GetEnumerator() => All().GetEnumerator();
IEnumerator IEnumerable.GetEnumerator() => All().GetEnumerator();
}
This proxy will take care to handle indexes and simplify the usage:
List<int> foo = new List<int>() { 1, 2, 3, 4, 5, 6, 7, 8 };
var bar1 = new Indexer<int>(foo, o => o % 2 == 0);
Console.WriteLine(string.Join(",", bar1)); // 2,4,6,8
bar1[0] = -1;
Console.WriteLine(string.Join(",", bar1)); // 4,6,8
Console.WriteLine(string.Join(",", foo)); // 1,-1,3,4,5,6,7,8

Using Group by with x amount of elements

Here's a list, think of it as rows and columns where rows are going down and columns are side ways. the column count will always be the same for all rows.
var dataValues = new List<List<string>>()
{
//row 1
new List<string>(){"A","12","X","P8" },
//row 2
new List<string>(){"B","13","Y","P7" },
//row 3
new List<string>(){"C","12","Y","P6" },
//row 4
new List<string>(){"A","14","X","P5" },
//....
new List<string>(){"D","15","Z","P4" },
new List<string>(){"A","13","X","P3" },
new List<string>(){"B","14","Y","P2" },
new List<string>(){"C","13","Z","P1" },
};
The user providers a list of indexes to group by.
var userParam= new List<int>() { 0, 2 };
my question is how do i dynamically group dataValues by the userParam where user param is n amount of index. In the example above it will gorup by the first column and the 3rd. However the index can change and the amount of indexes can change aswell
example
var userParam2 = new List<int>() { 0, 2};
var userParam3 = new List<int>() { 0};
var userParam4 = new List<int>() { 0,1,2};
i know how to group by when i know how many indexes there will be (the the case below it's 2 index parameters), however when it's dynamic (x amount) then i do not know how to do this
var result = dataValues.GroupBy(e => new { G1 = e[userParam2 [0]], G2 = e[userParam2 [1]] });

You could use a Custom Comparer to achieve this :
1 - Declaration of GroupByComparer that inherit from IEqualityComparer :
public class GroupByComparer : IEqualityComparer<List<string>>
{
private static List<int> _intList;
public GroupByComparer(List<int> intList)
{
_intList = intList;
}
public bool Equals(List<string> x, List<string> y)
{
foreach (int item in _intList)
{
if (x[item] != y[item])
return false;
}
return true;
}
public int GetHashCode(List<string> obj)
{
int hashCode = 0;
foreach (int item in _intList)
{
hashCode ^= obj[item].GetHashCode() + item;
}
return hashCode;
}
}
2 - Call group by with EqualityComparer like :
var userParam = new List<int>() { 0, 2 };
var result = dataValues.GroupBy(e => e, new GroupByComparer(userParam));
I hope you find this helpful.

I believe i have something but this looks slow please let me know if there is anyway better of doing this.
var userParams = new List<int>() { 0, 2 };
var dataValues = new List<List<string>>()
{
new List<string>(){"A","12","X","P8" },
new List<string>(){"B","13","Y","P7" },
new List<string>(){"C","12","Y","P6" },
new List<string>(){"A","14","X","P5" },
new List<string>(){"D","15","Z","P4" },
new List<string>(){"A","13","X","P3" },
new List<string>(){"B","14","Y","P2" },
new List<string>(){"C","13","Z","P1" },
};
var result = new List<(List<string> Key, List<List<string>> Values)>();
result.Add((new List<string>(), dataValues));
for (int index = 0; index < userParams.Count; index++)
{
var currentResult = new List<(List<string> Key, List<List<string>> Values)>();
foreach (var item in result)
{
foreach (var newGroup in item.Values.GroupBy(e => e[userParams[index]]))
{
var newKey = item.Key.ToList();
newKey.Add(newGroup.Key);
currentResult.Add((newKey, newGroup.ToList()));
}
}
result = currentResult;
}
foreach(var res in result)
{
Console.WriteLine($"Key: {string.Join(#"\", res.Key)}, Values: {string.Join(" | ", res.Values.Select(e=> string.Join(",",e)))}");
}
final result
Key: A\X, Values: A,12,X,P8 | A,14,X,P5 | A,13,X,P3
Key: B\Y, Values: B,13,Y,P7 | B,14,Y,P2
Key: C\Y, Values: C,12,Y,P6
Key: C\Z, Values: C,13,Z,P1
Key: D\Z, Values: D,15,Z,P4

Building dynamic query in a loop using Expression trees

I have a system that allows different criteria pertaining to Sales to be stored in the database. When the criteria are loaded, they are used to build a query and return all applicable Sales. The criteria objects look like this:
ReferenceColumn (The column in the Sale table they apply to)
MinValue (Minimum value the reference column must be)
MaxValue (Maximum value the reference column must be)
A search for Sales is done using a collection of the aforementioned criteria. ReferenceColumns of the same type are OR'd together, and ReferenceColumns of different types are AND'd together. So for example if I had three criteria:
ReferenceColumn: 'Price', MinValue: '10', MaxValue: '20'
ReferenceColumn: 'Price', MinValue: '80', MaxValue: '100'
ReferenceColumn: 'Age', MinValue: '2', MaxValue: '3'
The query should return all Sales where the price was between 10-20 or between 80-100, but only if those Sales's Age is between 2 and 3 years old.
I have it implemented using a SQL query string and executing using .FromSql:
public IEnumerable<Sale> GetByCriteria(ICollection<SaleCriteria> criteria)
{
StringBuilder sb = new StringBuilder("SELECT * FROM Sale");
var referenceFields = criteria.GroupBy(c => c.ReferenceColumn);
// Adding this at the start so we can always append " AND..." to each outer iteration
if (referenceFields.Count() > 0)
{
sb.Append(" WHERE 1 = 1");
}
// AND all iterations here together
foreach (IGrouping<string, SaleCriteria> criteriaGrouping in referenceFields)
{
// So we can always use " OR..."
sb.Append(" AND (1 = 0");
// OR all iterations here together
foreach (SaleCriteria sc in criteriaGrouping)
{
sb.Append($" OR {sc.ReferenceColumn} BETWEEN '{sc.MinValue}' AND '{sc.MaxValue}'");
}
sb.Append(")");
}
return _context.Sale.FromSql(sb.ToString();
}
And this is fact works just fine with our database, but it doesn't play nice with other collections, particulary the InMemory database we use for UnitTesting, so I'm trying to rewrite it using Expression trees, which I've never used before. So far I've gotten this:
public IEnumerable<Sale> GetByCriteria(ICollection<SaleCriteria> criteria)
{
var referenceFields = criteria.GroupBy(c => c.ReferenceColumn);
Expression masterExpression = Expression.Equal(Expression.Constant(1), Expression.Constant(1));
List<ParameterExpression> parameters = new List<ParameterExpression>();
// AND these...
foreach (IGrouping<string, SaleCriteria> criteriaGrouping in referenceFields)
{
Expression innerExpression = Expression.Equal(Expression.Constant(1), Expression.Constant(0));
ParameterExpression referenceColumn = Expression.Parameter(typeof(Decimal), criteriaGrouping.Key);
parameters.Add(referenceColumn);
// OR these...
foreach (SaleCriteria sc in criteriaGrouping)
{
Expression low = Expression.Constant(Decimal.Parse(sc.MinValue));
Expression high = Expression.Constant(Decimal.Parse(sc.MaxValue));
Expression rangeExpression = Expression.GreaterThanOrEqual(referenceColumn, low);
rangeExpression = Expression.AndAlso(rangeExpression, Expression.LessThanOrEqual(referenceColumn, high));
innerExpression = Expression.OrElse(masterExpression, rangeExpression);
}
masterExpression = Expression.AndAlso(masterExpression, innerExpression);
}
var lamda = Expression.Lambda<Func<Sale, bool>>(masterExpression, parameters);
return _context.Sale.Where(lamda.Compile());
}
It's currently throwing an ArgumentException when I call Expression.Lamda. Decimal cannot be used there and it says it wants type Sale, but I don't know what to put there for Sales, and I'm not sure I'm even on the right track here. I'm also concerned that my masterExpression is duplicating with itself each time instead of appending like I did with the string builder, but maybe that will work anyway.
I'm looking for help on how to convert this dynamic query to an Expression tree, and I'm open to an entirely different approach if I'm off base here.

I think this will work for you
public class Sale
{
public int A { get; set; }
public int B { get; set; }
public int C { get; set; }
}
//I used a similar condition structure but my guess is you simplified the code to show in example anyway
public class Condition
{
public string ColumnName { get; set; }
public ConditionType Type { get; set; }
public object[] Values { get; set; }
public enum ConditionType
{
Range
}
//This method creates the expression for the query
public static Expression<Func<T, bool>> CreateExpression<T>(IEnumerable<Condition> query)
{
var groups = query.GroupBy(c => c.ColumnName);
Expression exp = null;
//This is the parametar that will be used in you lambda function
var param = Expression.Parameter(typeof(T));
foreach (var group in groups)
{
// I start from a null expression so you don't use the silly 1 = 1 if this is a requirement for some reason you can make the 1 = 1 expression instead of null
Expression groupExp = null;
foreach (var condition in group)
{
Expression con;
//Just a simple type selector and remember switch is evil so you can do it another way
switch (condition.Type)
{
//this creates the between NOTE if data types are not the same this can throw exceptions
case ConditionType.Range:
con = Expression.AndAlso(
Expression.GreaterThanOrEqual(Expression.Property(param, condition.ColumnName), Expression.Constant(condition.Values[0])),
Expression.LessThanOrEqual(Expression.Property(param, condition.ColumnName), Expression.Constant(condition.Values[1])));
break;
default:
con = Expression.Constant(true);
break;
}
// Builds an or if you need one so you dont use the 1 = 1
groupExp = groupExp == null ? con : Expression.OrElse(groupExp, con);
}
exp = exp == null ? groupExp : Expression.AndAlso(groupExp, exp);
}
return Expression.Lambda<Func<T, bool>>(exp,param);
}
}
static void Main(string[] args)
{
//Simple test data as an IQueriable same as EF or any ORM that supports linq.
var sales = new[]
{
new Sale{ A = 1, B = 2 , C = 1 },
new Sale{ A = 4, B = 2 , C = 1 },
new Sale{ A = 8, B = 4 , C = 1 },
new Sale{ A = 16, B = 4 , C = 1 },
new Sale{ A = 32, B = 2 , C = 1 },
new Sale{ A = 64, B = 2 , C = 1 },
}.AsQueryable();
var conditions = new[]
{
new Condition { ColumnName = "A", Type = Condition.ConditionType.Range, Values= new object[]{ 0, 2 } },
new Condition { ColumnName = "A", Type = Condition.ConditionType.Range, Values= new object[]{ 5, 60 } },
new Condition { ColumnName = "B", Type = Condition.ConditionType.Range, Values= new object[]{ 1, 3 } },
new Condition { ColumnName = "C", Type = Condition.ConditionType.Range, Values= new object[]{ 0, 3 } },
};
var exp = Condition.CreateExpression<Sale>(conditions);
//Under no circumstances compile the expression if you do you start using the IEnumerable and they are not converted to SQL but done in memory
var items = sales.Where(exp).ToArray();
foreach (var sale in items)
{
Console.WriteLine($"new Sale{{ A = {sale.A}, B = {sale.B} , C = {sale.C} }}");
}
Console.ReadLine();
}

Using LINQ, how would you filter out all but one item of a particular criteria from a list?

I realize my title probably isn't very clear so here's an example:
I have a list of objects with two properties, A and B.
public class Item
{
public int A { get; set; }
public int B { get; set; }
}
var list = new List<Item>
{
new Item() { A = 0, B = 0 },
new Item() { A = 0, B = 1 },
new Item() { A = 1, B = 0 },
new Item() { A = 2, B = 0 },
new Item() { A = 2, B = 1 },
new Item() { A = 2, B = 2 },
new Item() { A = 3, B = 0 },
new Item() { A = 3, B = 1 },
}
Using LINQ, what's the most elegant way to collapse all the A = 2 items into the first A = 2 item and return along with all the other items? This would be the expected result.
var list = new List<Item>
{
new Item() { A = 0, B = 0 },
new Item() { A = 0, B = 1 },
new Item() { A = 1, B = 0 },
new Item() { A = 2, B = 0 },
new Item() { A = 3, B = 0 },
new Item() { A = 3, B = 1 },
}
I'm not a LINQ expert and already have a "manual" solution but I really like the expressiveness of LINQ and was curious to see if it could be done better.

How about:
var collapsed = list.GroupBy(i => i.A)
.SelectMany(g => g.Key == 2 ? g.Take(1) : g);
The idea is to first group them by A and then select those again (flattening it with .SelectMany) but in the case of the Key being the one we want to collapse, we just take the first entry with Take(1).

One way you can accomplish this is with GroupBy. Group the items by A, and use a SelectMany to project each group into a flat list again. In the SelectMany, check if A is 2 and if so Take(1), otherwise return all results for that group. We're using Take instead of First because the result has to be IEnumerable.
var grouped = list.GroupBy(g => g.A);
var collapsed = grouped.SelectMany(g =>
{
if (g.Key == 2)
{
return g.Take(1);
}
return g;
});

One possible solution (if you insist on LINQ):
int a = 2;
var output = list.GroupBy(o => o.A == a ? a.ToString() : Guid.NewGuid().ToString())
.Select(g => g.First())
.ToList();
Group all items with A=2 into group with key equal to 2, but all other items will have unique group key (new guid), so you will have many groups having one item. Then from each group we take first item.

Yet another way:
var newlist = list.Where (l => l.A != 2 ).ToList();
newlist.Add( list.First (l => l.A == 2) );

An alternative to other answers based on GroupBy can be Aggregate:
// Aggregate lets iterate a sequence and accumulate a result (the first arg)
var list2 = list.Aggregate(new List<Item>(), (result, next) => {
// This will add the item in the source sequence either
// if A != 2 or, if it's A == 2, it will check that there's no A == 2
// already in the resulting sequence!
if(next.A != 2 || !result.Any(item => item.A == 2)) result.Add(next);
return result;
});

What about this:
list.RemoveAll(l => l.A == 2 && l != list.FirstOrDefault(i => i.A == 2));
if you whould like more efficient way it would be:
var first = list.FirstOrDefault(i => i.A == 2);
list.RemoveAll(l => l.A == 2 && l != first);

LINQ C# - Combining multiple groups

LINQ Groupby query creates a new group for each unique key. I would like to combine multiple groups into a single group based on the key value.
e.g.
var customersData = new[]
{
new { id = 1, company = "ABC" },
new { id = 2, company = "AAA" },
new { id = 3, company = "ABCD" },
new { id = 4, company = "XYZ" },
new { id = 5, company = "X.Y.Z." },
new { id = 6, company = "QQQ" },
};
var groups = from d in customersData
group d by d.company;
Let's say I want ABC, AAA, and ABCD in the same group, and XYZ, X.Y.Z. in the same group.
Is there anyway to achieve this through LINQ queries?

You will need to use the overload of GroupBy that takes an IEqualityComparer.
var groups = customersData.GroupBy(k => k.company, new KeyComparer());
where KeyComparer could look like
public class KeyComparer : IEqualityComparer
{
public bool Equals(string x, string y)
{
// put your comparison logic here
}
public int GetHashCode(string obj)
{
// same comparison logic here
}
}
You can comparer the strings any way you like in the Equals method of KeyComparer.
EDIT:
You also need to make sure that the implementation of GetHashCode obeys the same rules as the Equals method. For example if you just removed the "." and replaced with "" as in other answers you need to do it in both methods like this
public class KeyComparer : IEqualityComparer
{
public bool Equals(string x, string y)
{
return x.Replace(".", "") == y.Replace(".", "");
}
public int GetHashCode(string obj)
{
return obj.Replace(".", "").GetHashCode();
}
}

I am assuming the following:
You meant to have quotes surrounding the company "names" (as below).
Your problem is simply solved by removing the '.'s from each company name.
If these assumptions are correct, the solution is simply the following:
var customersData = new[] {
new { id = 1, company = "ABC" },
new { id = 2, company = "A.B.C." },
new { id = 3, company = "A.B.C." },
new { id = 4, company = "XYZ" },
new { id = 5, company = "X.Y.Z." },
new { id = 6, company = "QQQ" },
};
var groups = from d in customersData
group d by d.company.Replace(".", "");
If these assumptions are not correct, please clarify and I can help work closer to a solution.

var groups = from d in customersData
group d by d.company.Replace(".", "");

public void int GetId(Company c)
{
int result = //whatever you want
return result;
}
then later:
var groups = from d in customersData
group d by GetId(d.company);

I think this is what you want:
var customersData = new[]
{
new { id = 1, company = "ABC" },
new { id = 2, company = "AAA" },
new { id = 3, company = "ABCD" },
new { id = 4, company = "XYZ" },
new { id = 5, company = "X.Y.Z." },
new { id = 6, company = "QQQ" },
};
var groups = from d in customersData
group d by d.company[0];
foreach (var group in groups)
{
Console.WriteLine("Group " + group.Key);
foreach (var item in group)
{
Console.WriteLine("Item " + item.company);
}
}
Console.ReadLine();