We Keep Coding

C++ vector vs C# List. Why are they producing different results?

I'm translating some code from C++ to C#. The code is simple enough and most of the syntax involved is almost the same, however the C++ version uses a vector<vector<int>> structure that doesn't exists in C#, so I replaced it with a List<List<int>> which I initialized with empty "rows" since I need it to represent a table with a fixed number of rows (See the code below).
The logic is exactly the same but somehow i'm not getting the same outputs. I suspect it has to do with the behaviour of List<List<int>> but I'm not sure. Any hints would be appreciated.
This is the original C++ code:
void LIS(int arr[], int n)
{
vector<vector<int>> L(n);
L[0].push_back(arr[0]);
for (int i = 1; i < n; i++)
{
for (int j = 0; j < i; j++)
{
if ((arr[i] > arr[j]) && (L[i].size() < L[j].size() + 1)){
L[i] = L[j];
}
}
L[i].push_back(arr[i]);
}
vector<int> max = L[0];
for (vector<int> x : L)
if (x.size() > max.size())
max = x;
for(int e : max){
cout<<e<<endl;
}
}
This is my C# version:
public static void LIS(int[] arr, int n)
{
// Here I replace the vector<vector<int>> with a List<List<int>>
// and I fill it with n empty lists.
List<List<int>> L = new List<List<int>>();
for(int x = 0; x < n; x++)
{
L.Add(new List<int>());
}
L[0].Add(arr[0]);
for (int i = 1; i < n; i++)
{
for (int j = 0; j < i; j++)
{
if ((arr[i] > arr[j]) && (L[i].Count < L[j].Count + 1))
{
L[i] = L[j];
}
}
L[i].Add(arr[i]);
}
List<int> max = L[0];
foreach (List<int> x in L)
{
if (x.Count > max.Count)
{
max = new List<int>(x);
}
}
foreach(int e in max){
Console.WriteLine(e);
}
I'm testing with this array:
int[] arr = { 0, 8, 4, 12, 2, 10, 6, 14, 1, 9, 5, 13, 3, 11, 7, 15 };
int n = arr.Length;
In the C++ version I get:
0 2 6 9 11 15
and in the C# version I get:
0 8 4 12 2 10 6 14 1 9 5 13 3 11 7 15

In the C++ code doesn't L[i] = L[j]; make a copy of the vector at L[j] and store it in L[i] (similar thing for the variable max)? To achieve a similar result in C#, you could do L[i] = L[j].ToList() to copy the list.

print diagonal routes array

I make this code to print array if the number of array is
{
1, 2, 3, 4, 5,
6, 7, 8, 9,
10,11,12,13,
14,15,16,17,
18,19,20,21,
22,23,24,25
}
the out put of this code is
1,2,6,3,7,11,4,6,12,16,5,9,13,17,21,10,14,18,21,15,19,23,20,24,25
I want to make change in this my code to be the out put begging from
21 16 22 11 17 23 6 12 18 24 1 7 13 19 25 2 8 14 203 9 15 4 10 5
and this my Code
int [,] p = new int [5,5];
int sum = 1;
for (int i = 1; i <= 5; i++)
{
for (int j = 1; j <= 5; j++)
{
p[i, j] = sum;
richTextBox1.Text += p[i, j].ToString();
sum++;
}
}
int C;
int R=1;
for (int i = 1; i <= 5; i++)
{
C = i;
for (int r = 1; r <= i; r++)
{
Output = Output + p[r, C];
C--;
}
}
richTextBox2.Text += Output.ToString();
for (int i = 2; i >= 5; i++)
{
R = i;
for (C = 5; C >= i; C--)
{
Output = Output + p[R, C];
R++;
}
}
richTextBox2.Text += Output.ToString();

This is more fiddly than you might think!
You want to traverse the diagonals of this matrix:
1 2 3 4 5
6 7 8 9 10
11 12 13 14 15
16 17 18 19 20
21 22 23 24 25
starting at the bottom left and traversing each diagonal as depicted in this very badly-drawn diagram:
I've written a helper method called DiagonalIndices() that for a given maximum index (in the case of your 5x5 matrix, it will be 4) will produce a sequence of (row,col) indices in the correct order to traverse all the diagonals.
Note that the input array MUST be square - I'm assuming that is the case for your data! It clearly is for a 5x5 matrix.
The output of the following program is
21 16 22 11 17 23 6 12 18 24 1 7 13 19 25 2 8 14 20 3 9 15 4 10 5
Here's the code:
using System;
using System.Collections.Generic;
namespace ConsoleApplication4
{
public sealed class Index
{
public Index(int row, int col)
{
Row = row;
Col = col;
}
public readonly int Row;
public readonly int Col;
}
public static class Program
{
private static void Main()
{
int [,] p = new int[5, 5];
for (int i = 0, n = 1; i < 5; ++i)
for (int j = 0; j < 5; ++j, ++n)
p[i, j] = n;
// This is the bit you will use in your program.
// Replace the Console.WriteLine() with your custom code
// that uses p[index.Row, index.Col]
int maxIndex = p.GetUpperBound(1);
foreach (var index in DiagonalIndices(maxIndex))
Console.Write(p[index.Row, index.Col] + " ");
Console.WriteLine();
}
public static IEnumerable<Index> DiagonalIndices(int maxIndex)
{
for (int i = 0; i <= maxIndex; ++i)
for (int j = 0; j <= i; ++j)
yield return new Index(maxIndex-i+j, j);
for (int i = 0; i < maxIndex; ++i)
for (int j = 0; j < maxIndex-i; ++j)
yield return new Index(j, i+j+1);
}
}
}

This is a more efficient of printing only the diagonal values in a positive direction in a matrix.
int [,] p = new int [5,5]
{
{1, 2, 3, 4,5},
{6, 7, 8, 9},
{10,11,12,13},
{14,15,16,17},
{18,19,20,21},
{22,23,24,25}
};
for (int i = 4, j = 4; i >= 0 && j > = 0; i--, j--)
{
Console.Writeline(p[ i, j ]);
}

Converting a 2D Array of 2D Arrays into a single 2D Array

So I have an object called, for lack of a better word, MatricesMatrix which is a Matrix of Matrices (everything is stored as a double[,]). I want to strip all of the values from the inner matrices into one big Matrix. Here is what I have so far:
public Matrix ConvertToMatrix()
{
//Figure out how big the return matrix should be
int totalRows = this.TotalRows();
int totalColumns = this.TotalColumns();
Matrix returnMatrix = new Matrix(totalRows, totalColumns);
List<object> rowElementsList = new List<object>();
//"outer" index means an index of the MatricesMatrix
//"inner" index means an index of a Matrix within the Matrices Matrix
//outer row loop
for (int outerRowIndex = 0; outerRowIndex < NumberOfRows; outerRowIndex++)
{
//outer column loop
for (int outerColumnIndex = 0; outerColumnIndex < NumberOfColumns; outerColumnIndex++)
{
Matrix currentMatrix = GetElement(outerRowIndex, outerColumnIndex);
object element = null;
//inner row loop
for (int innerRowIndex = 0; innerRowIndex < currentMatrix.NumberOfRows; innerRowIndex++)
{
//inner column loop
for (int innerColumnIndex = 0; innerColumnIndex < currentMatrix.NumberOfColumns; innerColumnIndex++)
{
element = currentMatrix.GetElement(innerRowIndex, innerColumnIndex);
}
}
returnMatrix.SetElement(outerRowIndex, outerColumnIndex, (double)element);
}
}
return returnMatrix;
}
Note that I have determined programmatically the total number of rows and columns the returnMatrix needs to have.
Here are some more guidelines and output cases:
Each element of the big matrix should be in the same position relative to the other elements of the big matrix that came from the Matrix inside of MatricesMatrix that the element came from.
Each "matrix" (no longer in matrix form) inside of the big matrix should be in the same position relative to the other matrices inside of the big matrix as it was inside of the MatricesMatrix (with no overlapping, and 0's in any spaces left empty).
CASE 1
Given this input: a MatricesMatrix(2,2) with [0,0] = (2x2 matrix), [0,1] = (2x3 matrix), [1,0] = (2x2 matrix), and [1,1] = (2x3 matrix). That is,
Output must be:
CASE 2
Given this input: a MatricesMatrix(2,2) with [0,0] = (1x1 matrix), [0,1] = (3x3 matrix), [1,0] = (2x2 matrix), and [1,1] = (4x4 matrix). That is,
Output should be something like:
Any assistance would be greatly appreciated!
UPDATE:
Here is a unit test for Case 1 that should pass:
[TestMethod]
public void MatricesMatrix_ConvertToMatrixTest()
{
Matrix m1 = new Matrix(2);
Matrix m2 = new Matrix(2, 3);
Matrix m3 = new Matrix(2);
Matrix m4 = new Matrix(2, 3);
double[] m1Row1 = { 1, 1 };
double[] m1Row2 = { 1, 1 };
double[] m2Row1 = { 2, 2, 2 };
double[] m2Row2 = { 2, 2, 2 };
double[] m3Row1 = { 3, 3 };
double[] m3Row2 = { 3, 3 };
double[] m4Row1 = { 4, 4, 4 };
double[] m4Row2 = { 4, 4, 4 };
m1.SetRowOfMatrix(0, m1Row1);
m1.SetRowOfMatrix(1, m1Row2);
m2.SetRowOfMatrix(0, m2Row1);
m2.SetRowOfMatrix(1, m2Row2);
m3.SetRowOfMatrix(0, m3Row1);
m3.SetRowOfMatrix(1, m3Row2);
m4.SetRowOfMatrix(0, m4Row1);
m4.SetRowOfMatrix(1, m4Row2);
MatricesMatrix testMatricesMatrix = new MatricesMatrix(2, 2);
testMatricesMatrix.SetElement(0, 0, m1);
testMatricesMatrix.SetElement(0, 1, m2);
testMatricesMatrix.SetElement(1, 0, m3);
testMatricesMatrix.SetElement(1, 1, m4);
Matrix expectedResult = new Matrix(4, 5);
double[] expectedRow1 = { 1, 1, 2, 2, 2 };
double[] expectedRow2 = { 1, 1, 2, 2, 2 };
double[] expectedRow3 = { 3, 3, 4, 4, 4 };
double[] expectedRow4 = { 3, 3, 4, 4, 4 };
expectedResult.SetRowOfMatrix(0, expectedRow1);
expectedResult.SetRowOfMatrix(1, expectedRow2);
expectedResult.SetRowOfMatrix(2, expectedRow3);
expectedResult.SetRowOfMatrix(3, expectedRow4);
Matrix actualResult = testMatricesMatrix.ConvertToMatrix();
(actualResult == expectedResult).Should().BeTrue();
}

I started with a simple Matrix class to hold the double[,]s. Nothing too fancy, just a simple array-of-arrays with a row and column count and array accessor.
class Matrix<T>
{
public int Rows { get; private set; }
public int Cols { get; private set; }
private T[,] mat;
public Matrix(int rowCount, int colCount)
{
Rows = rowCount;
Cols = colCount;
mat = new T[Rows, Cols];
}
public T this[int r, int c]
{
get { return mat[r, c]; }
set { mat[r, c] = value; }
}
}
Your second case looks more difficult (and like a better test of correctness) than the first, so I set up a metamatrix to match that.
public static Matrix<double[,]> BuildMetaMatrix()
{
Matrix<double[,]> m = new Matrix<double[,]>(2, 2);
m[0, 0] = new double[,]
{
{ 1 }
};
m[0, 1] = new double[,]
{
{ 3, 3, 3 },
{ 3, 3, 3 },
{ 3, 3, 3 }
};
m[1, 0] = new double[,]
{
{ 2, 2 },
{ 2, 2 }
};
m[1, 1] = new double[,]
{
{4, 4, 4, 4},
{4, 4, 4, 4},
{4, 4, 4, 4},
{4, 4, 4, 4}
};
return m;
}
For convenience, I made a Place function that puts one matrix into another one at the given location.
static void Place(double[,] src, double[,] dest, int destR, int destC)
{
for (int row = 0; row < src.GetLength(ROW_DIM); row++)
{
for (int col = 0; col < src.GetLength(COL_DIM); col++)
{
dest[row + destR, col + destC] = src[row, col];
}
}
}
The magic numbers fed into GetLength() were just asking for mistakes, so I defined some constants for them (ROW_DIM = 0 and COL_DIM = 1). I decided to handle the padding by figuring out how wide a column is and how tall a row is and skipping any extra elements after Place()ing the sub-matrix in. A GetRowHeight() and GetColWidth() method figure out the values.
public static int GetRowHeight(Matrix<double[,]> m, int row)
{
int maxSeen = 0;
for (int col = 0; col < m.Cols; col++)
{
if (m[row, col].GetLength(ROW_DIM) > maxSeen)
{
maxSeen = m[row, col].GetLength(ROW_DIM);
}
}
return maxSeen;
}
public static int GetColWidth(Matrix<double[,]> m, int col)
{
int maxSeen = 0;
for (int row = 0; row < m.Rows; row++)
{
if (m[row, col].GetLength(COL_DIM) > maxSeen)
{
maxSeen = m[row, col].GetLength(COL_DIM);
}
}
return maxSeen;
}
A Flatten() function loops through all the sub-matrices, Place()ing them at the appropriate row and column in a new matrix. It updates the next row and column after each Place() using the GetRowHeight() and GetColWidth() functions.
Matrix<double> Flatten(Matrix<Matrix<double>> src)
{
// (7, 6) == (this.TotalRows(), this.TotalColumns())
// from your code.
Matrix<double> dest = new Matrix<double>(7, 6);
int nextRow = 0;
int nextCol = 0;
for (int row = 0; row < src.Rows; row++)
{
for (int col = 0; col < src.Rows; col++)
{
dest.Place(src[row, col], nextRow, nextCol);
nextCol += GetColWidth(src, col);
}
nextRow += GetRowHeight(src, row);
nextCol = 0;
}
return dest;
}
A little glue to test it out...
static void Main(string[] args)
{
Matrix<double[,]> src = BuildMetaMatrix();
double[,] dest = Flatten(src);
Print(dest);
Console.ReadLine();
}
static void Print(double[,] matrix)
{
for (int row = 0; row < matrix.GetLength(ROW_DIM); row++)
{
for (int col = 0; col < matrix.GetLength(COL_DIM); col++)
{
Console.Write(matrix[row, col] + "\t");
}
Console.Write("\n");
}
}
...and you get an output just like your second case with all the oddly fitting matrices and 0s in the empty places.*
1 0 3 3 3 0
0 0 3 3 3 0
0 0 3 3 3 0
2 2 4 4 4 4
2 2 4 4 4 4
0 0 4 4 4 4
0 0 4 4 4 4
*The destination matrix gets its values initialized to default(double), which happens to be 0 (the value you wanted). If you need something other than default(double) for the empty places, you can probably get them by iterating over the new matrix and writing the new default value everywhere before Flatten()ing the metamatrix.
(Thanks to Jeff Mercado for pointing out that multidimensional arrays' GetLength() method can be used to find their dimensions.)

I think it would be beneficial to you to break up the solution into the quadrants you are trying to fill. This will all be under the assumption that we will only be combining 4 matrices in this 2x2 configuration. The same strategies explined here can be applied to other dimensions of matrices to be combined.
So given 4 matrices A, B, C and D, we will try to build a resulting matrix in this arrangement:
+---+---+
| A | B |
+---+---+
| C | D |
+---+---+
Before we can start, we will need to figure out the dimensions of the final result. This should hopefully make sense. We'll have a top half, bottom half, left half and right half.
rows_top = max(rows_A, rows_B)
rows_bottom = max(rows_C, rows_D)
rows_result = rows_top + rows_bottom
cols_left = max(cols_A, cols_C)
cols_right = max(cols_B, cols_D)
cols_result = cols_left + cols_right
Then we will want to consider which regions of the result matrix we want to copy each of the 4 matrices. Considering the origin at the top-left, everything on the right half will be shifted over by the size of the left half, everything on the bottom half will be shifted over by the size of the top half. The offsets for each of the matrices would be:
offset_A = (0, 0)
offset_B = (0, cols_left)
offset_C = (rows_top, 0)
offset_D = (rows_top, cols_left)
Now with all this information, we can start building up the result matrix. Just copy over the values from each matrix to the result, with the offsets applied.
So in code, I would do this:
// I'm just going to use plain 2D arrays here
public T[,] Combine<T>(T[,] a, T[,] b, T[,] c, T[,] d)
{
// get the total rows
var rows_top = Math.Max(a.GetLength(0), b.GetLength(0));
var rows_bottom = Math.Max(c.GetLength(0), d.GetLength(0));
var rows_result = rows_top + rows_bottom;
// get the total columns
var cols_left = Math.Max(a.GetLength(1), c.GetLength(1));
var cols_right = Math.Max(b.GetLength(1), d.GetLength(1));
var cols_result = cols_left + cols_right;
// get the offsets
var offset_a = Tuple.Create(0, 0);
var offset_b = Tuple.Create(0, cols_left);
var offset_c = Tuple.Create(rows_top, 0);
var offset_d = Tuple.Create(rows_top, cols_left);
// fill 'er up
var result = new T[rows_result, cols_result];
Fill(result, a, offset_a);
Fill(result, b, offset_b);
Fill(result, c, offset_c);
Fill(result, d, offset_d);
return result;
}
public void Fill<T>(T[,] result, T[,] source, Tuple<int, int> offset)
{
for (var i = 0; i < source.GetLength(0); i++)
for (var j = 0; j < source.GetLength(1); j++)
result[offset.Item1 + i, offset.Item2 + j] = source[i, j];
}
Then to demonstrate the result in terms of case 2:
const string A = "A", B = "B", C = "C", D = "D";
var a = new string[1,1]
{
{ A },
};
var b = new string[3, 3]
{
{ B, B, B },
{ B, B, B },
{ B, B, B },
};
var c = new string[2, 2]
{
{ C, C },
{ C, C },
};
var d = new string[4, 4]
{
{ D, D, D, D },
{ D, D, D, D },
{ D, D, D, D },
{ D, D, D, D },
};
var result = Combine(a, b, c, d);
This of course can be generalized to any size matrix of matrices. The concept is the same in every step of the process.
Given m x n matrices, we will try to build a resulting matrix in this arrangement:
+-----+-----+-----+
| 0,0 | ... | 0,n |
+-----+-----+-----+
| ... | | ... |
+-----+-----+-----+
| m,0 | ... | m,n |
+-----+-----+-----+
Get the dimensions of each of the slices.
rows_0 = max(rows_0_0, ..., rows_0_n)
...
rows_m = max(rows_m_0, ..., rows_m_n)
rows_result = sum(rows_0, ..., rows_m)
cols_0 = max(cols_0_0, ..., cols_m_0)
...
cols_n = max(cols_0_n, ..., cols_m_n)
cols_result = sum(cols_0, ..., cols_m)
Get the offsets for each of the matrices. Each vertical slice is offset to the left by the total amount of columns in the previous vertical slices. Each horizontal slice is offset to the down by the total amount of rows in the previous horizontal slices.
offset_0_0 = (0, 0)
...
offset_m_n = (sum(rows_0, ..., rows_m-1), sum(cols_0, ..., cols_n-1))
So now we can build up the result matrix.
public T[,] Combine<T>(T[,][,] m)
{
// get the rows
var rows = GetSliceRows(m);
var rows_result = rows.Sum();
// get the cols
var cols = GetSliceCols(m);
var cols_result = cols.Sum();
// get the offsets
var offsets = GetOffsets(rows, cols);
// fill 'er up
var result = new T[rows_result, cols_result];
Fill(result, m, offsets);
return result;
}
public int[] GetSliceRows<T>(T[,][,] m)
{
var sliceRows = new int[m.GetLength(0)];
var segments = m.GetLength(1);
for (var i = 0; i < sliceRows.Length; i++)
{
sliceRows[i] = Enumerable.Range(0, segments)
.Select(j => m[i, j].GetLength(0))
.Max();
}
return sliceRows;
}
public int[] GetSliceCols<T>(T[,][,] m)
{
var sliceCols = new int[m.GetLength(1)];
var segments = m.GetLength(0);
for (var j = 0; j < sliceCols.Length; j++)
{
sliceCols[j] = Enumerable.Range(0, segments)
.Select(i => m[i, j].GetLength(1))
.Max();
}
return sliceCols;
}
public Tuple<int, int>[,] GetOffsets(int[] rows, int[] cols)
{
var offsets = new Tuple<int, int>[rows.Length, cols.Length];
for (var i = 0; i < rows.Length; i++)
for (var j = 0; j < cols.Length; j++)
offsets[i, j] = Tuple.Create(
rows.Take(i).Sum(),
cols.Take(j).Sum()
);
return offsets;
}
public void Fill<T>(T[,] result, T[,][,] m, Tuple<int, int>[,] offsets)
{
for (var i = 0; i < m.GetLength(0); i++)
for (var j = 0; j < m.GetLength(1); j++)
Fill(result, m[i, j], offsets[i, j]);
}
public void Fill<T>(T[,] result, T[,] source, Tuple<int, int> offset)
{
for (var i = 0; i < source.GetLength(0); i++)
for (var j = 0; j < source.GetLength(1); j++)
result[offset.Item1 + i, offset.Item2 + j] = source[i, j];
}

I think you have to give array elements corresponding rowid and columnid to achieve the indexing issue of the outer matrix. Assuming you already have a Array to Matrix object conversion;
Being not sure if I undestood the rules correctly, but here is what I implemented so far:
I implemented Matrix and MatrixList classes as follows:
public class Matrix
{
public int row { get; set; }
public int column { get; set; }
public double value { get; set; }
}
public class MatrixList
{
public List<Matrix> matrixList = new List<Matrix>();
}
Using those classes, I implemented the algorithm below:
List<MatrixList> matricesMatrix = new List<MatrixList>();
init(matricesMatrix);
int totalRows = 10;//as you stated, this is already known
int totalColumns = 10;//as you stated, this is already known
List<Matrix> ResultMatrix = new List<Matrix>();
foreach (MatrixList matrixListItem in matricesMatrix)
{
for (int i = 0; i < totalRows; i++)
{
List<Matrix> matrixItemList = matrixListItem.matrixList.FindAll(s => s.row == i);
foreach(Matrix matrixItem in matrixItemList)
for (int j = 0; j < totalColumns; j++)
{
if (matrixItem.column == j)
ResultMatrix.Add(new Matrix { row = i, column = j, value = matrixItem.value });
}
}
}
where init is a method to fill the objects, implemented as follows:
private void init(List<MatrixList> matricesMatrix)
{
MatrixList ml = new MatrixList();
for (int i = 0; i < 10; i++)
{
for (int j = 0; j < 10; j++)
{
ml.matrixList.Add(new Matrix { row = i, column = j, value = i + j });
}
}
matricesMatrix.Add(ml);
}
I was on a windows forms dummy app, so used a richtextbox to test the code above.
for (int i = 0; i < totalRows; i++)
{
foreach (Matrix item in ResultMatrix)
{
if (item.row == i)
{
for (int j = 0; j < totalColumns; j++)
if (item.column == j)
richTextBox1.Text += item.value + " ";
}
}
richTextBox1.Text += Environment.NewLine;
}
and the result is:
0 1 2 3 4 5 6 7 8 9
1 2 3 4 5 6 7 8 9 10
2 3 4 5 6 7 8 9 10 11
3 4 5 6 7 8 9 10 11 12
4 5 6 7 8 9 10 11 12 13
5 6 7 8 9 10 11 12 13 14
6 7 8 9 10 11 12 13 14 15
7 8 9 10 11 12 13 14 15 16
8 9 10 11 12 13 14 15 16 17
9 10 11 12 13 14 15 16 17 18
I don't have much time to give pretty numbers to array items to show beriefly at the moment, but I think you can get the idea.

How can I generate the list of number adding 5 to starting number?

This is the simple question but want to make sure that I do it right way!
I want to generate the list of numbers where I start with number 3 and add 5 to every next number.
Result should be like this:
var list = new List<int> { 3, 8, 13, 18, 23, 28 ..... 103 };
My Code:
var uniqueInts = new List<int>();
addBy = 5;
for (int i = 3; i < 1000; i++)
{
uniqueInts.Add(addBy);
}
Am I trying correctly or there is any easy way?

Increase your for loop index by 5 instead of 1.
for (int i = 3; i < 10000; i += 5)
{
uniqueInts.Add(i);
}

Just for comparison, here's a way to do it using Linq. In this case I think I prefer your loop though :)
var list = Enumerable.Range(0, 21).Select(x => x*5+3).ToList();
EDIT: Not sure what you mean by your comment below about wanting output as "var list = ...", but if you want the string (ie the source code) as output, you can use a Linq aggregate;
string str = Enumerable.Range(0, 21).
Aggregate(new StringBuilder("var list = new List<int> {"),
(builder, i) => builder.Append(i*5+3 + ", ")).Append("};").ToString();
Console.WriteLine(str);

You can try the following:
IList<int> uniqueInts = new List<int>();
for (int i = 3; i < 10000; i+=5)
{
uniqueInts.Add(i);
}
Of course you would have to define an upper limit because i < 10000 will only give you (10000 / 5) - 1 numbers.

Change your incremental amount:
i += 5
so, applied:
for (var i = 3; i < 10000; i += 5){
uniqueIds.Add(i);
}
// 3, 8, 13, 18, 23, ...
Assuming that's what you meant, but your question's kind of ambiguous.
unless you want to add 5 to every number in the list, in which case:
var list = new List<int> { 3, 8, 13, 18, 23, 28 ..... 103 };
var uniqueIds = new List<int>();
foreach (var n in list)
{
uniqueIds.Add(n + 5);
}
// 8, 13, 23, 33, ...

Firstly, if you want to know if you are doing it right, test it yourself in a new console project and debug it.
Secondly,
Your code will result in a list of {5, 5, 5, 5...}.
What you should do is:
var uniqueInts = new List<int>();
addBy = 5;
for (int i = 3; i < 1000; i += addBy)
{
uniqueInts.Add(i);
}
This way i will always grow by 5 at the end of every loop and it will be added to the list..
This will give you all results between 3 and 1000 with jumps of 5, if that is what you wanted.

You can try something like this:
class Program
{
static void Main(string[] args)
{
List<int> numList = new List<int> { 3 };
for (int i = 0; i < 100; i++)
{
Console.WriteLine(numList[i]);
numList.Add(numList[i] + 5);
}
Console.Read();
}
}

Your code is wrong, it will add the number 5 every time.
Instead do this:
var uniqueInts = new List<int>();
addBy = 5;
for (int i = 3; i < 10000; i += 5)
{
uniqueInts.Add(i);
}

You can try something like this:
int num = 3;
List<int> listofnumbers = new List<int>();
listofnumbers.Add(num);
for (int i = 0; i < 10000; i++)
{
num += 5;
listofnumbers.Add(num);
}

Looks like perfect fit for infinite sequence with IEnumerable<int>:
IEnumerable<int> GeometricProgression(int start, int step)
{
while(true)
{
yield return start;
start += step;
}
}
Usage:
var result = GeometricProgression(3, 5).Take(10).ToList();

Different combinations of an array (C#)

how can we find out different combination of the elements of an array using c# code.
are there any inbuilt library function for this.?
for eg: suppose an array has elements {2,3,4,5,6,7}
then the possible combination would be 2,3,4,5,6,7,2 3,2 3 4,2 3 4 5, etc
so basically wat i need is a function which gives different combination based on its input for eg: comb(array,2) gives output 2 3,1 2,3 4 and comb(array,3) gives output 1 2 3,2 3 4,3 4 5 and so on
Eg: valid comnbination for array= {1, 2, 3} and length = 2 are 1 2,1 3,2 3 .....

static void Main()
{
var cnk = comb(new [] {1,2,3},2);
foreach ( var c in cnk)
{
}
}
public static IEnumerable<int[]> comb(int[] a, int k)
{
if (a == null || a.Length == 0 || k < 1 || k > a.Length)
yield break;
int n = a.Length;
// 1
if ( k == 1)
for ( int i = 0; i < n; i++)
{
yield return new int[] {a[i]};
}
else
{
// k
for ( int i = 0; i < n - k + 1; i++)
{
var res = new int[k];
for (int t = i, c = 0; t < i + k - 1; t++, c++)
res[c] = a[t];
for (int j = i + k - 1; j < n; j++)
{
res[k-1] = a[j];
yield return res;
}
}
}
}
You should take the algorithm from here, my answer doesn't solve your problem
Algorithm to return all combinations of k elements from n

Seemed logic is not absolutely correct as:
var cnk = comb(new[] { 1, 2, 3, 4 }, 3);
This gives 3 variants, but as a matter of fact it is 4:
1 2 3
1 2 4
1 3 4
2 3 4
I guess comb is better to be implemented in recursive way.