Within the main function I have the following.
int numRows = rowSequence.length() + 1;
int numCols = columnSequence.length() + 1;
int** twoDimTable = new int* [numRows];
for (int rowIndex = 0; rowIndex < numRows; rowIndex++)
{
twoDimTable[rowIndex] = new int [numCols];
}
//updating table
for (int i = 0; i <= numRows; i++)
{
for (int j = 0; j <= numCols; j++)
{
if (i == 0 || j == 0)
twoDimTable[i][j] = 0;
// when I start running my code I receive an unhandled exception right at the 'if'
// statement: Access violation writing location. I looked at other similar
// situations, but cannot seem to understand the specific issue
else if (rowSequence[i - 1] == columnSequence[j - 1])
twoDimTable[i][j] = twoDimTable[i - 1][j - 1] + 1;
else
twoDimTable[i][j] = max(twoDimTable[i - 1][j], twoDimTable[i][j - 1]);
}
}
One problem is that your for loops are wrong; numRows and numCols are not valid indices, so they should not be included in your iterations.
That is, instead of this:
for (int i = 0; i <= numRows; i++)
{
for (int j = 0; j <= numCols; j++)
{
[...]
}
}
... you should have this:
for (int i = 0; i < numRows; i++)
{
for (int j = 0; j < numCols; j++)
{
[...]
}
}
I'm trying to implement a quick program to solve a system of linear equations. The program reads the input from a file and then writes the upper-triangular system and solutions to a file. It is working with no pivoting, but when I try to implement the pivoting it produces incorrect results.
As example input, here is the following system of equations:
w+2x-3y+4z=12
2w+2x-2y+3z=10
x+y=-1
w-x+y-2z=-4
I expect the results to be w=1, x=0, y=-1 and z=2. When I don't pivot, I get this answer (with some rounding error on x). When I add in the pivoting, I get the same numbers but in the wrong order: w=2,x=1,y=-1 and z=0.
What do I need to do to get these in the correct order? Am I missing a step somewhere? I need to do column swapping instead of rows because I need to adapt this to a parallel algorithm later that requires that. Here is the code that does the elimination and back substitution:
void gaussian_elimination(double** A, double* b, double* x, int n)
{
int maxIndex;
double temp;
int i;
for (int k = 0; k < n; k++)
{
i = k;
for (int j = k+1; j < n; j++)
{
if (abs(A[k][j]) > abs(A[k][i]))
{
i = j;
}
}
if (i != k)
{
for (int j = 0; j < n; j++)
{
temp = A[j][k];
A[j][k] = A[j][i];
A[j][i] = temp;
}
}
for (int j = k + 1; j < n; j++)
{
A[k][j] = A[k][j] / A[k][k];
}
b[k] = b[k] / A[k][k];
A[k][k] = 1;
for (i = k + 1; i < n; i++)
{
for (int j = k + 1; j < n; j++)
{
A[i][j] = A[i][j] - A[i][k] * A[k][j];
}
b[i] = b[i] - A[i][k] * b[k];
A[i][k] = 0;
}
}
}
void back_substitution(double**U, double*x, double*y, int n)
{
for (int k = n - 1; k >= 0; k--)
{
x[k] = y[k];
for (int i = k - 1; i >= 0; i--)
{
y[i] = y[i] - x[k]*U[i][k];
}
}
}
I believe what you implemented is actually complete pivoting.
With complete pivoting, you must keep track of the permutation of columns, and apply the same permutation to your answer.
You can do this with an array {0, 1, ..., n}, where you swap the i'th and k'th values in the second loop. Then, rearange the solution using this array.
If what you were trying to do is partial pivoting, you need to look for the maximum in the respective row, and swap the rows and the values of 'b' accordingly.
Edit: I have uploaded the vector to Drive as a text file, in case anyone want to have a look: https://drive.google.com/file/d/0B0wsPU8YebRQbDUwNFYza3ljSnc/view?usp=sharing
I'm trying to reshape my vector h into a 3D array. h contains 295788 elements. In this case height = 314, width = 314 and depth = 3.
Basically what I'm trying to do is what MATLAB does with its reshape function.
h = reshape(h, height, width, depth)
This is my attempt so far, but when I print it all I see is zeroes, which is not right. I have double checked that h contains the numbers I'm expecting.
vector<vector<vector<double> > > array3D;
int height = 314, width = 314, depth = 3;
// Set up sizes
array3D.resize(height);
for (int i = 0; i < height; ++i) {
array3D[i].resize(width);
for (int j = 0; j < width; ++j)
array3D[i][j].resize(depth);
}
for (int i = 0; i < height; i++)
{
array3D[i][0][0] = h[i];
for (int j = 0; j < width; j++)
{
array3D[i][j][0] = h[i+j];
for (int k = 0; k < depth; k++)
{
array3D[i][j][k] = h[i+j+k];
}
}
}
Printing:
for (vector<vector<vector<double>>>::const_iterator i = array3D.begin(); i != array3D.end(); ++i)
{
for (vector<vector<double>>::const_iterator j = i->begin(); j != i->end(); ++j)
{
for (vector<double>::const_iterator k = j->begin(); k != j->end(); ++k)
{
cout << *k << ' ';
}
}
}
So my question is, how do I convert my vector into a 3D array properly?
I managed to do this by using Eigen::Tensor as suggested by Henri Menke. I ended up creating an array for the initial 314x314x3 matrix, and then another one for the 300x300x3 matrix. It's neither fast nor pretty, but for now that is what I could come up with. Looks like this.
For clarification: margin is calculated further up in the code, but in this example with the 314x314x3 matrix it's margin=7. h is a vector with 295788 elements. nrh=314, nch=314 and nradii=3.
Tensor<int, 3> t(nrh, nch, nradii);
int counter = 0;
for (int k = 0; k < nradii; k++)
{
for (int col = 0; col < nch; col++)
{
for (int row = 0; row < nrh; row++)
{
t(row, col, k) = h[counter];
counter += 1;
}
}
}
int height = nrh - margin * 2;
int width = nch - margin * 2;
int depth = nradii;
Tensor<int, 3> out(height, width, depth);
int count1 = 0, count2 = 0, count3 = 0;
for (int k = 0; k < depth; k++)
{
for (int j = margin; j < nch - margin; j++)
{
for (int i = margin; i < nrh - margin; i++)
{
out(count1, count2, count3) = t(i, j, k);
count1 += 1;
}
count1 = 0;
count2 += 1;
}
count2 = 0;
count3 += 1;
}
Edit: Solution #2 with Tensor.slice()
int height = nrh - margin * 2;
int width = nch - margin * 2;
int depth = nradii;
Tensor<int, 3> tensor(height, width, depth);
DSizes<ptrdiff_t, 3> indices(margin, margin, 0);
DSizes<ptrdiff_t, 3> sizes(height, width, nradii);
tensor = t.slice(indices, sizes);
How about:
array3D[i][j][k] = h[i*(depth*width)+j*depth+k];
That may or may not be scanning the vector in the correct order.
Notice how when the index k resets the index j increments so you move on exactly one until the index j resets in which case i increments and the same. It's easy to show this calculation reads every element exactly once.
I'd normally expect a width, height then depth and you're scanning in the opposite order!
Footnote: Depending on the application is may be worthwhile to just access the vector using this approach. In general it turns out to be faster than accessing a vector of vectors of vectors. That can be relevant when dealing with massive arrays.
Actually, your the structure of your code is already ok, however, there are two mistakes:
The lines
array3D[i][0][0] = h[i];
and
array3D[i][j][0] = h[i+j];
are pointless. You are overwriting those entries later on with the line
array3D[i][j][k] = h[i+j+k];
The index calculation for h[] is wrong: You must multiply the row index by the length of a row before adding the cell index. The assignment should look like this:
array3D[i][j][k] = h[(i*width+j)*depth+k];
Otherwise, you will get the same result for (i, j, k) == (3, 2, 1) as for (i, j, k) == (1, 3, 2), which is obviously wrong. In the index calculation above, I have assumed that k is the fastest changing dimension. If that is not the order in which your data is stored in h, you need to change the positions of i, j, and k and adjust the factors accordingly.
Putting this together, your assignment loop should read:
for (int i = 0; i < height; i++) {
for (int j = 0; j < width; j++) {
for (int k = 0; k < depth; k++) {
array3D[i][j][k] = h[(i*width+j)*depth+k];
}
}
}
Slightly off-topic:
If you were using C instead of C++, you could "simply" do this:
size_t dataSize;
//Create a real 3D array with the dimensions (height, width, depth).
double (*array3D)[width][depth] = malloc(dataSize = height*sizeof(*array3D));
//Copy over the data from the file.
memcpy(array3D, h, dataSize);
//Print the array contents:
for (int i = 0; i < height; i++) {
for (int j = 0; j < width; j++) {
for (int k = 0; k < depth; k++) {
printf("%d ", array3D[i][j][k]);
}
}
}
This uses a real 3D array instead of an array of pointers to arrays of pointers to arrays of doubles (which is roughly what a vector<vector<vector<double>>> is). However, this cannot be done in C++ as C++ does not allow for array types with dynamic sizes as C does.
I have the following problem. I have written code for alpha-trimmed filter in opencv library. I think that it is properly constructed but I don't know how to compare two 3 channels pixels during sorting a 'window with pixels'. In my code it is done but comparing two but it is impossible for vectors. I assume that i should compare it one channel and after second and so on. Have you any hints for me, or could you propose some modifications in my code. This is my code.
int alphatrimmed(Mat img, int alpha)
{
Mat img9 = img.clone();
const int start = alpha;
const int end = 9 - alpha;
//going through whole image
for (int i = 1; i < img.rows - 1; i++)
for (int j = 1; j < img.cols-1; j++)
{
int k = 0;
Vec3b element[9];
//selecting elements
for (int m = i - 1; m < i + 2; m++)
for (int n = j - 1; n < j + 2; n++)
element[k++] = img.at<Vec3b>(m*img.cols + n);
for (int i = 0; i < end; i++)
{
int min = i;
for (int j = i + 1; j < 9; j++)
if (element[j] < element[min])
min = j;
Vec3b temp = element[i];
element[i] = element[min];
element[min] = temp;
}
const int result = (i - 1)*(img.cols - 2) + j - 1;
img9.at<Vec3b>(result) = element[start];
for (int j = start + 1; j < end; j++)
img9.at<Vec3b>(result) += element[j];
img9.at<Vec3b>(result) /= 9 - alpha;
}
namedWindow("AlphaTrimmed Filter", WINDOW_AUTOSIZE);
imshow("AlphaTrimmed Filter", img9);
return 0;
}
Thank you for your time spent on solving my problem.
So what I am trying to do is multiply one 2d vector by another 2d vector.
I come from Java,Python and C# so I am pretty much learning C++ as I go along.
I have the code down to generate the vector and display the vector but I can't seem to finish the multiplication part.
v1 is another matrix that is already generated.
vector<vector<int> > v2 = getVector();
int n1 = v1[0].size();
int n2 = v2.size();
vector<int> a1(n2, 0);
vector<vector<int> > ans(n1, a1);
for (int i = 0; i < n1; i++) {
for (int j = 0; j < n2; j++) {
for (int k = 0; k < 10; k++) {
// same as z[i][j] = z[i][j] + x[i][k] * y[k][j];
ans[i][j] += v1[i][k] * v2[k][j];
}
}
}
displayVector(ans);
My guess for where I am going wrong is in the inner-most loop. I can't figure out what to actually put in place of that 10 I have there now.
When you multiply matrices, the number of columns of the matrix on the left side must equal the number of rows of the matrix on the right side. You need to check that that is true, and use that common number for your size of the k variable:
int nCommon = v1.size();
assert(v2[0].size() == nCommon);
for (int i = 0; i < n1; i++) {
for (int j = 0; j < n2; j++) {
for (int k = 0; k < nCommon ; k++) {
ans[i][j] += v1[i][k] * v2[k][j];
}
}
}
For you inner loop, you should do something like this
ans[i][j] = 0;
for (int k = 0; k < n2; k++) {
ans[i][j] += v1[i][k] * v2[k][j];
}
I don't know where the 10 comes from.