I have this array :
Point[] arr = samples.pointsArray;
I using this row to retrieve all elements that satisfy condition:
var maxXCol = arr.Where( p => maxX.X == p.X );
Any idea how to modify row above, to get only the indexes of these elements?
Thank you in advance!
EDIT
Use the version of Select that takes both the index and the object and create an anonymous object with the object and index inside it. It would look like this:
someEnumerable.Select((obj, idx) => new {Item = obj, Index = idx})
You'll need to do this before you use Where so that the original index remains intact after the filter operation.
In the following operations you can use the item like so:
x => x.Item
and the index like so:
x => x.Index
var maxXCol = arr.Select((p, inx) => new { p,inx})
.Where(y => maxX.X == y.p.X)
.Select(z => z.inx);
You may select the value first with the index in anonymous type, later you filter it with your condition and then select the index.
var result = arr.Select((g, index) => new { g, index })
.Where(r => maxX.X == r.X)
.Select(t => t.index);
You can use Select overload which takes an index, and project that index together with the original row. Then take only the index for the result collection.
var maxXCol = arr
.Select((p, index) => new { Item = p, Index = index })
.Where(p => maxX.X == p.Item.X)
.Select(x => x.Index);
Try this:
arr.Select((e,i)=>new{index=i, value=e}).Where(ei=>ei.value.X==maxX.X).Select(ei=>ei.index);
var maxXCol = arr
.Select((a, b) => new { b, a })
.Where(p => maxX.X == p.a.X)
.Select(i=>i.b);
Related
I have a requirement to rank the array elements and the array has duplicate values. I tried following this Ranking items in a list with LINQ but this doesn't work when the array has duplicate values in it. Any easy way to do it in c#?
For Example :
input = [650,150,150,200]
output = [1,3,3,2]
For Example :
input = [650,200,200,150]
output = [1,2,2,3]
Update: The requirement is as below, what if I add one more element to the array
Ex: [650,150,150,200,100] output needs to be [1,3,3,2,5] instead of [1,3,3,2,4]
You can create a dictionary as rank-lookup source:
int[] array = new[] {650,150,150,200};
Dictionary<int, int> numRanks = array
.GroupBy(i => i)
.OrderByDescending(g => g.Key)
.Select((g, index) => (num:g.Key, rank:index+1))
.ToDictionary(x => x.num, x => x.rank);
int[] result = array.Select(i => numRanks[i]).ToArray();
For your updated requirement you could use a similar approach using a Lookup<TKey, TValue>:
var rankLookup = array
.OrderByDescending(i => i)
.Select((num, index) => (num, index))
.ToLookup(x => x.num, x => x.index + 1);
int[] result = array.Select(i => rankLookup[i].First()).ToArray();
The lookup is like a dictionary that allows duplicate keys. You need to use First here because you are just interested in the rank. If you'd use Count() you'd know how many duplicates it had.
You could create an array of items, distinct and in order, then use the indices to determine the rank of each item.
var ranks = input.Distinct().OrderByDescending(x => x).ToArray();
var ranked = input.Select(x => Array.IndexOf(ranks, x) + 1);
Working example
Update after comment
If rankings need to be skipped, just remove the Distinct:
var ranks = input.OrderByDescending(x => x).ToArray();
var ranked = input.Select(x => Array.IndexOf(ranks, x) + 1);
Array.IndexOf will take the first element when there are duplicates.
Working example
I'm new to c# so go easy on me. Anyways, I made a list of numbers
List<int> numbers = new List<int>();
and I want to make a list of each number and its count/frequency.
var grouped = numbers
.GroupBy(i => i)
.Select(i => new { Number = i.Key, Count = i.Count() });
In locals, I can see the group, which has an IEnumerator interface with all of the numbers and their count values image of what I'm talking about. So is there a way to make a list with the numbers and their frequency/count?
Thank you.
IEnumerable<T> is a sequence so it doesn't own a count. But Enumerable.Count is an extension method of IEnumerable<T>
That is, you don't necessarily need to convert an IEnumerable<T> into a List<T>:
var grouped = numbers
.GroupBy(i => i)
.Select(i => new { Number = i.Key, Count = i.Count() });
var groupedCount = grouped.Count();
// You may iterate grouped
foreach(var value in grouped)
{
Console.WriteLine($"{value.Number} {value.Count}");
}
If you really need List<T> semantics, you just need to call Enumerable.ToList:
var grouped = numbers
.GroupBy(i => i)
.Select(i => new { Number = i.Key, Count = i.Count() })
.ToList();
In the other hand, you may directly convert everything into a string as follows:
var groupText = string.Join("\n", numbers
.GroupBy(i => i)
.Select(i => $"Number: {i.Key} Count: {i.Count()}"))
To get a list, you just need to call ToList(), for example:
var grouped = numbers
.GroupBy(i => i)
.Select(i => new { Number = i.Key, Count = i.Count() })
.ToList();
However, you really don't need to do that, you can simply loop over the enumerable as it stands:
foreach(var item in grouped)
{
Console.WriteLine($"{item.Number} occurs {item.Count} times");
}
Sounds like you want ToDictionary with the number as key and the frequency as value:
var grouped = numbers
.GroupBy(i => i)
.Select(i => new { Number = i.Key, Count = i.Count() })
.ToDictionary(x => x.Number, x => x.Count);
Now you can easily print every number and its frequency by looping the dictionary.
In fact you don´t even need neither ToDictionary nor your Select, as the IGrouping returned from GroupBy also derives from IEnumerable which is why you can iterate over it.
foreach(var g in grouped = numbers.GroupBy(i => i))
{
var number = g.Key;
var freq = g.Count();
}
I have the following LINQ query to receive indexes:
fieldIndexes = this.record.Fields.Where(a => !a.IsCodeField)
.OrderBy(a => a.DatabaseIndex)
.Select(a => a.DatabaseIndex - 1)
.ToArray();
But I want to replace the a.DatabaseIndex with the actual index of the search. I am aware of the syntax .Select((a, index) => new (index, a))... but I am not sure how to cast the a here to be of my type which in this case is Field. I have tried:
fieldIndexes = this.record.Fields.Select((a, index) => new {index, a})
.Where(a => !a.IsCodeField) // <- Invalid Cast.
.OrderBy(a => a.DatabaseIndex)
.Select(a => a.DatabaseIndex - 1)
.ToArray();
How can I cast a to my type within the LINQ statement?
Thanks for your time.
In the Where clause you are working with your newly created anonymous objects with properties a and index, which you can use:
.Where(a => !a.a.IsCodeField)
Of course this can be done in more readable fasion:
fieldIndexes = this.record.Fields.Select((a, index) => new {Index = index, Field = a})
.Where(a => !a.Field.IsCodeField)
...
You are projecting sequence items to anonymous objects with properties index and a. Original item will be accessible via property a:
fieldIndexes = this.record.Fields.Select((a, index) => new {index, a})
.Where(x => !x.a.IsCodeField)
About the homework:
There are casters(witch(0)/fairy(1)) and they have spellpower(int). I stored them in a list.
I'm to find the best of both types. (There can be multiple casters with the same spellpower)
I've come up with this code, but there is a problem. If the caster with the most spellpower is a 1, then the first FindAll won't return anything, because it tries to find the caster with type 0 AND with the most spellpower. How can I get a list containing type 0 caster(s) with the most spellpower, if the caster with the most overall spellpower is type 1?
private List<Caster> BestCasters()
{
List<Caster> temp = new List<Caster>();
temp = casters.FindAll(x => x.SpellPower == casters.Max(y => y.SpellPower) && (x.TypeOfCaster == 0));
temp.AddRange(casters.FindAll(x => x.SpellPower == casters.Max(y => y.SpellPower) && (x.TypeOfCaster == 1)));
temp.OrderBy(x => x.TypeOfCaster).ThenBy(y => y.CasterName);
return temp;
}
The LINQ GroupBy behavior is perfect for this:
var strongest_casters = casters.GroupBy(c => c.TypeOfCaster)
.Select(grp => grp.OrderByDescending(x => x.SpellPower)
.First()
);
Or to return more than one of each type:
var strongest_casters = casters.GroupBy(c => c.TypeOfCaster)
.SelectMany(grp => grp.Where(y.SpellPower == grp.Max(x => x.SpellPower))
);
private List<Caster> BestCasters()
{
var witches = casters.Where(x => x.TypeOfCaster == 0).ToList();
var fairies = casters.Where(x => x.TypeOfCaster == 1).ToList();
int witchesMax = witches.Max(x => x.SpellPower);
int fairiesMax = fairies.Max(x => x.SpellPower);
var temp = witches.Where(x => x.SpellPower == witchesMax).ToList();
temp.AddRange(fairies.Where(x => x.SpellPower == fairiesMax));
return temp.OrderBy(x => x.TypeOfCaster).ThenBy(y => y.CasterName).ToList();
}
If you have to use FindAll like this you should invoke the Max on a subset only containing the casters of the right kind. Of course it would make more sense to split the initial list first and then fetch the strongest caster of each kind.
Since you did not tell what exactly you have to do I can only hope that you are allowed to split :-)
Lets say i have an array
byte[] myarr = {1,4,3,4,1,2,1,2,4,3,1,4,2};
myarr will be of length 13 (0-12 Index) which will also be the length of int[] val.
int[] val = new int[13];
I want to check index of myarr where its value is 4 i.e. 1,3,8,11.
Then i want
val[1]++;
val[3]++;
val[8]++;
val[11]++;
One way of doing this is using for loop
for(int i=0; i<myarr.length; i++)
{
if(myarr[i] == 4)
val[i]++;
}
We can use Array.indexof but it returns the first index of that value meaning that value has to be unique and my myarr has lots of same values.
Can this be done using linq?
This is what I ended up doing in LINQ (update included):
myarr.Select((b, i) => b == 4 ? i : -1)
.Where(i => i != -1)
.ToList().ForEach(i => val[i]++);
Your non-LINQ version is obviously much more succinct and readable, so I think you should use that.
You can, but it won't be simpler. LINQ will only help you with the query part, the update part has to be done in a foo loop, but since the array contains value types you need to get indexes from your LINQ-query and not the actual values and you have won nothing.
You can use an anonymous type to store the index:
int[] newIndices = myarr.Select((i, index) => new { Index = index, Value = i })
.Where (x => x.Value == 4)
.Select(x => x.Index + 1)
.ToArray();
Edit: Finally i've understood your question ;)
myarr.Select((i, index) => new { Index = index, Value = i })
.Where(x => x.Value == 4)
.Select(x => x.Index)
.ForEach(i => myarr[i]++);