Develop Reference

Calculate Mode Using LINQ C#

I'm new with using Linq and was wondering how I could print out multiple values of my Mode value. At the minute I can only get 1 value from the Mode but I want it to show multiples ones.
string[] list = TextBox1.Text.Split(new string[] { "," },
StringSplitOptions.RemoveEmptyEntries);
int[] numbers = new int[list.Length];
for (int i = 0; i < numbers.Length; i++)
{
numbers[i] = Convert.ToInt32(list[i].Trim());
}
int mode = numbers.GroupBy(v => v)
.OrderByDescending(g => g.Count())
.First()
.Key;

You need to save off the collection before taking the item(s) you want.
string[] list = TextBox1.Text.Split(new string[] { "," },
StringSplitOptions.RemoveEmptyEntries);
IEnumerable<IGrouping<int, int>> modes = list.GroupBy(v => v);
IEnumerable<IGrouping<int, IGrouping<int, int>>> groupedModes = modes.GroupBy(v => v.Count());
var sortedGroupedModes = groupedModes.OrderByDescending(g => g.Key).ToList();
TextBox2.Text = string.Join(" ", sortedGroupedModes[0].Select(x => x.Key)));

You could get all of the groups and just extract those with the highest count (including ties):
var counts = numbers.GroupBy(v => v)
.Select(g => g.Key, Count = g.Count())
.OrderByDescending(g => g.Count);
var modes = numbers.Where(g => g.Count == counts.First().Count)
.Select(g => g.Key);

Converting IEnumerable<IEnumerable<T>> to List<string>

I’m trying to convert from this answer the code:
static IEnumerable<IEnumerable<T>> GetKCombs<T>(IEnumerable<T> list, int length) where T : IComparable
{
if (length == 1) return list.Select(t => new T[] { t });
return GetKCombs(list, length - 1)
.SelectMany(t => list.Where(o => o.CompareTo(t.Last()) > 0),
(t1, t2) => t1.Concat(new T[] { t2 }));
}
Into a list of strings. For example I want this output {1,2} {1,3} to convert it to "1,2","1,3" (this is 2 seperate string) but I cant get it. I cant even understand how I can read the results of the above function. this is my code:
int[] numbers = ListEditText.Text.Split(',').Select(x => int.Parse(x)).ToArray();
var combinations = GetKCombs(numbers, 2);
stringCombinations = combinations.Select(j => j.ToString()).Aggregate((x, y) => x + "," + y);
In the end all the results i will add them on a List with all the possible unique combinations
For example for the numbers {1,2,3} i want this List:
'1','2','3','1,2','1,3','2,3','1,2,3'
This is my code right now:
List<string> stringCombinations = new List<string>();
for (int i = 0; i < numbers.Count(); i++)
{
combinations = GetKCombs(numbers, i + 1).Select(c => string.Join(",", c));
stringCombinations.AddRange(combinations);
}

You can try first joining the results of the inner IEnumerables
var combinations = GetKCombs(numbers, 2).Select(c => string.Join(",", c));
and then concatenating them into a single string
var combinationString = string.Join("; ", combinations); // "1,2; 1,3"
Based on your edits -- if I got you right -- you can try doing
var combinationStrings =
numbers
.SelectMany((_, i) =>
GetKCombs(numbers, i + 1) // get combinations for each 'length'
.Select(c => string.Join(",", c))) // join them to a string
.ToList();

Try
var stringCombinations = string.Join(",", combinations.Select(j => $#"""{string.Join(",", j)}"""));
It prints exactly the output you want.

Group the indexes of the same elements in a array in C#

There is a int[] array that stores different numbers.
What I want is to group the indexes of those same numbers in the array to the same groups.
For exmaple, the array is int[5]{1,2,5,1,5}
I would like to see the output is List<List<int>> { {0,3}, {1}, {2,4} } // don't mind syntax
It's better if Linq (or a more efficient way) can be used, thanks for help.

You can simply use GroupBy and the position obtained from the Select overload:
int[] array;
var result = array.Select((v, idx) => new { Value = v, Index = idx })
.GroupBy(g => g.Value)
.Select(g => g.ToArray()) // inner array
.ToArray(); // outer array

One of ways:
var result = myArray.Select((elem, idx) => new { Value = elem, Idx = idx})
.GroupBy(proxy => proxy.Value);
foreach (var grouped in result)
{
Console.WriteLine("Element {0} has indexes: {1}",
grouped.Key,
string.Join(", ", grouped.Select(proxy => proxy.Idx).ToArray()));
}
var myFinalList = result.Select(proxy => proxy.ToArray()).ToList();

You can use Enumerable.Range combined with GroupBy:
int[] arr = { 1, 2, 5, 1, 5 };
var result = Enumerable.Range(0, arr.Length)
.GroupBy(i => arr[i])
.Select(x => x.ToList()).ToList();
DEMO HERE

Find multiple index in array

Say I have an array like this
string [] fruits = {"watermelon","apple","apple","kiwi","pear","banana"};
Is there an built in function that allows me to query all the index of "apple" ?
For example,
fruits.FindAllIndex("apple");
will return an array of 1 and 2
If there is not, how should I implement it?
Thanks!

LINQ version:
var indexes = fruits.Select((value, index) => new { value, index })
.Where(x => x.value == "apple")
.Select(x => x.index)
.ToList();
Non-LINQ version, using Array<T>.IndexOf() static method:
var indexes = new List<int>();
var lastIndex = 0;
while ((lastIndex = Array.IndexOf(fruits, "apple", lastIndex)) != -1)
{
indexes.Add(lastIndex);
lastIndex++;
}

One way would be to write like this:
var indices = fruits
.Select ((f, i) => new {f, i})
.Where (x => x.f == "apple")
.Select (x => x.i);
Or the traditional way:
var indices = new List<int>();
for (int i = 0; i < fruits.Length; i++)
if(fruits[i] == "apple")
indices.Add(i);

Pretty easy with an extension method.
var fruits = new[] { "watermelon","apple","apple","kiwi","pear","banana" };
var indexes = fruits.FindAllIndexes("apple");
public static class Extensions
{
public static int[] FindAllIndexes(this string[] array, string search) => array
.Select((x, i) => (x, i))
.Where(value => value.x == search)
.Select(value => value.i)
.ToArray();
}

Grouping each 500 elements of array according to array elements

I have an array of 2000 strings. The strings are: "art", "economy", "sport" and "politic". I want to group each 500 elements and get their counts
Could anyone help please?

Another solution:
var count = 0;
var dictionaries =
strings.GroupBy(s => count++ / 500)
.Select(g => g.Distinct().ToDictionary(k => k, k => g.Count(s => s == k)))
.ToList();
This will create a List<Dictionary<string, int>>. Each dictionary represents a tally of 500 elements (or possibly less for the last dictionary), where the keys are strings and the values are the number of occurrences of the string among the 500 elements the dictionary represents.
There is no requirement to hardcode all the possible values that may be encountered.
For the maximum possible performance you can also use this version:
var count = 0;
var dictionaries =
strings.GroupBy(s => count++ / 500)
.Select(g => g.Aggregate(
new Dictionary<string, int>(),
(d, w) => { d[w] = (d.ContainsKey(w) ? d[w] + 1 : 1); return d; })
)
.ToList();
This version iterates over each element in your source array exactly once. The output is in the same format as the first version.

var result = strings.Select((s, i) => new { s, i })
.GroupBy(x => x.i / 500)
.Select(x => x.GroupBy(y => y.s)
.Select(z => new {
Name=z.Key,
Count=z.Count()
}).ToList())
.ToList();

Try
var grouping = Enumerable.Range(0,2000)
.Select(i => i / 500)
.Zip(Strings, (i,s) => new { Group = i, Str = s})
.GroupBy(anon => anon.Group,
anon => anon.Str,
(key,g) => new
{
Key = key,
Art = g.Count(str => str == "art"),
Economy = g.Count(str => str == "economy"),
Politic = g.Count(str => str == "politic"),
Sport= g.Count(str => str == "sport")
});
foreach(anon in grouping)
{
//textbox logic OP will have to change to suit
TextBox1.WriteLine(String.Format("Group: {0}", anon.Key));
TextBox1.WriteLine(String.Format("Art: {0}",anon.Art));
TextBox1.WriteLine(String.Format("Economy: {0}",anon.Economy ));
TextBox1.WriteLine(String.Format("Politic: {0}",anon.Politic ));
TextBox1.WriteLine(String.Format("Sport: {0}",anon.Sport));
}
Alternatively (as per Snowbear)
var grouping = Strings.Select((s,i) => new { Group = i / 500, Str = s})
.GroupBy(anon => anon.Group,
anon => anon.Str,
(key,g) => new
{
Key = key,
Art = g.Count(str => str == "art"),
Economy = g.Count(str => str == "economy"),
Politic = g.Count(str => str == "politic"),
Sport= g.Count(str => str == "sport")
});
foreach(anon in grouping)
{
//textbox logic OP will have to change to suit
TextBox1.WriteLine(String.Format("Group: {0}",anon.Key + 1));
TextBox1.WriteLine(String.Format("Art: {0}",anon.Art));
TextBox1.WriteLine(String.Format("Economy: {0}",anon.Economy ));
TextBox1.WriteLine(String.Format("Politic: {0}",anon.Politic ));
TextBox1.WriteLine(String.Format("Sport: {0}",anon.Sport));
}

int CountElementsInGroup = 500;
//from 500 to 1000
int NumberGroup = 2;
string[] GroupTypes = new string[4] { "art", "economy", "sport", "politic" };
//Fill example array
string[] arr = new string[2000];
Random rand = new Random();
for (int i = 0; i < arr.Length;i++ )
arr[i] = GroupTypes[rand.Next(0, 3)];
var res = (from p in arr.Skip((NumberGroup - 1) * CountElementsInGroup).Take(CountElementsInGroup)
group p by p into g
select new GroupCountClass { GroupName = g.Key, GroupCount = g.Count() });
textBox1.Text = "";
foreach (GroupCountClass c in res)
{
textBox1.Text += String.Format("GroupName:{0} Count:{1};",c.GroupName,c.GroupCount);
}