Develop Reference

Gather all possible combinations of 2 from a list

I have a list with 10 items in it. I am trying to output to console every possible pairing of 2. But it cannot pair with itself. For example
1,2
1,3
1,4 etc...
I found this to find all possible combinations within a list. Can someone help me modify it please?
private static void GetCombination(IList list)
{
var count = Math.Pow(2, list.Count);
for (var i = 1; i <= count - 1; i++)
{
var str = Convert.ToString(i, 2).PadLeft(list.Count, '0');
for (var j = 0; j < str.Length; j++)
{
if (str[j] == '1')
{
Console.Write(list[j]);
}
}
Console.WriteLine();
}
}

So if you have a list with 1 to 10, you need 1,2 1,3 1,4...1,10 - 2,1 2,3..2,10 and so on.
You just have to use bubble and check if the first index is different from the second.
For more clarification, here is an example:
List<int> mylist = new List<int>(new int[] { 1,2,3,4,5,6,7,8,9 });
GetCombination(mylist);
private static void GetCombination(IList<int> values)
{
for (int i = 0; i < values.Count; i++)
{
for (int j = 0; j < values.Count; j++)
{
if (i != j)
{
Console.WriteLine(values[i] + " " + values[j]);
}
}
}
}

This is such a easy question.Adding to the answer of #FirstOne
you can also return the List containing all the combinations from the function:
List mylist = new List(new int[] { 1,2,3,4,5,6,7,8,9 });
GetCombination(mylist);
public static IList<Tuple<int,int>> GetCombination(IList<int> values)
{
List<Tuple<int,int>> _temp=new List<Tuple<int, int>>();
for (int i = 0; i < values.Count; i++)
{
for (int j = 0; j < values.Count; j++)
{
if (i != j)
{
_temp.Add(Tuple.Create(i, j));
}
}
}
return _temp;
}

C#: Build 3D-Array and fill it with data

The code I tried so far below:
private static List<List<List<int>>> threeDArrayToThreeDList(int [,,] letters) {
// 3d-array to 3d-list
List<List<List<int>>> letterslist = new List<List<List<int>>>();
List<List<int>> sublist = new List<List<int>> ();
List<int> subsublist = new List<int> ();
for (int i = 0; i < 2; i++) {
letterslist.Add (sublist);
for (int j = 0; j < 2; j++) {
letterslist[i].Add (subsublist);
for (int k = 0; k < 2; k++) {
Console.WriteLine (letterslist [i][j][k]); // Element not found
Console.WriteLine (letters [i,j,k]);
letterslist [i] [j] [k] = letters [i,j,k];
}
}
}
return letterslist;
}
Why letterslist [i][j][k] isn't found?

Your code is wrong. You need to create a list for each "index". You're code only creates 3 lists altogether.
Here's how it should work:
private static List<List<List<int>>> threeDArrayToThreeDList(int [,,] letters) {
// 3d-array to 3d-list
List<List<List<int>>> letterslist = new List<List<List<int>>>();
for (int i = 0; i < 2; i++) {
letterslist.Add (new List<List<int>> ());
for (int j = 0; j < 2; j++) {
letterslist[i].Add (new List<int> ());
for (int k = 0; k < 2; k++) {
Console.WriteLine (letters [i,j,k]);
letterslist [i] [j].Add(letters [i,j,k]);
}
}
}
return letterslist;
}

This because the letterslist [i][j] list has no elements in it.
Add an element to it, and it will get you through the line that causes the exception.
Change the code in the innermost loop as follows:
for (int k = 0; k < 2; k++) {
letterslist[i][j].Add (letters[i, j, k]);
Console.WriteLine(letterslist[i][j][k]); // Should work fine
Console.WriteLine(letters[i, j, k]);
}

What is the most efficient way to find the largest element in a matrix along with position? Also, the largest element in each column with position

I have written the following code but it looks to be far from efficient.
//Find largest in tempRankingData
int largestIntempRankingData = tempRankingData[0, 0];
for (int i = 0; i < count; i++)
{
for (int j = 0; j < count; j++)
{
if (tempRankingData[i, j] > largestIntempRankingData)
{
largestIntempRankingData = tempRankingData[i, j];
}
}
}
//Find position of largest in tempRankingData
List<string> positionLargestIntempRankingData = new List<string>();
for (int i = 0; i < count; i++)
{
for (int j = 0; j < count; j++)
{
if (tempRankingData[i, j] == largestIntempRankingData)
{
positionLargestIntempRankingData.Add(i + "," + j);
}
}
}
//Find largest in each column
int largestInColumn = 0;
List<string> positionOfLargestInColumn = new List<string>();
Dictionary<int, List<string>> position = new Dictionary<int, List<string>>();
for (int i = 0; i < count; i++)
{
largestInColumn = tempRankingData[0, i];
positionOfLargestInColumn = new List<string>();
for (int j = 0; j < count; j++)
{
if (tempRankingData[j, i] > largestInColumn)
{
largestInColumn = tempRankingData[j, i];
}
}
for (int j = 0; j < count; j++)
{
if (tempRankingData[j, i] == largestInColumn)
{
positionOfLargestInColumn.Add(j + "," + i);
}
}
position.Add(i, positionOfLargestInColumn);
}
So, I wanted to check about the most efficient way to do this.

Whilst you're finding the largest in each column, you could also be finding the largest overall. You can also capture the positions as you go:
//Find largest in each column
int largestInColumn = 0;
int largestOverall = int.MinValue;
List<string> positionOfLargestInColumn;
Dictionary<int, List<string>> position = new Dictionary<int, List<string>>();
List<string> positionLargestIntempRankingData = new List<string>();
for (int i = 0; i < count; i++)
{
largestInColumn = tempRankingData[0, i];
positionOfLargestInColumn = new List<string>();
positionOfLargestInColumn.Add("0," + i);
for (int j = 1; j < count; j++)
{
if (tempRankingData[j, i] > largestInColumn)
{
largestInColumn = tempRankingData[j, i];
positionOfLargestInColumn.Clear();
positionOfLargestInColumn.Add(j + "," + i);
}
else if(tempTankingData[j,i] == largestInColumn)
{
positionOfLargestInColumn.Add(j + "," + i);
}
}
position.Add(i, positionOfLargestInColumn);
if(largestInColumn > largestOverall)
{
positionLargestIntempRankingData.Clear();
positionLargestIntempRankingData.AddRange(positionOfLargestInColumn);
largestOverall = largestInColumn;
}
else if(largestInColumn == largestOverall)
{
positionLargestIntempRankingData.AddRange(positionOfLargestInColumn);
}
}

1). You can find largest element and its position in one method and retrieve.
Would be caller of your method concerned about position or actual value, is a matter of concrete case.
2) You can use `yield return' technique in your matrix search (for column based search), so do not compute all column's maximas and push them into the dictionary. Dictionaries are not that fast as arrays, if you can avoid use them, do that.
3) You can keep a matrix in single dimension, long array. Have [] access operator overload, to "emulate" matrix access. Why ? If finding maximum is something frequent you might need to do during program run, having one foreach loop is faster then having 2 nested once. In case of a big matrices, single array search can be easily parallelized among different cores.
If big matrices and/or frequent calls are not your concern, just simplify your code like in points (1), (2).

For your fist two itterations you could replace with this:
//Find largest in tempRankingData
int largestIntempRankingData = tempRankingData[0, 0];
List<KeyValuePair<double,string>> list = new List<KeyValuePair<double,string>>();
for (int i = 0; i < count; i++)
{
for (int j = 0; j < count; j++)
{
if (tempRankingData[i, j] > largestIntempRankingData)
{
largestIntempRankingData = tempRankingData[i, j];
list.Add(new KeyValuePair<double, string>(largestIntempRankingData, i + "," + j)); //Add the value and the position;
}
}
}
//This gives a list of strings in which hold the position of largestInItemRankingData example "3,3"
//Only positions where the key is equal to the largestIntempRankingData;
list.Where(w => w.Key == largestIntempRankingData).ToList().Select(s => s.Value).ToList();

You can get all these pieces of information in a single scan with a little fiddling around. Something like this (converting the rows and columns to a string is trivial and better done at the end anyway):
int? largestSoFar = null; // you could populate this with myMatrix[0,0]
// but it would fail if the matrix is empty
int largestCol = 0;
int largestRow = 0;
int?[] largestPerColumn = new int?[numOfCols]; // You could also populate this with
// the values from the first row but
// it would fail if there are no rows
int[] largestColumnRow = new int[numOfCols];
for (int i = 0; i < numOfRows; i++)
{
for (int j = 0; j < numOfCols; i++)
{
if (largestSoFar < myMatrix[i,j])
{
largestSoFar = myMatrix[i,j];
largestCol = j;
largestRow = i;
}
if (largestPerColumn[j] < myMatrix[i,j])
{
largestPerColumn[j] = myMatix[i,j];
largestColumnRow[j] = i;
}
}
}
// largestSoFar is the biggest value in the whole matrix
// largestCol and largestRow is the column and row of the largest value in the matrix
// largestPerColumn[j] is the largest value in the jth column
// largestColumnRow[j] is the row of the largest value of the jth column
If you do need to capture all the "maxima" (for want of a better word, because that's not really what you are doing) in a column, you could just change the above code to something like this:
int? largestSoFar = null; // you could populate this with myMatrix[0,0]
// but it would fail if the matrix is empty
int largestCol = 0;
int largestRow = 0;
int?[] largestPerColumn = new int?[numOfCols]; // You could also populate this with
// the values from the first row but
// it would fail if there are no rows
List<int>[] largestColumnRow = new List<int>[numOfCols];
for (int i = 0; i < numOfRows; i++)
{
for (int j = 0; j < numOfCols; i++)
{
if (largestSoFar < myMatrix[i,j])
{
largestSoFar = myMatrix[i,j];
largestCol = j;
largestRow = i;
}
if (largestPerColumn[j] < myMatrix[i,j])
{
largestPerColumn[j] = myMatix[i,j];
largestColumnRow[j].Add(i);
}
}
}
// Now largestColumnRow[j] gives you a list of all the places where you found a larger
// value for the jth column

Remove Nulls from string[,]

I have a string array defined in c# as
string[,] options = new string[100,3];
Throughout the code it gets populated with data but not always filled.
So if I have 80 parts of it filled and 20 parts of it not filled. The 20 parts have nulls in them or 60 nulls at the end. Is there an easy way to resize the array so that after filling it the array is the same as
String[,] options = new string[80,3];
It would have to be resized based on the position of the first set of 3 nulls it found.
If this was a jagged array I would have done
options = options.Where(x => x != null).ToArray();

The method is quite long, because it has to check every row twice...
public static string[,] RemoveEmptyRows(string[,] strs)
{
int length1 = strs.GetLength(0);
int length2 = strs.GetLength(1);
// First we count the non-emtpy rows
int nonEmpty = 0;
for (int i = 0; i < length1; i++)
{
for (int j = 0; j < length2; j++)
{
if (strs[i, j] != null)
{
nonEmpty++;
break;
}
}
}
// Then we create an array of the right size
string[,] strs2 = new string[nonEmpty, length2];
for (int i1 = 0, i2 = 0; i2 < nonEmpty; i1++)
{
for (int j = 0; j < length2; j++)
{
if (strs[i1, j] != null)
{
// If the i1 row is not empty, we copy it
for (int k = 0; k < length2; k++)
{
strs2[i2, k] = strs[i1, k];
}
i2++;
break;
}
}
}
return strs2;
}
Use it like:
string[,] options = new string[100, 3];
options[1, 0] = "Foo";
options[3, 1] = "Bar";
options[90, 2] = "fiz";
options = RemoveEmptyRows(options);
As suggested by Alexei, there is another way of doing this:
public static string[,] RemoveEmptyRows2(string[,] strs)
{
int length1 = strs.GetLength(0);
int length2 = strs.GetLength(1);
// First we put somewhere a list of the indexes of the non-emtpy rows
var nonEmpty = new List<int>();
for (int i = 0; i < length1; i++)
{
for (int j = 0; j < length2; j++)
{
if (strs[i, j] != null)
{
nonEmpty.Add(i);
break;
}
}
}
// Then we create an array of the right size
string[,] strs2 = new string[nonEmpty.Count, length2];
// And we copy the rows from strs to strs2, using the nonEmpty
// list of indexes
for (int i1 = 0; i1 < nonEmpty.Count; i1++)
{
int i2 = nonEmpty[i1];
for (int j = 0; j < length2; j++)
{
strs2[i1, j] = strs[i2, j];
}
}
return strs2;
}
This one, in the tradeoff memory vs time, chooses time. It is probably faster, because it doesn't have to check every row twice, but it uses more memory, because it puts somewhere a list of the non-empty indexes.

I went for all rows until you find an row with all null values:
Needs some clean up and will obviously remove non-null rows that occur after the first all null row. The requirement wasn't too clear here
EDIT: Just seen the comment clarifying requirement to remove all null rows - I've tweaked the below to avoid downvotes but a more comprehensive answer is already accepted (and is more efficient) :)
void Main()
{
string[,] options = new string[100,3];
options[0,0] = "bleb";
options[1,1] = "bleb";
options[2,0] = "bleb";
options[2,1] = "bleb";
options[3,2] = "bleb";
options[4,1] = "bleb";
string[,] trimmed = TrimNullRows(options);
Console.WriteLine(trimmed);
}
public string[,] TrimNullRows(string[,] options)
{
IList<string[]> nonNullRows = new List<string[]>();
for (int x = 0; x < options.GetLength(0); x++)
{
bool allNull = true;
var row = new string[options.GetLength(1)];
for (int y = 0; y < options.GetLength(1); y++)
{
row[y] = options[x,y];
allNull &= options[x,y] == null;
}
if (!allNull)
{
nonNullRows.Add(row);
}
}
var optionsTrimmed = new string[nonNullRows.Count, options.GetLength(1)];
for (int i=0;i<nonNullRows.Count;i++)
{
for (int j=0;j<options.GetLength(1);j++)
{
optionsTrimmed[i, j] = nonNullRows[i][j];
}
}
return optionsTrimmed;
}

You can also get yourself some helpers to convert between jagged and multi-dimensional representations. This is pretty silly, of course, but for arrays as small as the ones you're showing (and also, very sparse arrays), it'll be fine.
void Main()
{
string[,] options = new string[100,3];
options[3, 1] = "Hi";
options[5, 0] = "Dan";
var results =
options
.JagIt()
.Where(i => i.Any(j => j != null))
.UnjagIt();
results.Dump();
}
static class Extensions
{
public static IEnumerable<IEnumerable<T>> JagIt<T>(this T[,] array)
{
for (var i = 0; i < array.GetLength(0); i++)
yield return GetRow(array, i);
}
public static IEnumerable<T> GetRow<T>(this T[,] array, int rowIndex)
{
for (var j = 0; j < array.GetLength(1); j++)
yield return array[rowIndex, j];
}
public static T[,] UnjagIt<T>(this IEnumerable<IEnumerable<T>> jagged)
{
var rows = jagged.Count();
if (rows == 0) return new T[0, 0];
var columns = jagged.Max(i => i.Count());
var array = new T[rows, columns];
var row = 0;
var column = 0;
foreach (var r in jagged)
{
column = 0;
foreach (var c in r)
{
array[row, column++] = c;
}
row++;
}
return array;
}
}
The JagIt method is pretty simple of course - we'll just iterate over the rows, and yield the individual items. This gives us an enumerable of enumerables, which we can use in LINQ quite easily. If desired, you could transform those into arrays, of course (say, Select(i => i.ToArray()).ToArray()).
The UnjagIt method is a bit more talkative, because we need to create the target array with the correct dimensions first. And there's no unyield instruction to simplify that :D
This is pretty inefficient, of course, but that isn't necessarily a problem. You could save yourself some of the iterations by keeping the inner enumerable an array, for example - that will save us having to iterate over all the inner items.
I'm mostly keeping this as the memory-cheap, CPU-intensive alternative to #xanatos' memory-intensive, CPU-cheap (relatively).
Of course, the main bonus is that it can be used to treat any multi-dimensional arrays as jagged arrays, and convert them back again. General solutions usually aren't the most efficient :D

Yet another variant with linq
static string[,] RemoveNotNullRow(string[,] o)
{
var rowLen = o.GetLength(1);
var notNullRowIndex = (from oo in o.Cast<string>().Select((x, idx) => new { idx, x })
group oo.x by oo.idx / rowLen into g
where g.Any(f => f != null)
select g.Key).ToArray();
var res = new string[notNullRowIndex.Length, rowLen];
for (int i = 0; i < notNullRowIndex.Length; i++)
{
Array.Copy(o, notNullRowIndex[i] * rowLen, res, i * rowLen, rowLen);
}
return res;
}

How to sort a 2D array filld with random numbers

I'm doing a class assignment,
I need to create an 2D array of random numbers and sort them either bubble or other sorting codes. I'm fine with single array, but the problem is a 2D array filled with random numbers, I just don't get it.
Random numbers should be made of (-I,I) interval it's a user input. Sorry for bad english, haven't gotten any degree. In working on visual C# windows form.
looking for simple couple cicles method.
example. : A[MxN] ->>> B[MxN] (Sorted 1.....n)

Getting the random numbers is trivial:
Random rnd;
for (int y = 0; y < h; y++)
for (int x = 0; x < w; x++)
array[y][x] = l - rnd.Next(2 * l + 1);
Random.Next() will return a random value between 0 and the given parameter (excluding the parameter; which is the reason for the + 1).
A 2D array can be sorted just like a 1D array, it only depends how you want to handle multiple lines, e.g. is it just for display or do you want to sort every line for itself, etc.

Here's a solution which first sorts each row's columns into order, then sorts each row comparing by first column, then second, etc:
namespace StackOverflow.Demos
{
class Program
{
public static void Main(string[] args)
{
new Program();
Console.WriteLine("Done");
Console.ReadKey();
}
Program()
{
double[,] data = GenerateData();
OutputData(data, "Before");
SortData(ref data);
OutputData(data, "After");
}
double[,] GenerateData()
{
Random randomGenerator = new Random(DateTime.UtcNow.Millisecond);
double[,] data = new double[5, 5];
for (int i = 0; i < data.GetLength(0); i++)
{
for (int j = 0; j < data.GetLength(1); j++)
{
data[i, j] = (randomGenerator.NextDouble() * 2) - 1;
}
}
return data;
}
void OutputData(double[,] data, string message)
{
Console.WriteLine("=====================");
Console.WriteLine(message);
Console.WriteLine("=====================");
for (int i = 0; i < data.GetLength(0); i++)
{
for (int j = 0; j < data.GetLength(1); j++)
{
Console.Write(data[i, j]);
Console.Write("\t");
}
Console.WriteLine();
}
}
void SortData(ref double[,] data)
{
//sort sub arrays
SortDataRows(ref data);
//sort this array
for (int i = 0; i < data.GetLength(0)-1; i++)
{
for (int j = i; j < data.GetLength(0); j++)
{
for (int k = 0; k < data.GetLength(1); k++)
{
if (data[i, k].CompareTo(data[j, k]) < 0) //if already in order exit loop
{
break;
} else if (data[i, k].CompareTo(data[j, k]) > 0) //if out of order switch and loop
{
SwapRows(ref data, i, j);
break;
}//else orders are equal so far; continue to loop
}
}
}
}
void SortDataRows(ref double[,] data)
{
for (int row = 0; row < data.GetLength(0); row++)
{
for (int i = 0; i < data.GetLength(1) - 1; i++)
{
for (int j = i; j < data.GetLength(1); j++)
{
if (data[row, i].CompareTo(data[row, j]) > 0)
{
Swap<double>(ref data[row, i], ref data[row, j]);
}
}
}
}
}
void Swap<T>(ref T a, ref T b)
{
T temp = a;
a = b;
b = temp;
}
void SwapRows(ref double[,]data, int i, int j)
{
for (int k = 0; k < data.GetLength(1); k++)
{
Swap<double>(ref data[i, k], ref data[j, k]);
}
}
}
}
The code's not the best (haven't had a cup of tea yet), but should do what you're after.
Here's a better solution (not using a 2D array as such, but using a structure which can easily be converted to/from such an array):
sing System.Diagnostics;
namespace StackOverflow.Demos
{
class Program
{
public static void Main(string[] args)
{
new Program();
Console.WriteLine("Done");
Console.ReadKey();
}
Program()
{
List<List<double>> data = GenerateData(5, 5).ToList<List<double>>();
OutputData(data,"Before");
foreach (List<double> item in data)
{
item.Sort();
}
data.Sort(CompareListOfDoubles);
OutputData(data,"After");
}
private IEnumerable<List<double>> GenerateData(int index1, int index2)
{
Random rnd = new Random(DateTime.UtcNow.Millisecond);
List<double> result;
for (int i = 0; i < index1; i++)
{
result = new List<double>(index2);
for (int j = 0; j < index2; j++)
{
result.Add(rnd.NextDouble() * 2 - 1);
}
yield return result;
}
}
private void OutputData(List<List<double>> data, string message)
{
Console.WriteLine(message);
foreach (List<double> list in data)
{
foreach (double datum in list)
{
Console.Write(datum);
Console.Write("\t");
}
Console.WriteLine();
}
}
static int CompareListOfDoubles(List<double> a, List<double> b)
{
for (int i = 0; i < a.Count; i++)
{
if (i > b.Count) return -1;
if (a[i] > b[i]) return -1;
if (a[i] < b[i]) return 1;
}
if (b.Count > a.Count) return 1;
return 0;
}
double[,] ConvertListListDoubleTo2DArray(List<List<double>> data)
{
double[,] result = new double[data.Count, data[0].Count];
for (int i = 0; i < result.GetLength(0); i++)
{
for (int j = 0; j < result.GetLength(1); j++)
{
result[i, j] = data[i][j];
}
}
return result;
}
}